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Telecommunication Reference Book
700 Acronyms & Terminologies
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Dedicated
To my mother Because of whose struggle and sacrifice I am able to attain this position
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Preface
After I completed the reference book on Internet, I realized that the task would remain incomplete if no similar effort is made with reference to telecommunication, as convergence of Internet to telecommunication media is occurring at very rapid pace that these two can not remain separated for long. A typical example is VOIP (Voice Over Internet Protocol). There is so much common between the two now that study of one will remain incomplete without the study of the other. This realization prompted me to compile a similar book on Telecommunication terminologies and acronyms.
The book contains about 700 terms and acronyms and is organized in alphabetical order on the pattern of any dictionary. Each term has been presented in the form of a continuous paragraph for continuity purpose. Wherever necessary, bullet points have been used to describe features, types and characteristics. The terms and acronyms are not merely defined but are explained in detail. Any term/acronym used within each of the explanation is underlined so that the reader not only becomes aware but also registers the same in mind subconsciously. The reader will be able to study the related terms so underlined generally within the same book elsewhere. The book also describes, certain products, services and protocols, which are offered by various international organizations of the industry and have become world industry standards. All the terms have been presented without any changes or amendments as they were downloaded from highly authentic websites (Please refer to bibliography at the end).
I am thankful to all my friends who assisted me in organizing this book, especially Mr. Naseemullah Malik, who designed its beautiful and meaningful cover.
I hope this small effort will be appreciated.
Ahmad Nadeem Syed [email protected]
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Table of Contents A ...... 1 B ...... 9 C ...... 17 D ...... 30 E ...... 51 F ...... 60 G ...... 75 H ...... 83 I ...... 90 J...... 94 K ...... 95 L ...... 96 M ...... 103 N ...... 110 O ...... 115 P ...... 124 Q ...... 133 R ...... 134 S...... 142 T ...... 161 U ...... 170 V ...... 174 W ...... 180 X ...... 188 Numbers & Signs ...... 191
vi A A Access Access is simply being able to get to what you need. Data access is being able to get to (usually having permission to use) particular data on a computer. Web access means having a connection to the World Wide Web through an access provider or an online service provider such as America Online. For data access, access is usually specified as read-only access and read/write access.
ACD An Automatic Call Distributor (ACD) is a telephone facility that manages incoming calls and handles them based on the number called and an associated database of handling instructions. Many companies offering sales and service support use ACDs to validate callers, make outgoing responses or calls, forward calls to the right party, allow callers to record messages, gather usage statistics, balance the use of phone lines, and provide other services. ACDs often provide some form of Automatic Customer/Caller Identification (ACIS) such as that provided by Direct Inward Dialing (DID), Dialed Number Identification Service (DNIS), or Automatic Number Identification (ANI).
ACTS ACTS (Automatic Coin Telephone System) is a public coin-operated telephone service that completes a variety of phone calls, times the calls, and collects payment without the aid of an operator
Adaptive Differential Pulse-Code Modulation Pl. see ADPCM
ADPCM ADPCM (adaptive differential pulse-code modulation) is a technique for converting sound or analog information to binary information (a string of 0's and 1's) by taking frequent samples of the sound and expressing the value of the sampled sound modulation in binary terms. ADPCM is used to send sound on fiber- optic long-distance lines as well as to store sound along with text, images, and code on a CD-ROM
ADSL ADSL (Asymmetric Digital Subscriber Line) is a technology for transmitting digital information at a high bandwidth on existing phone lines to homes and businesses. Unlike regular dialup phone service, ADSL provides continously-available, "always on" connection. ADSL is asymmetric in that it uses most of the channel to transmit downstream to the user and only a small part to receive information from the user. ADSL simultaneously accommodates analog (voice) information on the same line.
1 ADSL is generally offered at downstream data rates from 512 Kbps to about 6 Mbps. A form of ADSL, known as Universal ADSL or G.lite, has been approved as a standard by the ITU-TS. ADSL was specifically designed to exploit the one-way nature of most multimedia communication in which large amounts of information flow toward the user and only a small amount of interactive control information is returned. Several experiments with ADSL to real users began in 1996. In 1998, wide-scale installations began in several parts of the U.S. In 2000 and beyond, ADSL and other forms of DSL are expected to become generally available in urban areas. With ADSL (and other forms of DSL), telephone companies are competing with cable companies and their cable modem services. Also see Fast Guide to DSL.
Advanced Intelligent Network Pl. see AIN
AF AF (audio frequency) (also abbreviated af or a.f.) refers to alternating current (AC) having a frequency such that, if applied to a transducer such as a loudspeaker or headset, will produce acoustic waves within the range of human hearing. The AF range is generally considered to be from 20 Hz to 20,000 Hz. All telephone circuits operate with AF signals in a restricted range of approximately 200 Hz to 3000 Hz. A telephone-line modem is an AF device that converts binary digital data into analog signals that can be transmitted over the telephone circuit, and also converts incoming AF signals into binary digital data.
AIN The Advanced Intelligent Network (AIN) is a telephone network architecture that separates service logic from switching equipment, allowing new services to be added without having to redesign switches to support new services. It encourages competition among service providers since it makes it easier for a provider to add services and it offers customers more service choices. Developed by Bell Communications Research, AIN is recognized as an industry standard in North America. Its initial version, AIN Release 1, is considered a model toward which services will evolve. Meanwhile, evolutionary subsets of AIN Release 1 have been developed. These are shown in the AIN Release Table below. Elsewhere, the International Telecommunications Union (see ITU-T), endorsing the concepts of AIN, developed an equivalent version of AIN called Capability Set 1 (CS-1). It comes in evolutionary subsets called the Core INAP capabilities. How It Works A telephone caller dials a number that is received by a switch at the telephone company central office. The switch - known as the Service Switching Point (SSP) - forwards the call over a Signaling System 7 (SS7) network to a Service Control Point (SCP) where the service logic is located. The Service Control Point identifies the service requested from part of the number that was dialed and returns information about how to handle the call to the Service Switching Point. Examples of services that the SCP might provide include area number calling service, disaster recovery service, do not disturb service, and 5-digit extension dialing service. In some cases, the call can be handled more quickly by an Intelligent Peripheral (IP) that is attached to the Service Switching Point over a high-speed connection. For
2 example, a customized voice announcement can be delivered in response to the dialed number or a voice call can be analyzed and recognized. In addition, an "adjunct" facility can be added directly to the Service Switching Point for high-speed connection to additional, undefined services. One of the services that AIN makes possible is Local Number Portability (LNP).
The AIN Release Table
AIN Release Capabilities Trigger checkpoints at off-hook, digit collection and analysis, and routing points of call Code gapping to check for overload Release 0 conditions at SCP 75 announcements at the switching system Based on ANSI TCAP issue 1 Adds a formal call model that distinguishes the originating half of the call from the terminating half Additional triggers 254 Release 0.1 announcements at the switching system Based on ANSI TCAP issue 2 Adds Phase 2 Personal Communication Service (PCS) support Voice Activated Dialing (VAD) ISDN-based SSP-IP interface Release 0.2 Busy and no-answer triggers Next events list processing at SCP Default routing Release 1 A full set of capabilities
Air Interface In cellular telephone communications, the air interface is the radio-frequency portion of the circuit between the cellular phone set or wireless modem (usually portable or mobile) and the active base station. As a subscriber moves from one cell to another in the system, the active base station changes periodically. Each changeover is known as a handoff. A cellular connection is only as good as its weakest link, which is almost always the air interface. Radio-frequency (RF)circuits are subject to many variables that affect signal quality. Factors that can cause problems include: Use of the handheld phone set or portable wireless modem inside buildings, cars, buses, trucks, or trains Proximity to human-made, steel-frame obstructions, especially large buildings and freeway overpasses Abundance of utility wires that can reflect radio signals and/or generate noise that interferes with reception Irregular terrain, particularly canyons and ravines Inadequate transmitter power in phone set or wireless modem Poorly designed antenna in phone set or wireless modem
In addition to these variables, some cellular networks have inadequate coverage in certain geographic areas. Usually this is because there are not enough base stations to ensure continuous communications for subscribers using portable (handheld) phone sets. As a network evolves, more base stations may be installed
3 in a given region, and in that case, this problem will diminish with time. Conversion of a network from analog to digital can result in dramatic improvement.
Aloha Aloha, also called the Aloha method, refers to a simple communications scheme in which each source (transmitter) in a network sends data whenever there is a frame to send. If the frame successfully reaches the destination (receiver), the next frame is sent. If the frame fails to be received at the destination, it is sent again. This protocol was originally developed at the University of Hawaii for use with satellite communication systems in the Pacific. In a wireless broadcast system or a half-duplex two-way link, Aloha works perfectly. But as networks become more complex, for example in an Ethernet system involving multiple sources and destinations that share a common data path, trouble occurs because data frames collide (conflict). The heavier the communications volume, the worse the collision problems become. The result is degradation of system efficiency, because when two frames collide, the data contained in both frames is lost. To minimize the number of collisions, thereby optimizing network efficiency and increasing the number of subscribers that can use a given network, a scheme called slotted Aloha was developed. This system employs signals called beacons that are sent at precise intervals and tell each source when the channel is clear to send a frame. Further improvement can be realized by a more sophisticated protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
ADSI ADSI (Analog Display Services Interface) is the standard protocol for enabling alternate voice and data services, such as a visual display at the phone, over the analog telephone network. Developed by Bellcore in 1993, ADSI is now built into devices such as special telephones with small display screens, cable TV set-top box, personal digital assistants (personal digital assistant), pagers, and personal computers with telephone applications. A popular application enabled by ADSI is Call Waiting Deluxe, an application that displays the name and number of an incoming call while you are on the phone. If you have an ADSI screen phone, several options are displayed on your screen including switching to the new call, forwarding the new call to your voice mail, putting the new caller on hold, playing a recorded message, or dropping the current call and switching to the new call. Other ADSI applications include: Visual voice mail, the display of telephone voice mail menu options and a list of your voice mail messages Visual directory, a service that allows you to locate the telephone number of an individual or business and, possibly at extra charge, to download the address of that individual to your screen phone E-mail browsing, allowing you to send and receive e-mail messages via an ADSI-enabled device. Schedule-based services, faxing abilities, notification of incoming e-mail messages, home banking, ticket purchasing, and access to train and plane schedules
4 Analog Display Services Interface Pl. see ADSI
Amateur Radio Amateur radio, also known as ham radio, is a hobby enjoyed by several hundred thousand people in the United States and by over a million people worldwide. Amateur radio operators call themselves "radio hams" or simply "hams." To become a radio ham, you must pass an examination. Wireless amateur communication is done on numerous bands (relatively narrow frequency segments) extending from 1.8 MHz (a wavelength of about 160 meters) upwards through several hundred gigahertz (wavelengths in the millimeter range). There are several license classes. The more privileges a class of license conveys, the more difficult is the examination that one must pass to obtain it. Amateur radio operation is fun, and that is one of the main reasons hams do it. But ham radio can provide communication during states of emergency. Ham radio works when all other services fail. After Hurricane Andrew struck South Florida in 1992, the utility grid was destroyed over hundreds of square miles. All cellular towers and antennas were blown down. Only amateur radio, the Citizens Radio Service ("Citizens Band"), and a few isolated pay phones with underground lines provided communication between the outside world and the public in the affected area. Amateur radio operators are known as technical innovators, and have been responsible for important discoveries. For example, in the early part of the 20th century, government officials believed that all the frequencies having wavelengths shorter than 200 meters (1.5 MHz) were useless for radio communications, so they restricted radio amateurs to these frequencies. It was not long before ham radio operators discovered the truth, and were communicating on a worldwide scale using low-power transmitters. Thus the shortwave radio era began.
Analog In telecommunications, an analog signal is one in which a base carrier's alternating current frequency is modified in some way, such as by amplifying the strength of the signal or varying the frequency, in order to add information to the signal. Broadcast and telephone transmission have conventionally used analog technology. An analog signal can be represented as a series of sine waves. The term originated because the modulation of the carrier wave is analogous to the fluctuations of the human voice or other sound that is being transmitted. Analog describes any fluctuating, evolving, or continually changing process.
Apogee Pl. see Satellite
Asia Cellular Satellite System Asia Cellular Satellite System(ACeS) is a combined cellular telephone and satellite wireless system from Ericsson that provides digital communication service to mobile phone and computer users in the Asia Pacific Region. Adding satellite communication to the terrestrial Global System for Mobile (GSM) communication system, ACeS is billed as the first integrated satellite-GSM system in the world.
5 Users with Ericsson dual-mode terminals will be able to roam within the region switching as necessary between cellular (local) service and satellite service. ACeS is expected to be available in an area from Indonesia in the South; Papua, New Guinea in the East; Japan in the North; and Pakistan in the West, an area with a combined population of three billion. ACeS wilI make it possible for many people to have telecommunication services for the first time. ACeS has signed over 19 roaming service agreements with GSM operators. ACeS subscribers are provided with a GSM subscriber identify module (SIM) and a network access code (which is a telephone number) that can be used outside the region or within the region when blockage of satellite signals occur (typically, by nearby buildings). GSM subscribers visiting the region can also reach other GSM services via satellite if they have an ACeS SIM and an ACeS terminal.
ASR Automated speech recognition (ASR) is a technology that allows users of information systems to speak entries rather than punching numbers on a keypad. ASR is used primarily to provide information and to forward telephone calls. In recent years, ASR has become popular in the customer service departments of large corporations. It is also used by some government agencies and other organizations. Basic ASR systems recognize single-word entries such as yes-or-no responses and spoken numerals. This makes it possible for people to work their way through automated menus without having to enter dozens of numerals manually with no tolerance for error. In a manual-entry situation, a customer might hit the wrong key after having entered 20 or 30 numerals at intervals previously in the menu, and give up rather than call again and start over. ASR virtually eliminates this problem. Sophisticated ASR systems allow the user to enter direct queries or responses, such as a request for driving directions or the telephone number of a hotel in a particular town. This shortens the menu navigation process by reducing the number of decision points. It also reduces the number of instructions that the user must receive and comprehend. For institutions that rely heavily on customer service, such as airlines and insurance companies, ASR makes it possible to reduce the number of human call-center employees. Those people can then be trained for other jobs that are more profitable and interesting, such as complaint resolution, customer retention, or sales. The technology of speech recognition has been around for some time. It is improving, but problems still exist. An ASR system cannot always correctly recognize the input from a person who speaks with a heavy accent or dialect, and it has major problems with people who combine words from two languages by force of habit. Marginal cell-phone connections can cause the system to misinterpret the input. And, although the cost is gradually diminishing, ASR systems are still too expensive for some organizations.
Asymmetric Digital Subscriber Line Pl. see ADSL
Asymmetric DSL Pl. see ADSL
ATM
6 ATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path. Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN). ATM also stands for automated teller machine, a machine that bank customers use to make transactions without a human teller.
Attempt In a telecommunications system, an attempt is a user request to get connected to the system or to initiate a call, whether or not the connection is made or the call is initiated
Attachment Unit Interface Pl. see AUI
AUI The AUI (attachment unit interface) is the 15-PIN physical connector interface between a computer's network interface card (NIC) and an Ethernet cable. On 10BASE-5 ("thicknet") Ethernet, a short cable is used to connect the AUI on the computer with a transceiver on the main cable. In 10BASE-2 or "thinnet" Ethernet networks, the NIC connects directly to the Ethernet coaxial cable at the back of the computer. IEEE 802.3, the Ethernet standard, defines the AUI 15-pin physical layer interface. This interface is also called a DB-15 interface or a DIX interface (DIX refers to the three major companies who helped standardize Ethernet: Digital Equipment Corporation, Intel, and Xerox).
Audible Ring In a telephone system, an audible ring is the tone that is returned from the called party's switching device and heard by the caller. This tone indicates to the caller that the desired party is being rung.
Automated Speech Recognition Pl. see ASR
Automatic Call Distributor Pl. see ACD
Automatic Coin Telephone System Pl. see ACTS Asynchronous transfer mode Pl. see ATM
7 ATM ATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path. Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN). ATM also stands for automated teller machine, a machine that bank customers use to make transactions without a human teller.
8 B B Backbone A backbone is a larger transmission line that carries data gathered from smaller lines that interconnect with it. At the local level, a backbone is a line or set of lines that local area networks connect to for a wide area network connection or within a local area network to span distances efficiently (for example, between buildings). On the Internet or other wide area network, a backbone is a set of paths that local or regional networks connect to for long-distance interconnection. The connection points are known as network nodes or telecommunication data switching exchanges (DSEs).
Band In telecommunication, a band - sometimes called a frequency band - is a specific range of frequencies in the radio frequency (RF) spectrum, which is divided among ranges from very low frequencies (vlf) to extremely high frequencies (ehf). Each band has a defined upper and lower frequency limit. Because two radio transmitters sharing the same frequency band cause mutual interference, band usage is regulated. International use of the radio spectrum is regulated by the International Telecommunication Union (ITU). Domestic use of the radio spectrum is regulated by national agencies such as the Federal Communications Commission (Fcc) in the U.S. Regulatory organizations assign each transmission source a band of operation, a transmitter radiation pattern, and a maximum transmitter power.
Bandwidth Bandwidth (the width of a band of electromagnetic frequencies) is used to mean (1) how fast data flows on a given transmission path, and (2), somewhat more technically, the width of the range of frequencies that an electronic signal occupies on a given transmission medium. Any digital or analog signal has a bandwidth. Generally speaking, bandwidth is directly proportional to the amount of data transmitted or received per unit time. In a qualitative sense, bandwidth is proportional to the complexity of the data for a given level of system performance. For example, it takes more bandwidth to download a photograph in one second than it takes to download a page of text in one second. Large sound files, computer programs, and animated videos require still more bandwidth for acceptable system performance. Virtual reality (VR) and full-length three-dimensional audio/visual presentations require the most bandwidth of all. In digital systems, bandwidth is expressed as data speed in bits per second (bps). Thus, a modem that works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps. In analog systems, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. frequency is measured in the number of cycles of change per second, or hertz. A typical voice signal has a bandwidth of approximately three kilohertz (3 kHz); an
9 analog television (TV) broadcast video signal has a bandwidth of six megahertz (6 MHz) -- some 2,000 times as wide as the voice signal. Communications engineers once strove to minimize the bandwidths of all signals, while maintaining a minimum acceptable level of system performance. This was done for at least two reasons: (1) low-bandwidth signals are less susceptible to noise interference than high- bandwidth signals; and (2) low-bandwidth signals allow for a greater number of communications exchanges to take place within a specified band of frequencies. However, this simple rule no longer applies in general. For example, in spread spectrum communications, the bandwidths of signals are deliberately expanded. In digital cable and fiber optic systems, the demand for ever-increasing data speeds outweighs the need for bandwidth conservation. In the electromagnetic radiation spectrum, there is only so much available bandwidth to go around, but in hard- wired systems, available bandwidth can literally be constructed without limit by installing more and more cables.
B channel In the Integrated Services Digital Network (ISDN), the B-channel is the channel that carries the main data. (The "B" stands for "bearer" channel.) In ISDN, there are two levels of service: the Basic Rate Interface, intended for the home and small enterprise, and the Primary Rate Interface, for larger users. Both rates include a number of B- (bearer) channels and a D-channel. The B-channels carry data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D- channel. Thus, a Basic Rate Interface user can have up to 128 Kbps service. The Primary Rate Interface consists of 23 B-channels and one 64 Kpbs D-channel in the United States or 30 B-channels and 1 D-channel in Europe.
B8ZS B8ZS (bipolar 8-zero substitution, also called binary 8-zero substitution, clear channel, and clear 64) is an encoding method used on T1 circuits that inserts two successive ones of the same voltage - referred to as a bipolar violation - into a signal whenever eight consecutive zeros are transmitted. The device receiving the signal interprets the bipolar violation as a timing mark, which keeps the transmitting and receiving devices synchronized. Ordinarily, when successive ones are transmitted, one has a positive voltage and the other has a negative voltage. B8ZS is based on an older encoding method called alternate mark inversion (AMI). AMI is used with Dataphone Digital Service, the oldest data service still in use that uses 64 Kbps channels. AMI, however, requires the use of 8 Kbps of the 64 Kbps of each channel to maintain synchronization. In a T1 circuit, there are 24 channels. This loss adds up to 192 Kbps, which means that in reality only 56 Kbps is available for data transmission. B8ZS uses bipolar violations to synchronize devices, a solution that does not require the use of extra bits, which means a T1 circuit using B8ZS can use the full 64 Kbps for each channel for data. B8ZS is not compatible with older AMI equipment. T1 technology is used in the United States and Japan. In Europe, a comparable technology called E1 provides 32 channels instead of 24 and uses an encoding scheme called high-density bipolar 3 (HDB3) instead of B8ZS.
10 Baseband Baseband has several usages: Describing a telecommunication system in which information is carried in digital form on a single unmultiplexed signal channel on the transmission medium. This usage pertains to a baseband network such as Ethernet and token ring local area networks. Same as the above, but allowing that the information could also be carried in analog form. Any frequency band on which information is superimposed, whether or not the band is multiplexed and information is sent on subbands. In this usage, there is sometimes the meaning that the frequency band is not shifted to some other frequency band but remains at its original place in the electromagnetic spectrum.
Basic Rate Interface Pl. see B-Channel
BER In telecommunication transmission, the bit error rate (BER) is the percentage of bits that have errors relative to the total number of bits received in a transmission, usually expressed as ten to a negative power. For example, a transmission might have a BER of 10 to the minus 6, meaning that, out of 1,000,000 bits transmitted, one bit was in error. The BER is an indication of how often a packet or other data unit has to be retransmitted because of an error. Too high a BER may indicate that a slower data rate would actually improve overall transmission time for a given amount of transmitted data since the BER might be reduced, lowering the number of packets that had to be resent. A BERT (bit error rate test or tester) is a procedure or device that measures the BER for a given transmission.
Binary Runtime Environment for Wireless Pl. see BREW
Binary 8-zero substitution Pl. see B8ZS
Bipolar Signaling Bipolar signaling, also called bipolar transmission, is a baseband method of sending binary data over wire or cable. There are two logic states, low and high, represented by the digits 0 and 1 respectively. The illustration shows a bipolar signal as it might appear on the screen of an oscilloscope. Each horizontal division represents one bit (binary digit). The logic 0 state is -3 volts and logic 1 is +3 volts. This is positive logic. Alternatively, logic 0 might be +3 volts, and logic 1 might be -3 volts; this would be negative logic. Whether positive or negative logic is used, the voltages representing the low and high states are equal and opposite; over time, the average voltage is approximately equal to 0. A bipolar signal resembles an alternating current (AC) rectangular wave, except that the frequency is not constant. The bandwidth of the signal is inversely proportional to the duration of each data bit. Typical data speeds in baseband are several megabits per second (Mbps); hence the duration of each bit is a fraction of a microsecond.
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Bipolar Transmission Pl. see Bipolar signaling
Bipolar 8-zero substitution Pl. see B8ZS
Bis The word (also used as a prefix or suffix) bis, applied to some modem protocol standards, is Old Latin for "repeat" (akin to Old High German "twice"). When a protocol ends with "bis," it means that it's the second version of that protocol. Similarly, ter is from Old Latin meaning "three times." The suffix terbo in the V.xx modem protocol is an invented word based on the Old Latin ter and the word turbo (Latin for "whirling top" or "whirlwind") meaning "speed." V.32terbo is the third version developed of the V.32 modem protocol.
Bit Error Rate Pl. ses BER
Bit Robbing Bit robbing is a technique used in signaling on the T-carrier system, the widely- used system for transmitting both voice and data in digital form in the public switched telephone network (PSTN) and in private networks. In the basic T-1 system, a 193 bit frame, serving 24 channels, is transmitted in a sequence of 12 frames that are referred to as a superframe. Special signaling information, such as whether a voice channel is on-hook or off-hook, is included within the superframe by using a bit (called the a bit) that is "robbed" from the sixth frame as a signaling bit and another bit (the b bit) that is robbed from the 12th frame. Bit robbing is
12 acceptable for voice conversations or data transmissions that are received by a modem but not for "pure" data transmission (where every bit is significant). Bit robbing is the reason that a 64 Kbps channel only carries 56 Kbps of usable data. Bit robbing is a form of in-band signalling.
Bit Stuffing In telecommunication, bit stuffing is the addition of a small number of binary digits to a transmission unit in order to fill it up to a standard size or to help synchronize signaling rates between points in a network. The receiver knows how to detect and remove or disregard the stuffed bits. For example, the timing or bit rate of T-carrier system signals is constantly synchronized between any terminal device and an adjacent repeater or between any two repeaters. The synchronization is achieved by detecting the transition in polarity for 1 bits in the data stream. (T-1 signalling uses bipolar signaling, where each successive bit with a value of 1 is represented by voltage with a reverse polarity from the previous bit. Bits with a value of 0 are represented by a no-voltage time slot.) If more than 15 bits in a row are sent with a 0 value, this "lull" in 1 bits that the system depends on for synchronization may be long enough for two end points to become out of synchronization. To handle this situation (the sequence of more than 15 0 bits), the signal is "stuffed" with a short, unique bit pattern (which includes some 1 bits) that is recognized as a synchronization pattern. The receiving end removes the stuffed bits and restores the bit stream to its original sequence.
Bits per second Pl. see BPS
Blue Tooth Bluetooth is a computing and telecommunications industry specification that describes how mobile phones, computers, and personal digital assistants (PDAs) can easily interconnect with each other and with home and business phones and computers using a short-range wireless connection. Using this technology, users of cellular phones, pagers, and personal digital assistants such as the PalmPilot will be able to buy a three-in-one phone that can double as a portable phone at home or in the office, get quickly synchronized with information in a desktop or notebook computer, initiate the sending or receiving of a fax, initiate a print-out, and, in general, have all mobile and fixed computer devices be totally coordinated. Bluetooth requires that a low-cost transceiver chip be included in each device. The tranceiver transmits and receives in a previously unused frequency band of 2.45 GHz that is available globally (with some variation of bandwidth in different countries). In addition to data, up to three voice channels are available. Each device has a unique 48-bit address from the IEEE 802 standard. Connections can be point-to-point or multipoint. The maximum range is 10 meters. Data can be exchanged at a rate of 1 megabit per second (up to 2 Mbps in the second generation of the technology). A frequency hop scheme allows devices to communicate even in areas with a great deal of electromagnetic interference. Built- in encryption and verification is provided.
13 BPS In data communications, bits per second (abbreviated bps) is a common measure of data speed for computer modem and transmission carriers. As the term implies, the speed in bps is equal to the number of bits transmitted or received each second. The duration d of a data bit, in seconds, is inversely proportional to the digital transmission speed s in bps: d = 1/s
Larger units are sometimes used to denote high data speeds. One kilobit per second (abbreviated Kbps in the U.S.; kbps elsewhere) is equal to 1,000 bps. One megabit per second (Mbps) is equal to 1,000,000 bps or 1,000 kbps. Computer modems for twisted pair telephone lines usually operate at speeds between 14.4 and 57.6 kbps. The most common speeds are 28.8 and 33.6 kbps. So-called "cable modems," designed for use with TV cable networks, can operate at more than 100 kbps. Fiberoptic modems are the fastest of all; they can send and receive data at many Mbps. The bandwidth of a signal depends on the speed in bps. With some exceptions, the higher the bps number, the greater is the nominal signal bandwidth. (Speed and bandwidth are, however, not the same thing.) Bandwidth is measured in standard frequency units of kHz or MHz. Data speed is sometimes specified in terms of baud, which is a measure of the number of times a digital signal changes state in one second. Baud, sometimes called the "baud rate," is almost always a lower figure than bps for a given digital signal. The terms are often used interchangeably, even though they do not refer to the same thing. If you hear that a computer modem can function at "33,600 baud" or "33.6 kilobaud," you can be reasonably sure that the term is being misused, and the figures actually indicate bps.
BREW BREW (Binary Runtime Environment for Wireless) is Qualcomm's open source application development platform for wireless devices equipped for code division multiple access (CDMA) technology. BREW makes it possible for developers to create portable applications that will work on any handsets equipped with CDMA chipsets. Because BREW runs in between the application and the chip operating system software, the application can use the device's functionality without the developer needing to code to the system interface or even having to understand wireless applications. Users can download applications - such as text chat, enhanced e-mail, location positioning, games (both online and offline), and Internet radio - from carrier networks to any BREW-enabled phone. BREW is competing for wireless software market share with J2ME (Java 2 Micro Edition), a similar platform from Sun Microsystems. The initial version of BREW is solely for CDMA networks; later versions could be enabled for time division multiple access (TDMA) and Global System for Mobile Communication (GSM) networks.
Broadband In general, broadband refers to telecommunication in which a wide band of frequencies is available to transmit information. Because a wide band of frequencies is available, information can be multiplexed and sent on many different frequencies or channels within the band concurrently, allowing more information to
14 be transmitted in a given amount of time (much as more lanes on a highway allow more cars to travel on it at the same time). Related terms are wideband (a synonym), baseband (a one-channel band), and narrowband (sometimes meaning just wide enough to carry voice, or simply "not broadband," and sometimes meaning specifically between 50 cps and 64 Kpbs). Various definers of broadband have assigned a minimum data rate to the term. Here are a few: Newton's Telecom Dictionary: "...greater than a voice grade line of 3 KHz...some say [it should be at least] 20 KHz." Jupiter Communications: at least 256 Kbps. IBM Dictionary of Computing: A broadband channel is "6 MHz wide."
It is generally agreed that Digital Subscriber Line (DSL) and cable TV are broadband services in the downstream direction. Also see bandwidth.
Broadcast In general, to broadcast (verb) is to cast or throw forth something in all directions at the same time. A radio or television broadcast (noun) is a program that is transmitted over airwaves for public reception by anyone with a receiver tuned to the right signal channel. The term is sometimes used in e-mail or other message distribution for a message sent to all members, rather than specific members, of a group such as a department or enterprise. On the Internet, certain Web sites deliver original or redistributed broadcasts from existing radio and television stations, using streaming sound or streaming video techniques, to Web users who visit the Web site or "tune it in" using a special program such as RealPlayer. Like publicly available radio and television broadcasts, Web broadcasts are available to anyone. The Web now offers live as well as prepackaged broadcasts and also plays back audio and video tapes. Some programming is scheduled and other prepackaged programs can be delivered on demand. Many Web users listen to music from a particular broacasting site as they surf other sites on the Web. Broadcast should not be confused with unicast, a transmission to a specific receiver (like most e-mail messages); multicast, a transmission to multiple specific receivers (as in e-mail to a distribution list or a Web transmission over the MBone network to a specific group of receiving addresses); or anycast, a transmission to the nearest of a group of routers, used in Internet Protocol Version 6 (IPv6) as a technique for chain-updating a group of routers with new routing information.
BRI Pl. see B-channel
Bridge In telecommunication networks, a bridge is a product that connects a local area network (LAN) to another local area network that uses the same protocol (for example, Ethernet or token ring). You can envision a bridge as being a device that decides whether a message from you to someone else is going to the local area network in your building or to someone on the local area network in the building across the street. A bridge examines each message on a LAN, "passing" those
15 known to be within the same LAN, and forwarding those known to be on the other interconnected LAN (or LANs). In bridging networks, computer or node addresses have no specific relationship to location. For this reason, messages are sent out to every address on the network and accepted only by the intended destination node. Bridges learn which addresses are on which network and develop a learning table so that subsequent messages can be forwarded to the right network. Bridging networks are generally always interconnected local area networks since broadcasting every message to all possible destinations would flood a larger network with unnecessary traffic. For this reason, router networks such as the Internet use a scheme that assigns addresses to nodes so that a message or packet can be forwarded only in one general direction rather than forwarded in all directions. A bridge works at the data-link (physical network) level of a network, copying a data frame from one network to the next network along the communications path. A bridge is sometimes combined with a router in a product called a brouter.
Bridge Tap A bridge tap is an extraneous length of dangling, unterminated cable on a communications line, usually left over from an earlier configuration, that can cause impedance mismatches and other undesired effects in transmissions. In a given cabling arrangement, allowance is usually made for a certain length of bridge taps. ISDN uses the standard line code, 2B1Q, because it has the ability to handle the incidence of bridge taps well.
Brouter A brouter (pronounced BRAU-tuhr or sometimes BEE-rau-tuhr) is a network bridge and a router combined in a single product. A bridge is a device that connects one local area network (LAN) to another local area network that uses the same protocol (for example, Ethernet or token ring). If a data unit on one LAN is intended for a destination on an interconnected LAN, the bridge forwards the data unit to that LAN; otherwise, it passes it along on the same LAN. A bridge usually offers only one path to a given interconnected LAN. A router connects a network to one or more other networks that are usually part of a wide area network (WAN) and may offer a number of paths out to destinations on those networks. A router therefore needs to have more information than a bridge about the interconnected networks. It consults a routing table for this information. Since a given outgoing data unit or packet from a computer may be intended for an address on the local network, on an interconnected LAN, or the wide area network, it makes sense to have a single unit that examines all data units and forwards them appropriately.
16 C C Cable head-end A cable head-end is the facility at a local cable TV office that originates and communicates cable TV services and cable modem services to subscribers. In distributing cable television services, the head-end includes a satellite dish antenna for receiving incoming programming. This programming is then passed on to the subscriber. (Cable TV companies may also play videotapes and originate live programming.) Normally, all signals are those that are sent downstream to the subscriber, but some are received upstream such as when a customer requests a pay-per-view program. When a cable company provides Internet access to subscribers, the head-end includes the computer system and databases needed to provide Internet access. The most important component located at the head-end is the Cable Modem Termination System (CMTS), which sends and receives digital cable modem signals on a cable network and is necessary for providing Internet services to cable subscribers.
Cable Modem Termination System Pl. see CMTS
Cable TV Cable TV is also known as "CATV" (community antenna television). In addition to bringing television programs to those millions of people throughout the world who are connected to a community antenna, cable TV will likely become a popular way to interact with the World Wide Web and other new forms of multimedia information and entertainment services.
CableLabs Certified Cable Modems Pl. see DOCSIS
Call Failure Rate Pl. see CFR
Callback Callback, also known as international callback, is a system for avoiding regular phone company long-distance charges by having a call initiated from within the United States with the originating caller joining in a conference call. Here's how the procedure works: A call originator (for example, someone in South America) calls a predesignated number in the United States, waits until it rings once, and then hangs up. A machine in the office where the phone rang recognizes that the phone number was called and knows the phone number of the party that called it (because it was the only party that knew the number).
17 The machine places a call (which may be a local or a long-distance call) that originates from the U.S. location and also calls the party who initiated this procedure, thus arranging a conference call but at the U.S. long-distance rate. In another variation, the automatically-generated call from the U.S. may call the originator and ask the originator to dial their desired number or provide a U.S. dial tone. The originator (who subscribes to this callback service) is billed by the U.S.- based service at its own rates. In localities where portable phone (cellphone) companies do not charge for incoming calls, callback is also sometimes used to avoid airtime charges for outgoing calls.
CAP Carrierless amplitude/phase (CAP) modulation was the original approach for modulation of a Digital Subscriber Line (DSL) signal. Discrete multitone (DMT) is now the preferred modulation alternative over CAP. CAP is closely related to quadrature amplitude modulation (QAM).
Carrier In information technology, a carrier (or carrier signal) is a transmitted electromagnetic pulse or wave at a steady base frequency of alternation on which information can be imposed by increasing signal strength, varying the base frequency, varying the wave phase, or other means. This variation is called modulation. With the advent of laser transmission over optical fiber media, a carrier can also be a laser-generated light beam on which information is imposed. Types of analog modulation of a carrier include amplitude modulation (AM), frequency modulation (FM), and phase modulation. Types of digital modulation include varieties of pulse code modulation (PCM), including pulse amplitude modulation (PAM), pulse duration modulation (PDM), and pulse position modulation (PPM). Carrier detect (see modem lights) is a control signal between a modem and a computer that indicates that the modem detects a "live" carrier that can be used for sending and receiving information. In the telecommunications industry, a carrier is a telephone or other company that sells or rents telecommunication transmission services. A local exchange carrier (LEC) is a local phone company and an inter- exchange carrier (IEC or IXC) carries long-distance calls.
Carrierless Amplitude Phase Modulation Pl. see CAP
CATV CATV (originally "community antenna television," now often "community access television") is more commonly known as "cable TV." In addition to bringing television programs to those millions of people throughout the world who are connected to a community antenna, cable TV is an increasingly popular way to interact with the World Wide Web and other new forms of multimedia information and entertainment services.
CB
18 The Citizen's Band (CB) Radio Service, also known simply as CB, is a public, two- way personal radio service. There are several classifications of CB operation. The best-known form of CB is voice communications that became a fad in the 1970s. Mobile CB operation, especially in cars and trucks, remains popular. To a lesser extent, "CB'ers" engage in fixed operation from homes, and in portable communications using handheld transceivers.Most CB operation takes place within a narrow band of frequencies near 27 MHz. There are 40 channels in this band. These channels are overcrowded. Congestion is worst during peaks in the 11-year sunspot cycle when the 27-MHz band supports worldwide communication known as "skip." Sunspot cycle peaks occurred in 1967-69, 1978-80, 1989-91, and 2000- 2002. During the long intervals between these peaks, the normal range of operation on the 27-MHz band is rarely more than 20 miles. Some CB operators illegally modify their equipment, or obtain illegally modified equipment, capable of higher transmitter output power and greater frequency coverage than is allowed by Federal Communications Commission (FCC) regulations. These operators use their equipment in an attempt to communicate over long distances, and/or to compete with other users of the service. This is called freebanding or bootlegging. The CB Radio Service has proven useful in disaster situations, both small-scale (for example, for a stranded motorist) and large-scale (such as after a hurricane or during a flash flood). An organization of expert communicators known as the Radio Emergency Associated Communications Teams (REACT) provides communications support in emergency situations. Channel 9 (at 27.065 MHz) is monitored by members of REACT, and is recognized as the CB emergency channel in the United States.
CBR Pl. see CB
CDMA CDMA (code-division multiple access) refers to any of several protocols used in so- called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands. CDMA employs analog-to- digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes; this enhances privacy and makes cloning difficult. The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes. CDMA is compatible with other cellular technologies; this allows for nationwide Roaming. The original CDMA standard, also known as CDMA One and still common
19 in cellular telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband CDMA deliver data many times faster.
CDMA 2000 CDMA2000, also known as IMT-CDMA Multi-Carrier or IS-136, is a code-division multiple access (CDMA) version of the IMT-2000 standard developed by the International Telecommunication Union (ITU). The CDMA2000 standard is third- generation (3-G) mobile wireless technology. CDMA2000 can support mobile data communications at speeds ranging from 144 Kbps to 2 Mbps. Deployment is in the planning stages. Versions have been developed by Ericsson and Qualcomm.
CDMA One CDMA One, also written cdmaOne, refers to the original ITU IS-95 (CDMA) wireless interface protocol that was first standardized in 1993. It is considered a second- generation (2G) mobile wireless technology. Today, there are two versions of IS- 95, called IS-95A and IS-95B. The IS-95A protocol employs a 1.25-MHz carrier, operates in radio-frequency bands at either 800 MHz or 1.9 GHz, and supports data speeds of up to 14.4 Kbps. IS-95B can support data speeds of up to 115 kbps by bundling up to eight channels.
CCTV CCTV (closed circuit television) is a television system in which signals are not publicly distributed; cameras are connected to television monitors in a limited area such as a store, an office building, or on a college campus. CCTV is commonly used in surveillance systems.
CDSL CDSL (Consumer Digital Subscriber Line) is a version of Digital Subscriber Line (DSL) service, trademarked by Rockwell Corp., that is somewhat slower than Asymmetric DSL (ADSL) - up to 1 Mbps downstream, probably less upstream - and has the advantage that a splitter does not need to be installed at the user's end. Rockwell no longer provides information about CDSL at its Web site and it does not appear to be marketing it.
Cell In wireless telephony, a cell is the geographical area covered by a cellular telephone transmitter. The transmitter facility itself is called the cell site. The cell provided by a cell site can be from one mile to twenty miles in diameter, depending on terrain and transmission power. Several coordinated cell sites are called a cell system. When you sign up with a cellular telephone service provider, you generally are given access to their cell system, which is essentially local. When travelling out of the range of this cell system, the cell system can enable you to be transferred to a neighboring company's cell system without your being aware of it. This is called roaming service. The cell sites in a system connect to a Mobile Telephone Switching Office (MTSO), which in turn connects to the standard landline telephone system. In a battery power source, a cell is a single energy or charge-storing unit
20 within a pack of cells that form the battery. Each cell has a voltage rating that is combined with the other cells' voltages to form the overall battery voltage rating.
Cell breathing Cell breathing is the constant change of the range of the geographical area covered by a cellular telephone transmitter based on the amount of traffic currently using that transmitter. When a cell becomes heavily loaded, it shrinks. Subscriber traffic is then redirected to a neighboring cell that is more lightly loaded, which is called load balancing. Cell breathing is a common phenomenon of 2G and 3G wireless systems including code-division multiple access (CDMA). CDMA2000 and wideband code-division multiple access (WCDMA) are designed to manage cell breathing.
Cell of Origin Pl. see COO
Cellular Telephone Cellular telephone is a type of short-wave analog or digital transmission in which a subscriber has a wireless connection from a mobile telephone to a relatively nearby transmitter. The transmitter's span of coverage is called a cell. Generally, cellular telephone service is available in urban areas and along major highways. As the cellular telephone user moves from one cell or area of coverage to another, the telephone is effectively passed on to the local cell transmitter. A cellular telephone is not to be confused with a cordless telephone (which is simply a phone with a very short wireless connection to a local phone outlet). A newer service similar to cellular is personal communications services (PCS).
Central office In telephone communication in the United States, a central office (CO) is an office in a locality to which subscriber home and business lines are connected on what is called a local loop. The central office has switching equipment that can switch calls locally or to long-distance carrier phone offices. In other countries, the term public exchange is used.
Centrex Centrex (central office exchange service) is a service from local telephone companies in the United States in which up-to-date phone facilities at the phone company's central (local) office are offered to business users so that they don't need to purchase their own facilities. The Centrex service effectively partitions part of its own centralized capabilities among its business customers. The customer is spared the expense of having to keep up with fast-moving technology changes (for example, having to continually update their private branch exchange infrastructure) and the phone company has a new set of services to sell. In many cases, Centrex has now replaced the private branch exchange. Effectively, the central office has become a huge branch exchange for all of its local customers. In most cases, Centrex (which is sold by different names in different localities) provides customers with as much if not more control over the services they have than PBX did. In some cases, the phone company places Centrex equipment on the customer premises. Typical Centrex service includes direct inward dialing (DID), sharing of the same
21 system among multiple company locations, and self-managed line allocation and cost-accounting monitoring.
CFR Call failure rate (CFR) is a statistical measure commonly used in assessing Internet service providers (ISPs) or any network provider. The call failure rate is the percentage of calls to an ISP or network provider that fail to get through. Companies can measure the CFR for their own employees who dial in for access to the company's network. Rating companies report on the CFRs for major ISPs like AOL, Ameritech, and Mindspring. Visual Networks, formerly Inverse Network Technology, is probably the best known benchmarking company of ISPs.
Channel The channel is a commonly used term in communication and broadcast services as follows: In telecommunications in general, a channel is a separate path through which signals can flow. In the public switched telephone network (PSTN), a channel is one of multiple transmission paths within a single link between network points. For example, the commonly used (in North America) T-carrier system line service provides 24 64 Kbps channels for digital data transmission. In radio and television, a channel is a separate incoming signal or program source that a user can select. In optical fiber transmission using dense wavelength-division multiplexing (DWDM), a channel is a separate wavelength of light within a combined, multiplexed light stream. On the World Wide Web, a channel is a preselected Web site that can automatically send updated information for immediate display or viewing on request. See push technology. In computer and Internet marketing, a channel is a "middleman" between a product creator and the marketplace. Value-added resellers (VAR) and retail store chains are examples of channels in this context. Using Internet Relay Chat, a channel is a specific chat group.
In IBM mainframe systems, a channel is a high bandwidth connection between a processor and other processors, workstations, printers, and storage devices within a relatively close proximity. It's also called a local connection as opposed to a remote (or telecommunication) connection.
CHTML Compact HTML (CHTML or cHTML) is a subset of standard Hypertext Markup Language (HTML) adapted for use with small computing devices such as personal digital assistants (PDAs), cellular phones, and smartphones. Access Company Ltd., a Japanese company, developed Compact HTML for use with i-Mode devices. Because handheld devices have display constraints, and limited power, storage, and memory resources, Compact HTML was created as a stripped-down version of the standard, excluding support for the more demanding features of HTML pages,
22 such as image maps, backgrounds, varieties of fonts, frames , style sheets, and JPEG images. Compact HTML includes support for GIF images, and uses four buttons for operation, rather than two-dimensional cursor movement. Scrolling is not featured, because it is assumed that pages properly designed will fit the screen. Because Compact HTML is based on the universally used standard, it will enable small devices to connect to the open Web, rather than merely a section of it. It is expected that, in the future, several different levels of Compact HTML will be developed to adapt to the requirements of different applications.
Channel 64 Pl. see B8ZS
Channel bank A channel bank is a device at a telephone company central office (public exchange) that converts analog signals from home and business users into digital signals to be carried over higher-speed lines between the central office and other exchanges. The analog signal is converted into a digital signal that transmits at a rate of 64 thousand bits per second (Kbps). This 64 Kbps signal is a standard known as a DS0 signal. The signal is multiplexed with other DS0 signals on the same line using time-division multiplexing (TDM) . Usually, the digital information is put on each DS0 signal using pulse code modulation (PCM).
Circular Mil The circular mil is a unit of area used especially when denoting the cross-sectional size of a wire or cable. It is the equivalent area of a circle whose diameter is 0.001 (10-3) inch, or approximately 2.54 x 10-5 meter. In plane geometry, area always varies in direct proportion to the square of the linear dimension. Thus, if wire A is twice the diameter of wire B, then wire A has four times the cross-sectional area, in circular mils, of B. If wire C is 1/5 of the diameter of wire D, then C has 1/25 the cross-sectional area, in circular mils, of D. In this respect, the circular mil is like any other unit for measurement of area. Because the circular mil is, as its name implies, based on a circular unit of measure rather than a rectangular unit, conversions between circular mils and square units such as the meter squared (m2) or centimeter squared (cm2) can be awkward. To convert circular mils approximately to meters squared (m2), multiply by 5.066 x 10-10. Conversely, multiply by 1.974 x 109. To convert circular mils approximately to centimeters squared (cm2), multiply by 5.066 x 10-6. Conversely, multiply by 1.974 x 105. Also see area, meter squared, centimeter squared, foot squared, International System of Units (SI), and Table of Physical Units.
Citizen's Band Radio Pl. see CB
Clear Channel Pl. see B8ZS
Cloud
23 In telecommunications, a cloud is the unpredictable part of any network through which data passes between two end points. Possibly the term originated from the clouds used in blackboard drawings or more formal illustrations to describe the non-specifiable or uninteresting part of a network. Clouds exist because between any two points in a packet-switched network, the physical path on which a packet travels can vary from one packet to the next and, in a circuit-switched network, the specific circuit that is set up can vary from one connection to the next.
CMTS A cable modem termination system (CMTS) is a component that exchanges digital signals with cable modems on a cable network. A cable modem termination system is located at the local office of a cable television company. A data service is delivered to a subscriber through channels in a coaxial cable or optical fiber cable to a cable modem installed externally or internally to a subscriber's computer or television set. One television channel is used for upstream signals from the cable modem to the CMTS, and another channel is used for downstream signals from the CMTS to the cable modem. When a CMTS receives signals from a cable modem, it converts these signals into Internet Protocol (IP) packets, which are then sent to an IP router for transmission across the Internet. When a CMTS sends signals to a cable modem, it modulates the downstream signals for transmission across the cable to the cable modem. All cable modems can receive from and send signals to the CMTS but not to other cable modems on the line.
CO Pl. see Central Office
Code-Division Multiple Access Pl. see CDMA
Coded Orthogonal Frequency Division Multiplexing Pl. see COFDM
COFDM COFDM is a modulation scheme that divides a single digital signal across 1,000 or more signal carriers simultaneously. The signals are sent at right angles to each other (hence, orthogonal) so they do not interfere with each other. COFDM is used predominately in Europe and is supported by the Digital Video Broadcasting (DVB) set of standards. In the U.S., the Advanced Television Standards Committee (ATSC) has chosen 8-VSB (8-level Vestigial Sideband) as its equivalent modulation standard. The main reason for Europe's decision to use COFDM is its ability to completely overcome multipath effects. When a signal is transmitted, it is met with obstructions such as canyons, buildings, and even people, which scatter the signal causing it to take two or more paths to reach its final destination, the television. The late arrival of the scattered portions of the signal cause ghost images. Multipath effects can occur simply by an individual walking into the room. For this very reason, some consumers in metropolitan areas or areas with rugged terrain opt for cable or satellite television instead of fighting their antennas for better reception.
24 COFDM is resistant to multipath effects because it uses multiple carriers to transmit the same signal. Instead of the signal scattering when met with an obstacle, it flows around the obstacle like a river flows around a rock making it perfect for free DTV programming and for mobile television viewing. Problems with multipath effects were often cited in early evaluations of 8-VSB, although it is expected that devices such as internal antennas will overcome them. In Europe, stations transmit the same signal 100 percent of the time across many borders using single frequency networks. A single frequency network is a network of several stations that broadcast the same signal simultaneously using multiple transmitters. This allows television viewers to watch the same broadcast anywhere in Europe without interference. COFDM is ideal for single frequency networks.
Compact HTML Pl. see CHTML
Colocation Colocation (sometimes spelled "co-location" or "collocation") is the provision of space for a customer's telecommunications equipment on the service provider's premises. For example, a Web site owner could place the site's own computer servers on the premises of the Internet service provider (ISP). Or an ISP could place its network routers on the premises of the company offering switching services with other ISPs. The alternative to colocation is to have the equipment and the demarc located at the customer's premises. Colocation is sometimes provided by companies that specialize in Web site hosting.
Computer-telephony integration Pl. see CTI
Conditional Access Conditional access (CA) is a technology used to control access to digital television (DTV) services to authorized users by encrypting the transmitted programming. CA has been used for years for pay-TV services. There are numerous ATSC and DVB- compliant CA systems available for a broadcaster to choose from. The CA system provider provides the equipment and software to the broadcaster who then integrates the CA system into his equipment. CA is not designed solely for DTV. It can be used for digital radio broadcasts, digital data broadcasts, and non-broadcast information and interactive services. A CA system consists of several basic components: Subscriber Management System The SMS is a subsystem of the CA system that manages the subscriber's information and requests entitlement management messages (EMM) from the Subscriber Authorization System (SAS). An EMM provides general information about the subscriber and the status of the subscription. The EMM is sent with the ECM. The ECM is a data unit that contains the key for decrypting the transmitted programs. Subscriber Authorization System (SAS) The SAS is a subsystem of the CA system that translates the information about the subscriber into an EMM at the request of the SMS. The SAS also ensures that the
25 subscriber's security module receives the authorization needed to view the programs, and the SAS acts as a backup system in case of failure. Security module The security module, usually in the form of a smart card, extracts the EMM and ECM necessary for decrypting the transmitted programs. The security module is either embedded within the set-top box or in a PC Card that plugs into the set-top box. Set-top box The set-top box houses the security module that gives authorization for decrypting the transmitted programs. The set-top box also converts the digital signal to an analogue signal so an older television can display the programs. There are two DVB protocols used by CA systems: SimulCrypt and MultiCrypt. SimulCrypt uses multiple set-top boxes, each using a different CA system, to authorize the programs for display. The different ECMs and EMMs required by each CA system are transmitted simultaneously. Each set-top box recognizes and uses the appropriate ECM and EMM needed for authorization. The ATSC standard uses SimulCrypt. MultiCrypt allows multiple CA systems to be used with one set-top box by using a PC card with an embedded smart card for each CA system used. Each card is then plugged into a slot in the set-top box. Each card recognizes the ECM and EMM needed for authorization. A typical CA process involves three basic elements: the broadcast equipment, the set-top box, and the security module. The broadcast equipment generates the encrypted programs that are transmitted to the subscriber. When these are transmitted, the set-top box filters out the signals and passes them to the security module. The security module then authorizes these programs for decryption. The programs are then decrypted in real time and sent back to the set-top box for display.
Connection In telecommunication and computing in general, a connection is the successful completion of necessary arrangements so that two or more parties (for example, people or programs) can communicate at a long distance. In this usage, the term has a strong physical (hardware) connotation although logical (software) elements are usually involved as well. A dialup (sometimes called a switched) connection is a telephonic arrangement that is set up only when needed, using shared, circuit- switched communication lines (as in "plain old telephone service"). A dedicated (sometimes called a nonswitched) connection is a continuous, always available connection (familiar to users of Digital Subscriber Line or DSL service). A leased line is a line rented from a telephone company that provides dedicated connection between two points (such as a headquarters office and a manufacturing plant). In computer programming, a connection is the setting up of resources (such as computer memory and buffers) so that a particular object such as a database or file can be read or written to. Typically, a programmer encodes an OPEN or similar request to the operating system that ensures that system resources such as memory are set up, encodes READs and WRITES or similar requests, and then encodes a CLOSE when a connection is no longer needed so that the resources are returned to the system for other users. A closely related term is session, which is sometime used to distinguish the ability to communicate for some duration in a
26 logical sense. In this usage, the connection is regarded as the physical setup and the session is regarded as the logical setup. A session could be terminated and the connection maintained with the expectation of a new session later.
CMTS A cable modem termination system (CMTS) is a component that exchanges digital signals with cable modems on a cable network. A cable modem termination system is located at the local office of a cable television company. A data service is delivered to a subscriber through channels in a coaxial cable or optical fiber cable to a cable modem installed externally or internally to a subscriber's computer or television set. One television channel is used for upstream signals from the cable modem to the CMTS, and another channel is used for downstream signals from the CMTS to the cable modem. When a CMTS receives signals from a cable modem, it converts these signals into Internet Protocol (IP) packets, which are then sent to an IP router for transmission across the Internet. When a CMTS sends signals to a cable modem, it modulates the downstream signals for tranmission across the cable to the cable modem. All cable modems can receive from and send signals to the CMTS but not to other cable modems on the line.
Consumer DSL Pl. See CDSL
COO Cell of Origin (COO) is a mobile positioning technique for finding a caller's cell (the basic geographical coverage unit of a cellular telephone system) location. It may be used by emergency services or commercial use. COO is the only positioning technique that is widely used in wireless networks and is used for Phase 1 of 911 service in the United States. For COO positioning, the location of the base station is ascertained and considered to be the location of the caller. COO is a variable and not a very precise locator; depending on the number of base stations in the search area, accuracy may be as close as within one hundred meters of the target (in an urban area) or as far off as thirty kilometers away from the target where base stations are less densely concentrated. For this reason, when precision is important COO is often used in conjunction with some other technology, such as the Global Positioning System (GPS) or Time of Arrival (TOA). Although COO positioning is not as precise as other methods, it offers unique advantages: it can very quickly identify the location (generally in about three seconds) and does not require equipment or network upgrades, which makes it easily deployed to existing customer bases. The American National Standards Institute (ANSI), and the European Telecommunications Standards Institute (ETSI) recently formed the T1P1 subcommittee dedicated to creating standardization for positioning systems using TOA, Assisted GPS, and Enhanced Observed Time Difference in addition to COO.
CPNI In the United States, CPNI (Customer Proprietary Network Information) is information that telecommunications services such as local, long distance, and wireless telephone companies acquire about their subscribers. It includes not only
27 what services they use but their amount and type of usage. The Telecommunications Act of 1996 together with clarifications from the Federal Communications Commission (FCC) generally prohibits the use of that information without customer permission, even for the purpose of marketing the customers other services . In the case of customers who switch to other service providers, the original service provider is prohibited from using the information to try to get the customer back. CPNI includes such information as optional services subscribed to, current charges, directory assistance charges, usage data, and calling patterns. The CPNI rules do not prohibit the gathering and publishing of aggregate customer information nor the use of customer information for the purpose of creating directories.
CTI CTI ( computer-telephony integration), or sometimes simply "computer telephony," is the use of computers to manage telephone calls. The term is used in describing the computerized services of call centers, such as those that direct your phone call to the right department at a business you're calling. It's also sometimes used to describe the ability to use your personal computer to initiate and manage phone calls (in which case you can think of your computer as your personal call center). CTI applications provide the ability to do one or more of the following: Authenticate callers. Using one of several standard methods, the telephone number of the caller can be screened against a database. Recognize a voice, either for authentication or for message forwarding Using live, recorded voice, or touch-tone entered input, determine how to process a call (for example, by forwarding it to the appropriate person or department) Provide interactive voice response (IVR) to callers Match the number of a caller with a customer record and display it for reference when talking to the caller Manage voice or video conferences Collect and display pending live calls or messages that have been left by callers Receive fax messages and route them to appropriate fax machines For outbound calling such as telemarketing, predial callers Based on call input, initiate a smart agent application to provide help with the caller's request
The Advanced Intelligent Network (AIN) is a telephone service architecture that separates CTI services from call switching and will make it easier to add new services. The Windows Telephony Application Program Interface (TAPI) and Novell's TSAPI are programming interfaces intended to make it easier to create applications that enable telephone services on a personal computer or in a local area network.
Customer Premises Equipment Pl. see CPE
28 CPE Customer premises equipment (CPE) is telephone or other service provider equipment that is located on the customer's premises (physical location) rather than on the provider's premises or in between. Telephone handsets, cable TV set-top boxes, and Digital Subscriber Line routers are examples. Historically, this term referred to equipment placed at the customer's end of the telephone line and usually owned by the telephone company. Today, almost any end-user equipment can be called customer premise equipment and it can be owned by the customer or by the provider.
29 D D Dark fiber Dark fiber is optical fiber infrastructure (cabling and repeaters) that is currently in place but is not being used. Optical fiber conveys information in the form of light pulses so the "dark" means no light pulses are being sent. For example, some electric utilities have installed optical fiber cable where they already have power lines installed in the expectation that they can lease the infrastructure to telephone or cable TV companies or use it to interconnect their own offices. To the extent that these installations are unused, they are described as dark. "Dark fiber service" is service provided by local exchange carriers (LECs) for the maintenance of optical fiber transmission capacity between customer locations in which the light for the fiber is provided by the customer rather than the LEC.
Data Over Cable Systems Interface Pl. see DOCSIS
Data Rates This table shows the stated data rates for the most important end-user and backbone transmission technologies Physical Technology Speed Application Medium Mobile telephone GSM mobile RF in space 9.6 to 14.4 Kbps for business and telephone service (wireless) personal use High-Speed Circuit- Mobile telephone RF in space Switched Data Up to 56 Kbps for business and (wireless) service (HSCSD) personal use Regular telephone Home and small Up to 56 Kbps twisted pair service (POTS) business access Business e-mail Dedicated 56Kbps 56 Kbps Various with fairly large on frame relay file attachments The base signal on a channel in DS0 64 Kbps All the set of Digital Signal levels Mobile telephone General Packet RF in space 56 to 114 Kbps for business and Radio System (GPRS) (wireless) personal use BRI: 64 Kbps to BRI: Twisted- BRI: Faster home ISDN 128 Kbps pair PRI: T-1 and small
30 PRI: 23 (T-1) or 30 or E1 line business access (E1) assignable PRI: Medium and 64-Kbps channels large enterprise plus control access channel; up to 1.544 Mbps (T-1) or 2.048 (E1) Faster home and IDSL 128 Kbps Twisted-pair small business access Local area network for Apple devices; several networks can be AppleTalk 230.4 Kbps Twisted pair bridged; non- Apple devices can also be connected Mobile telephone Enhanced Data GSM RF in space 384 Kbps for business and Environment (EDGE) (wireless) personal use 400 Kbps Faster home and RF in space Satellite (DirecPC and small enterprise (wireless) others) access Large company Twisted-pair backbone for 56 Kbps to 1.544 Frame Relay or coaxial LANs to ISP Mbps cable ISP to Internet infrastructure Twisted-pair, Large company coaxial cable, to ISP DS1/T-1 1.544 Mbps or optical ISP to Internet fiber infrastructure Mobile telephone Universal Mobile for business and RF in space Telecommunications Up to 2 Mbps personal use (wireless) Service (UMTS) (available in 2002 or later) Twisted-pair, 32-channel coaxial cable, E-carrier 2.048 Mbps European or optical equivalent of T-1 fiber Large company Twisted-pair, to ISP T-1C (DS1C) 3.152 Mbps coaxial cable, ISP to Internet or optical fiber infrastructure IBM token ring/802.5 4 Mbps (also 16 Twisted-pair, Second most
31 Mbps) coaxial cable, commonly-used or optical fiber local area network after Ethernet Large company Twisted-pair, to ISP DS2/T-2 6.312 Mbps coaxial cable, ISP to Internet or optical fiber infrastructure Twisted-pair Home, small (used as a business, and Digital Subscriber 512 Kbps to 8 digital, enterprise access Line (DSL) Mbps broadband using existing medium) copper lines Twisted-pair, Carries four E-2 8.448 Mbps coaxial cable, multiplexed E-1 or optical fiber signals Coaxial cable (usually uses Ethernet); in 512 Kbps to 52 some Mbps(see "Key Home, business, Cable Modem systems, and explanation" school access telephone below) used for upstream requests 10BASE-T (twisted-pair); Most popular 10BASE-2 or - business local Ethernet 10 Mbps 5 (coaxial area network cable); (LAN) 10BASE-F (optical fiber) Second most Twisted-pair, commonly-used 16 Mbps (also 4 IBM token ring/802.5 coaxial cable, local area Mbps) or optical fiber network after Ethernet Twisted-pair Carries 16 E-l E-3 34.368 Mbps or optical fiber signals ISP to Internet infrastructure DS3/T-3 44.736 Mbps Coaxial cable Smaller links within Internet infrastructure ISP to Internet OC-1 51.84 Mbps Optical fiber infrastructure Smaller links
32 within Internet infrastructure Between router hardware and WAN lines Short-range (50 High-Speed Serial Up to 53 Mbps HSSI cable feet) Interface (HSSI) interconnection between slower LAN devices and faster WAN lines 100BASE-T Workstations with (twisted pair); 10 Mbps Ethernet 100BASE-T Fast Ethernet 100 Mbps cards can plug (twisted pair); into a Fast 100BASE-T Ethernet LAN (optical fiber) Large, wide- range LAN Fiber Distributed- 100 Mbps Optical fiber usually in a large Data Interface (FDDI) company or a larger ISP ISP to Internet infrastructure T-3D (DS3D) 135 Mbps Optical fiber Smaller links within Internet infrastructure Carries 4 E3 channels Up to 1,920 E-4 139.264 Mbps Optical fiber simultaneous voice conversations Large company backbone OC-3/SDH 155.52 Mbps Optical fiber Internet backbone Carries 4 E4 channels Up to 7,680 E-5 565.148 Mbps Optical fiber simultaneous voice conversations Internet OC-12/STM-4 622.08 Mbps Optical fiber backbone Optical fiber Workstations/net Gigabit Ethernet 1 Gbps (and "copper" works with
33 up to 100 10/100 Mbps meters) Ethernet plug into Gigabit Ethernet switches Internet OC-24 1.244 Gbps Optical fiber backbone 2.325 Gbps (15 Part of the vBNS SciNet Optical fiber OC-3 lines) backbone Internet OC-48/STM-16 2.488 Gbps Optical fiber backbone OC-192/STM-64 10 Gbps Optical fiber Backbone OC-256 13.271 Gbps Optical fiber Backbone
Key and Explanation "Kbps" in US English is used as the abbreviation for "thousands of bits per second." In international English outside the U.S., the equivalent usage is "kbits s-1" or "kbits/s". Engineers use data rate rather than speed, but speed (as in "Why isn't my Web page getting here faster?") seems more meaningful for the less technically inclined. Many of us tend to think that the number of bits getting somewhere over a period of time is their speed of travel. Relative to data transmission, a related term, bandwidth or "capacity," means how wide the pipe is and how quickly the bits can be sent down the channels in the pipe. (The analogy of multiple lanes on a superhighway with cars containing speed governors may help. One reason why digital traffic flows faster than voice traffic on the same copper line is because digital has managed to convert a one-lane or narrowband highway into a many-lane or broadband highway.) These "speeds" are aggregate speeds. That is, the data on the multiple signal channels within the carrier is usually allocated by channel for different uses or among different users. Key: "T" = T-carrier system in U.S., Canada, and Japan...."DS"= digital signal (that travels on the T-carrier or E-carrier)..."E" = Equivalent of "T" that uses all 8 bits per channel; used in countries other than U.S. Canada, and Japan...."OC" = optical carrier (Synchronous Optical Network)...."STM" = Synchronous Transport Modules (see Synchronous Digital Hierarchy) Only the most common technologies are shown. "Physical medium" is stated generally and doesn't specify the classes or numbers of pairs of twisted pair or whether optical fiber is single-mode or multimode. The effective distance of a technology is not shown. There are published standards for many of these technologies. Some of these are indicated on pages linked to from the table. Cable modem note:The upper limit of 52 Mbps on a cable is to an ISP, not currently to an individual PC. Most of today's PCs are limited to an internal design that can accomodate no more than 10 Mbps (although the PCI bus itself carries data at a faster speed). The 52 Mbps cable channel is subdivided among individual users. Obviously, the faster the channel, the fewer channels an ISP will require and the lower the cost to support an individual user.
34 DECT Unlike the analog cordless phones you may have in your home, DECT (Digital Enhanced Cordless Telecommunications) is a digital wireless telephone technology that is expected to make cordless phones much more common in both businesses and homes in the future. Formerly called the Digital European Cordless Telecommunications standard because it was developed by European companies, DECT's new name reflects its global acceptance. Like another important wireless standard, Global System for Mobile communication, DECT uses time division multiple access (TDMA) to transmit radio signals to phones. Whereas GSM is optimized for mobile travel over large areas, DECT is designed especially for a smaller area with a large number of users, such as in cities and corporate complexes. A user can have a telephone equipped for both GSM and DECT (this is known as a dual-mode phone) and they can operate seamlessly. DECT has five major applications: The "cordless private branch exchange." A company can connect to a wired telephone company and redistribute signals by radio antenna to a large number of telephone users within the company, each with their own number. A cordless PBX would be especially useful and save costs in a company with a number of mobile employees such as those in a large warehouse. Wireless Local Loop (WLL). Users in a neighborhood typically served by a telephone company wired local loop can be connected instead by a cordless phone that exchanges signals with a neighborhood antenna. A standard telephone (or any device containing a telephone such as a computer modem or fax machine) is simply plugged into a fixed access unit (FAU), which contains a transceiver. The Wireless Local Loop would typically be installed in an urban area where many users could share the same antenna. Cordless Terminal Mobility. The arrangement used by businesses for a cordless PBX could also be used by a service that provided cordless phone numbers for individual subscribers. In general, the mobility would be less than that available for GSM users. Home cordless phones. A homeowner could install a single-cell antenna within the home and use it for a number of cordless phones throughout the home and garden. GSM/DECT internetworking. Part of the DECT standard describes how it can interact with the GSM standard so that users can be free to move with a telephone from the outdoors (and GSM signals) into an indoor environment (and a DECT system). It's expected that many GSM service providers may want to extend their service to support DECT signals inside buildings. A dual- mode phone would automatically search first for a DECT connection, then for a GSM connection if DECT is not available.
Dedicated Line A dedicated line is a telecommunications path between two points that is available 24 hours a day for use by a designated user (individual or company). It is not shared in common among multiple users as dial-up lines are. A dedicated line can be a physical path owned by the user or rented from a telephone company, in
35 which case it is called a leased line. A synonym is nonswitched line (as opposed to a switched or dial-up line).
Deep Space Network Pl. see DSN
Dense Wavelength Division Multiplexing Pl. see DWDM
Digital Pulse Wireless Ultra wideband (also known as UWB or as digital pulse wireless) is a wireless technology for transmitting large amounts of digital data over a wide spectrum of frequency bands with very low power for a short distance. Ultra wideband radio not only can carry a huge amount of data over a distance up to 230 feet at very low power (less than 0.5 milliwatts), but has the ability to carry signals through doors and other obstacles that tend to reflect signals at more limited bandwidths and a higher power. Ultra wideband can be compared with another short-distance wireless technology, Bluetooth, which is a standard for connecting handheld wireless devices with other similar devices and with desktop computers. Ultra wideband broadcasts digital pulses that are timed very precisely on a carrier signal across a very wide spectrum (number of frequency channels) at the same time. Transmitter and receiver must be coordinated to send and receive pulses with an accuracy of trillionths of a second. On any given frequency band that may already be in use, the ultra wideband signal has less power than the normal and anticipated background noise so theoretically no interference is possible. Time Domain, a company applying to use the technology, uses a microchip manufactured by IBM to transmit 1.25 million bits per second, but says there is the potential for a data rate in the billions of bits per second. Ultra wideband has two main types of application: Applications involving radar, in which the signal penetrates nearby surfaces but reflects surfaces that are farther away, allowing objects to be detected behind walls or other coverings. Voice and data transmission using digital pulses, allowing a very low powered and relatively low cost signal to carry information at very high rates within a restricted range.
Digital Subscriber Line Access Multiplexer Pl. see DSL in Local Loop
D-channel Pl. see B-channel
Dedicated line A dedicated line is a telecommunications path between two points that is available 24 hours a day for use by a designated user (individual or company). It is not shared in common among multiple users as dial-up lines are. A dedicated line can be a physical path owned by the user or rented from a telephone company, in which case it is called a leased line. A synonym is nonswitched line (as opposed to a switched or dial-up line).
36 Dedicated Short Range Communication Pl. see RFID
Demarc A demarc (an abbreviation for demarcation point) marks the point where communications facilities owned by one organization interface with that of another organization. In telephone terminology, this is the interface between customer- premises equipment and network service provider equipment. A synonym is network terminating interface (NTI
Dial-up Dial-up pertains to a telephone connection in a system of many lines shared by many users. A dial-up connection is established and maintained for a limited time duration. The alternative is a dedicated connection, which is continuously in place. Dial-up lines are sometimes called switched lines and dedicated lines are called nonswitched lines. A dedicated line is often a leased line that is rented from a telephone company. A dial-up connection can be initiated manually or automatically by your computer's modem or other device
DID Direct Inward Dialing (DID) is a service of a local phone company (or local exchange carrier) that provides a block of telephone numbers for calling into a company's private branch exchange (PBX) system. Using DID, a company can offer its customers individual phone numbers for each person or workstation within the company without requiring a physical line into the PBX for each possible connection. For example, a company might rent 100 phone numbers from the phone company that could be called over eight physical telephone lines (these are called "trunk lines"). This would allow up to eight ongoing calls at a time; additional inbound calls would get a busy signal until one of the calls completed or be able to leave a voice mail message. The PBX automatically switches a call for a given phone number to the appropriate workstation in the company. A PBX switchboard operator is not involved. A DID system can be used for fax and voice mail as well as for live voice connections. Compared to regular PBX service, DID saves the cost of a switchboard operator, calls go through faster, and callers feel they are calling a person rather than a company.
Digital Digital describes electronic technology that generates, stores, and processes data in terms of two states: positive and non-positive. Positive is expressed or represented by the number 1 and non-positive by the number 0. Thus, data transmitted or stored with digital technology is expressed as a string of 0's and 1's. Each of these state digits is referred to as a bit (and a string of bits that a computer can address individually as a group is a byte). Prior to digital technology, electronic transmission was limited to analog technology, which conveys data as electronic signals of varying frequency or amplitude that are added to carrier waves of a given frequency. Broadcast and phone transmission has conventionally used analog technology. Digital technology is primarily used with new physical communications media, such as satellite and fiber optic transmission. A modem is
37 used to convert the digital information in your computer to analog signals for your phone line and to convert analog phone signals to digital information for your computer.
Digital Loop Carrier Pl. see DLC
Digital Powerline Pl. see DPL
Digital Subscriber Line Pl. see DSL
Digital Signal X Digital signal X is a term for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 Kbps, the bandwidth normally used for one telephone voice channel. Both the North American T-carrier system system and the European E-carrier systems of transmission operate using the DS series as a base multiple. The digital signal is what is carried inside the carrier system. DS0 is the base for the digital signal X series. DS1, used as the signal in the T-1 carrier, is 24 DS0 (64 Kbps) signals transmitted using pulse-code modulation (PCM) and time-division multiplexing (TDM). DS2 is four DS1 signals multiplexed together to produce a rate of 6.312 Mbps. DS3, the signal in the T-3 carrier, carries a multiple of 28 DS1 signals or 672 DS0s or 44.736 Mbps. Digital signal X is based on the ANSI T1.107 guidelines. The ITU-TS guidelines differ somewhat. The following table summarizes the set of signals and relates them to the T-carrier and E-carrier systems.
Digital Signal DS0 Data Rate T-Carrier E-Carrier Designator Multiple DS0 64 Kbps 1 - - DS1 1.544 Mbps 24 T-1 - - 2.048 Mbps 32 - E1 DS1C 3.152 Mbps 48 - - DS2 6.312 Mbps 96 T-2 - - 8.448 Mbps 128 - E2 - 34.368 Mbps 512 - E3 DS3 44.736 Mbps 672 T-3 - - 139.264 Mbps 2048 - E4 DS4/NA 139.264 Mbps 2176 - - 4032 DS4 274.176 Mbps - - 4 E4 - 565.148 Mbps - E5 channels
Digital Subscriber Line Access Multiplexer Pl. see DSLAM
Digital Switch
38 A digital switch is a device that handles digital signals generated at or passed through a telephone company central office and forwards them across the company's backbone network. It receives the digital signals from the office's channel banks that have been converted from users' analog signals and switches them with other incoming signals out to the wide area network. Digital switches are described in terms of classes based on the number of lines and features that are provided. A private branch exchange (PBX) is a digital switch owned by a private company. A centrex is a digital switch at the central office that manages switching for the private company from the central office.
Direct Inward Dialing Pl. see DID
Direct Sequence Spread Spectrum Pl. see DSCDMA
Discrete Multitone Pl. see DMT
Discontinuous Transmission Pl. see DTX
Dish Antenna A dish antenna, also known simply as a dish, is common in microwave systems. This type of antenna can be used for satellite communication and broadcast reception, space communications, radio astronomy, and radar. A dish antenna consists of an active, or driven, element and a passive parabolic or spherical reflector. The driven element can be a dipole antenna or a horn antenna. If a horn is used, it is aimed back at the center of the reflecting dish. The reflector has a diameter of at least several wavelengths. As the wavelength increases (and the frequency decreases), the minimum required dish diameter becomes larger. When the dipole or horn is properly positioned and aimed, incoming electromagnetic fields bounce off the reflector, and the energy converges on the driven element. If the horn or dipole is connected to a transmitter, the element emits electromagnetic waves that bounce off the reflector and propagate outward in a narrow beam. A dish antenna is usually operated with an unbalanced feed line. For satellite television reception, coaxial cable is used. In applications such as radar where a high-power signal is transmitted, a feed system is preferred.
DLC Digital loop carrier (DLC) is equipment that bundles a number of individual phone line signals into a single multiplexed digital signal for local traffic between a telephone company central office and a business complex or other outlying service area. Typically, up to 24 analog voice calls are combined into a single signal and transmitted over a single copper T-carrier system or E-carrier line, an optical fiber cable, or a wireless connection. In a home, business, or other installation using digital loop carrier, the analog phone lines of individual users are connected to a local DLC box which then converts the analog signals into digital and combines
39 (multiplexes) them into one signal that it sent to the phone company's central office on the single line. At the central office, the combined signal is separated back into the original signals. An estimated 20% of today's telephone users are being served by digital loop carriers. Digital loop carrier can carry traffic for regular phone calls (plain old telephone service) and Integrated Services Digital Network (ISDN) service. More recently, approaches have been developed for using DLC to handle the higher bandwidth of Digital Subscriber Line (DSL) service. Digital loop carrier is typically used as an efficient way to provide service to an office building or complex and to extend service to new areas outside the current local loop. DLC is also used to set up telephone service in emergency situations. Customers can easily migrate from a T-1 or E-1 line to fiber optic when it becomes needed and is available. DLC also is an abbreviation for data link control.
DOCSIS Now known as CableLabs Certified Cable Modems, DOCSIS (Data Over Cable Service Interface Specifications) is a standard interface for cable modems, the devices that handle incoming and outgoing data signals between a cable TV operator and a personal or business computer or television set. DOCSIS 1.0 was ratified by the International Telecommunication Union (ITU-TS) in March of 1998. Although "DOCSIS" continues to be used, the newer name emphasizes that the standard is now being used to certify the products of cable modem makers. Cable modems conforming to DOCSIS are now being marketed. Cable operators whose existing customers have non-standard cable modems can handle them by adding backwards-compatible support to the DOCSIS card at the cable operator's end. As DOCSIS continues to evolve to new versions, existing modems can be upgraded to the newer versions by changing the programming in the cable modem's EEPROM memory. DOCSIS-compliant cable modems are being integrated into set-top boxes for use with television sets. DOCSIS must also support or converge with the high definition television (HDTV) standard. The set-top box itself follows a standard known as OpenCable. DOCSIS specifies modulation schemes and the protocol for exchanging bidirectional signals over cable. It supports downstream-to-the-user data rates up to 27 Mbps (megabits per second). Since this data rate is shared by a number of users and because many cable operators will be limited by a T1 connection to the Internet, the actual downstream data rate to an individual business or home will be more like 1.5 to 3 Mbps. Since the upstream data flow has to support much smaller amounts of data from the user, it's designed for an aggregate data rate of 10 Mbps with individual data rates between 500 Kbps and 2.5 Mbps. Cisco and Microsoft have endorsed DOCSIS. They are collaborating on a DOCSIS-compliant cable hybrid fiber-coax (HFC) system, called the Multimedia Cable Network System (MCNS), that will deliver services to residential, commercial, and educational customers.
DoPa DoPa (DoCoMo Packet Transmission) is a packet-switched network service developed by NTT DoCoMo in Japan for Internet connection from mobile devices. NTT DoCoMo, Japan's leading wireless technology company, created their popular i-Mode service by adding features to DoPa that made it easier to use and allowed a
40 wider range of content to be delivered. The transmission of packets allows the service to charge by the number of packets transmitted, so the customer pays for the volume of data, rather than according to call duration and distance, as is almost always tbe case with circuit-switched services.
Downlink In satellite telecommunication, a downlink is the link from a satellite down to one or more ground stations or receivers, and an uplink is the link from a ground station up to a satellite. Some companies sell uplink and downlink services to television stations, corporations, and to other telecommunication carriers. A company can specialize in providing uplinks, downlinks, or both. The following table shows the main frequency bands used for satellite links.
Frequency Band Downlink Uplink C 3,700-4,200 MHz 5,925-6,425 MHz Ku 11.7-12.2 GHz 14.0-14.5 GHz Ka 17.7-21.2 GHz 27.5-31.0 GHz
The C band is the most frequently used. The Ka and Ku bands are reserved exclusively for satellite communication but are subject to rain attenuation. Some satellites carry transponders for both C and Ku bands.
Downstream In telecommunications generally, a transmission from an information server toward an end user is referred to as downstream and a transmission toward the server is referred to as upstream. In some transmission technologies, such as Digital Subscriber Line (DSL), the rates of data transfer upstream and downstream are not the same. In DSL, downstream data rates are higher since the kind of information that needs to get to the user (including still and video images and sound) requires a higher data rate. User responses back to the computer on the upstream path can be smaller since they are usually text-only. In a token ring network, a computer station is downstream from any station through which the token on the ring has already passed. In CATV, a downstream channel is one used to transmit signals from the headend to the user. An upstream channel is one in another frequency band that is used to send signals from the user back to the headend. This term should not be confused with downlink.
DPL Digital Powerline (DPL) technology provides the transmission of data to users over the same lines that bring electric power to homes and businesses. Using the Internet's TCP/IP protocol, companies using DPL across the mains electricity grid plan to deliver data at speeds up to 1 Mbps. DPL would allow a user to get Web pages and other Internet information over power lines with a 24-hour continuous connection (since your power lines are always connected). This would free your telephone for voice use. In addition, since many home appliances are attached to the power system, they could easily be addressed as Internet devices when
41 plugged in. NOR.WEB, a joint venture of Northern Telecom Limited (Nortel) and the UK-based United Utilities PLC, plans to offer a service to users in the UK. Trials of DPL are ongoing in Manchester, UK., and Milan, Italy. Utilities in Sweden and Germany have also shown interest and are planning trials. The DPL technology has, however, been dogged by allegations that it has failed to overcome line noise on the power grid; in addition concerns have been addressed in certain countries that DPL will cause the power grid to emit unwanted radio frequency (RF) interference. NOR.WEB says, however, that their technology has overcome the line noise problem.
DS-CDMA Direct sequence spread spectrum, also known as direct sequence code division multiple access (DS-CDMA), is one of two approaches to spread spectrum modulation for digital signal transmission over the airwaves. In direct sequence spread spectrum, the stream of information to be transmitted is divided into small pieces, each of which is allocated across to a frequency channel across the spectrum. A data signal at the point of transmission is combined with a higher data- rate bit sequence (also known as a chipping code) that divides the data according to a spreading ratio. The redundant chipping code helps the signal resist interference and also enables the original data to be recovered if data bits are damaged during transmission. Direct sequence contrasts with the other spread spectrum process, known as frequency hopping spread spectrum, or frequency hopping code division multiple access (FH-CDMA), in which a broad slice of the bandwidth spectrum is divided into many possible broadcast frequencies. In general, frequency-hopping devices use less power and are cheaper, but the performance of DS-CDMA systems is usually better and more reliable. Spread spectrum first was developed for use by the military because it uses wideband signals that are difficult to detect and that resist attempts at jamming. In recent years, researchers have turned their attention to applying spread spectrum processes for commercial purposes, especially in local area wireless networks.
Direct Sequence Spread Spectrum Pl. see DS-CDMA
Discrete Multitone Pl. see DMT
DMT Discrete multitone (DMT) is a method of separating a Digital Subscriber Line (DSL) signal so that the usable frequency range is separated into 256 frequency bands (or channels) of 4.3125 kHz each. DMT uses the fast Fourier transform (FFT) algorithm for modulation and demodulation. Dividing the frequency spectrum into multiple channels allows DMT to work better when AM radio transmitters are present. Within each channel, modulation uses quadratude amplitude modulation (QAM). By varying the number of bits per symbol within a channel, the modem can be rate-adaptive. Both G.DMT and G.lite use DMT. Other modulation technologies for DSL are carrierless amplitude modulation (CAP) and multiple virtual line (MVL).
42 However, DMT is the most widely used and appears to be becoming the industry standard.
DSL DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL, HDSL, and RADSL. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. DSL installations began in 1998 and will continue at a greatly increased pace through the next decade in a number of communities in the U.S. and elsewhere. Compaq, Intel, and Microsoft working with telephone companies have developed a standard and easier-to-install form of ADSL called G.lite that is accelerating deployment. DSL is expected to replace ISDN in many areas and to compete with the cable modem in bringing multimedia and 3-D to homes and small businesses. How It Works Traditional phone service (sometimes called POTS for "plain old telephone service") connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair. Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signaling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why your computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information. Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (thousands of bits per second). (With ISDN, which one might think of as a limited precursor to DSL, you can receive up to 128 Kbps.) The ability of your computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for your telephone line, and requires your modem to change it back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck. Digital Subscriber Line is a technology that assumes digital data does not require change into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for
43 transmitting it to you. Meanwhile, if you choose, the signal can be separated so that some of the bandwidth is used to transmit an analog signal so that you can use your telephone and computer on the same line and at the same time.
Various types of DSL technologies are described as follows: ATM (asynchronous transfer mode) - the protocol used to "gather" DSL traffic from users and forward it to a DSLAM, which consolidates traffic across the backbone network. Carries data in fixed-length frames of 53 bytes each. ATU-C (ADSL Termination Unit - Central Office) - the downstream channel. ATU-R (ADSL Termination Unit - Remote) - the upstream channel. CAP (carrierless amplitude/phase modulation) - the original ADSL modulation approach in which the signal frequency range is divided into voice (0-4 KHz), upstream data, and downstream data. G.DMT A form of ADSL that uses discrete multitone technology with a splitter. G.DMT is officially ITU-T standard G-992.1. DMT Pl. see above Splitter-based vs. Splitterless DSL Most DSL technologies require that a signal splitter be installed at a home or business, requiring the expense of a phone company visit and installation. However, it is possible to manage the splitting remotely from the central office. This is known as splitterless DSL, "DSL Lite," G.Lite, or Universal ADSL and has recently been made a standard. Modulation Technologies Several modulation technologies are used by various kinds of DSL, although these are being standardized by the International Telecommunication Union (ITU). Different DSL modem makers are using either Discrete Multitone Technology (DMT) or Carrierless Amplitude Modulation (CAP). A third technology, known as Multiple Virtual Line (MVL), is another possibility. Factors Affecting the Experienced Data Rate DSL modems follow the data rate multiples established by North American and European standards. In general, the maximum range for DSL without a repeater is 5.5 km (18,000 feet). As distance decreases toward the telephone company office, the data rate increases. Another factor is the gauge of the copper wire. The heavier 24 gauge wire carries the same data rate farther than 26 gauge wire. If you live beyond the 5.5 kilometer range, you may still be able to have DSL if your phone company has extended the local loop with optical fiber cable. The Digital Subscriber Line Access Multiplexer (DSLAM) To interconnect multiple DSL users to a high-speed backbone network, the telephone company uses a Digital Subscriber Line Access Multiplexer (DSLAM). Typically, the DSLAM connects to an asynchronous transfer mode (ATM) network that can aggregate data transmission at gigabit data rates. At the other end of each
44 transmission, a DSLAM demultiplexes the signals and forwards them to appropriate individual DSL connections. Sync Rate The data rate or speed that the DSLAM negotiates with your DSL modem. For a given service, the service provider may mandate a given maximum data rate.
Types of DSL There are various types of DSL as discussed below: ADSL The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media intensive Web data need lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice rather data, and you can hold phone conversations without requiring a separate line. Unlike a similar service over your cable TV line, using ADSL, you won't be competing for bandwidth with neighbors in your area. In many cases, your existing telephone lines will work with ADSL. In some areas, they may need upgrading. CDSL CDSL (Consumer DSL) is a version of DSL, trademarked by Rockwell Corp., that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) and has the advantage that a "splitter" does not need to be installed at the user's end. Rockwell no longer provides information about CSDL at its Web site and does not appear to be marketing it. G.Lite or DSL Lite G.lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company. This saves the cost of what the phone companies call "the truck roll." G.Lite, officially ITU-T standard G-992.2, provides a data rate from 1.544 Mbps to 6 Mpbs downstream and from 128 Kbps to 384 Kbps upstream. G.Lite is expected to become the most widely installed form of DSL. HDSL HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. HDSL can carry as much on a single wire of twisted-pair cable as can be carried on a T1 line (up to 1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in
45 Europe over a somewhat longer range and is considered an alternative to a T1 or E1 connection. IDSL IDSL (ISDN DSL) is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL. RADSL RADSL (Rate-Adaptive DSL) is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line. SDSL SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because the data rate is the same in both directions. Splitter Based DSL Any DSL service that requires that a signal splitter be manually installed at a home or business, usually requiring the expense of a phone company visit and installation. Splitterless DSL Any DSL service in which the signal plitting is provided remotely from the central office. G.Lite DSL is a splitterless service. UDSL UDSL (Unidirectional DSL) is a proposal from a European company. It's a unidirectional version of HDSL. VDSL VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments is not yet determined. A number of standards organizations are working on it. x2/DSL x2/DSL is a modem from 3Com that supports 56 Kbps modem communication but is upgradeable through new software installation to ADSL when it becomes available in the user's area. 3Com calls it "the last modem you will ever need."
A DSL Summary Table Data Rate Distance DSL Type Description Downstream; Application Limit Upstream Similar to the ISDN Digital 18,000 feet ISDN BRI IDSL Subscriber 128 Kbps on 24 gauge service but Line wire data only (no voice on the
46 same line) Splitterless home and Consumer 1 Mbps 18,000 feet small CDSL DSL from downstream; on 24 gauge business Rockwell less upstream wire service; similar to DSL Lite The standard ADSL; From 1.544 sacrifices "Splitterless" Mbps to 6 Mbps DSL Lite 18,000 feet speed for not DSL without downstream, (same as on 24 gauge having to the "truck depending on G.Lite) wire install a roll" the subscribed splitter at the service user's home or business The standard ADSL; From 1.544 sacrifices "Splitterless" G.Lite Mbps to 6 Mbps 18,000 feet speed for not DSL without (same as , depending on on 24 gauge having to the "truck DSL Lite) the subscribed wire install a roll" service splitter at the user's home or business T1/E1 service 1.544 Mbps between duplex on two server and High bit-rate twisted-pair 12,000 feet phone Digital HDSL lines;2.048 on 24 gauge company or Subscriber Mbps duplex on wire within a Line three twisted- company; pair lines WAN, LAN, server access 1.544 Mbps duplex (U.S. and Canada); Same as for 2.048 Mbps 12,000 feet HDSL but Symmetric SDSL (Europe) on a on 24 gauge requiring only DSL single duplex wire one line of line twisted-pair downstream and upstream Asymmetric 1.544 to 6.1 1.544 Mbps Used for ADSL Digital Mbps at 18,000 Internet and Subscriber downstream; 16 feet; 2.048 Web access,
47 Line to 640 Kbps Mbps at motion video, upstream 16,000 feet; video on 6.312 Mpbs demand, at 12,000 remote LAN feet; 8.448 access Mbps at 9,000 feet Adapted to the line, 640 Kbps Rate- to 2.2 Mbps Similar to RADSL Adaptive DSL downstream; Not provided ADSL from Westell 272 Kbps to 1.088 Mbps upstream Unidirectional DSL Similar to UDSL proposed by Not known Not known HDSL a company in Europe 12.9 to 52.8 4,500 feet at Mbps 12.96 Very high downstream;1.5 ATM Mbps;3,000 Digital to 2.3 Mbps networks; VDSL feet at 25.82 Subscriber upstream;1.6 Fiber to the Mbps; 1,000 Line Mbps to 2.3 Neighborhood feet at 51.84 Mbps Mbps downstream
DS (digital signal) Levels Pl. see Digital signal X
DS0 Pl. see Digital signal X
DS1 Pl. see Digital signal X
DS2 Pl. see Digital signal X
DS3 Pl. see Digital signal X
DS4 Pl. see Digital signal X
DSLAM
48 A Digital Subscriber Line Access Multiplexer (DSLAM) is a network device, usually at a telephone company central office, that receives signals from multiple customer Digital Subscriber Line (DSL) connections and puts the signals on a high-speed backbone line using multiplexing techniques. Depending on the product, DSLAM multiplexers connect DSL lines with some combination of asynchronous transfer mode (ATM), frame relay, or Internet Protocol networks. DSLAM enables a phone company to offer business or homes users the fastest phone line technology (DSL) with the fastest backbone network technology (ATM).
DSRC Pl. see RFID
DSSS Pl. see DS-CDMA
Duplex In telecommunication, duplex communication means that both ends of the communication can send and receive signals at the same time. full-duplex communication is the same thing. half-duplex is also bidirectional communication but signals can only flow in one direction at a time. Simplex communication means that communication can only flow in one direction and never flow back the other way. An ordinary telephone conversation is a duplex communication. Most inexpensive speakerphones in conference rooms are half-duplex communication. (If you're speaking, you can't hear anyone else interrupt. You have to pause to let others speak.)
DSN The Deep Space Network (DSN) is a sophisticated data communications system used by NASA (the U.S. National Aeronautics and Space Administration) in conjunction with manned and unmanned space missions. The DSN is also used by radio astronomers. The main terminal of the DSN is located at JPL (Jet Propulsion Laboratory) headquarters in Pasadena, California. There are three primary antennas, spaced equally on a great circle that slants around the world. All three are large paraboloid (dish) antennas that can be used for transmitting and receiving signals over a wide range of radio frequencies. One antenna is located in California, another is in Spain, and another is in Australia. The antennas are located in such a way that all existing operational spacecraft can be monitored and controlled, and communications maintained with them, almost 100 percent of the time. This is true of both earth-orbiting satellites and interplanetary space vehicles. Signals transmitted and received by DSN equipment include satellite control and telemetry, e-mail (including text, graphics, video, programs, and sound attachments), communications with the Space Shuttles, and radio-frequency emanations from distant celestial objects
DTX Discontinuous transmission (DTX) is a method of momentarily powering-down, or muting, a mobile or portable wireless telephone set when there is no voice input to the set. This optimizes the overall efficiency of a wireless voice communications
49 system. In a typical two-way conversation, each individual speaks slightly less than half of the time. If the transmitter signal is switched on only during periods of voice input, the duty cycle of the telephone set can be cut to less than 50 percent. This conserves battery power, eases the workload of the components in the transmitter amplifiers, and frees the channel so that time-division multiplexing (TDM) can take advantage of the available bandwidth by sharing the channel with other signals. A DTX circuit operates using voice activity detection (VAD). Sophisticated engineering is necessary to ensure that circuits of this type operate properly. In wireless transmitters, VAD is sometimes called voice-operated transmission (VOX).
DWDM Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a lightstream transmitted on a single optical fiber. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM). Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. Specifically, Internet (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all be travelling at the same time within the optical fiber. DWDM promises to solve the "fiber exhaust" problem and is expected to be the central technology in the all- optical networks of the future.
50 E
E E-1 E1 (or E-1) is a European digital transmission format devised by the ITU-TS and given the name by the Conference of European Postal and Telecommunication Administration (CEPT). It's the equivalent of the North American T-carrier system format. E2 through E5 are carriers in increasing multiples of the E1 format. E1 signal format carries data at a rate of 2.048 million bits per second and can carry 32 channels of 64 Kbps* each. E1 carries at a somewhat higher data rate than T-1 (which carries 1.544 million bits per second) because, unlike T-1, it does not do bit-robbing and all eight bits per channel are used to code the signal. E1 and T-1 can be interconnected for international use. E2 (E-2) is a line that carries four multiplexed E1 signals with a data rate of 8.448 million bits per second. E3 (E-3) carries 16 E1 signals with a data rate of 34.368 million bits per second. E4 (E-4) carries four E3 channels with a data rate of 139.264 million bits per second. E5 (E-5) carries four E4 channels with a data rate of 565.148 million bits per second. * In international English outside the U.S., the equivalent usage is "kbps" or "kbits s-1." To see the relationship between the E-carrier system, the T-carrier system, and DS0 multiples, see digital signal X.
E-2 Pl. see E-1
E-3 Pl. see E-1
E-4 Pl. see E-1
E-5 Pl. see E-1
E-carrier system Pl. see E-1
EDFA
51 An erbium amplifier, also called optical amplifier or an erbium-doped fiber amplifier or EDFA, is an optical or IR repeater that amplifies a modulated laser beam directly, without opto-electronic and electro-optical conversion. The device uses a short length of optical fiber doped with the rare-earth element erbium. When the signal- carrying laser beams pass through this fiber, external energy is applied, usually at IR wavelengths. This so-called pumping excites the atoms in the erbium-doped section of optical fiber, increasing the intensity of the laser beams passing through. The beams emerging from the EDFA retain all of their original modulation characteristics, but are brighter than the input beams. In fiber optic communications systems, problems arise from the fact that no fiber material is perfectly transparent. The visible-light or infrared (IR) beams carried by a fiber are attenuated as they travel through the material. This necessitates the use of repeaters in spans of optical fiber longer than about 100 kilometers. A conventional repeater puts a modulated optical signal through three stages: (1) optical-to-electronic conversion, (2) electronic signal amplification, and (3) electronic-to-optical conversion. (The term optical encompasses IR as well as visible-light energy in this context.) Repeaters of this type limit the bandwidth of the signals that can be transmitted in long spans of fiber optic cable. This is because, even if a laser beam can transmit several gigabits per second (Gbps) of data, the electronic circuits of a conventional repeater cannot. Besides eliminating complex and inefficient conversion and electronic amplification stages, the EDFA allows the transmission of signals that employ wavelength-division multiplexing (WDM). This increases the realizable bandwidth relative to conventional repeaters still further.
EDGE EDGE (Enhanced Data GSM Environment), a faster version of the Global System for Mobile (GSM) wireless service, is designed to deliver data at rates up to 384 Kbps and enable the delivery of multimedia and other broadband applications to mobile phone and computer users. The EDGE standard is built on the existing GSM standard, using the same time-division multiple access (TDMA) frame structure and existing cell arrangements. Ericsson notes that its base stations can be updated with software. EDGE is expected to be commercially available in 2001. It is regarded as an evolutionary standard on the way to Universal Mobile Telecommunications Service (UMTS).
ELF ELF (extremely low frequency) refers to an electromagnetic field having a frequency much lower than the frequencies of signals typically used in communications. The most common ELF field is radiated by utility power lines. In the United States, this frequency is 60 Hz. You are exposed to these fields whenever you are near electrical appliances of any kind. In recent years, ELF fields have become a subject of concern in computing applications where cathode-ray tube (CRT) displays are used. These displays, typically used in desktop computer workstations and television sets, generate electromagnetic fields because of the strong, fluctuating currents in the electron-beam deflecting coils. The frequencies of these fields are on the order of a few kilohertz or less. Some studies suggest that ELF fields might have detrimental health effects on humans exposed to them for
52 long periods of time. The claims vary from increased risk of cancer to premature births and miscarriages. However, as of this writing, conclusive proof has yet to be obtained that ELF fields are harmful at the levels encountered by computer users. The ELF fields surrounding a CRT display tend to be stronger off the sides of the CRT than directly in front of it or behind it. The fields diminish rapidly in intensity with increasing distance from the CRT. As a general rule, computer users should sit at least 18 inches away from a CRT display. Side-by-side workstations should be at least five feet apart. These considerations are important for visual comfort and "breathing room" as much as for minimizing the potential risk posed by ELF fields.
Electronic Program Guide Pl. see EPG
Electronic Worldwide Switch Digital Pl. see EWSD
EME Moonbounce, also called Earth-Moon-Earth (EME), is a form of wireless communication in which the moon is used as a passive satellite. To the uninitiated, this sounds a little like science fiction, but it has been done and continues to be done by experimentally-inclined amateur radio operators. There are several challenges and difficulties inherent in moonbounce operation. One of the most troublesome for two-way communication is the fact that the moon's distance introduces lag time. The moon is approximately 250,000 miles away from the earth, and radio waves travel at 186,282 miles per second. A signal sent to the moon does not return until 2.7 seconds have elapsed. If two people are engaged in a conversation and one person asks a question, that person cannot expect a reply until at least 5.4 seconds later (the answer must travel to the moon and back, as must the question). Besides propagation delay, the path loss to and from the moon is considerable. The moon is a relatively poor reflector of electromagnetic rays at any wavelength, including radio waves. Its surface is irregular, and it scatters, rather than focusing, reflected energy. Because of this, sophisticated equipment is necessary to successfully bounce a signal off the moon and hear it return. Another problem with moonbounce communication is libration fading and Doppler shifting. The moon does not always present exactly the same face; it "wobbles" a few degrees back and forth. This "wobbling," called libration, produces a constant change in every component of any signal reflected from the moon. The returned signal consists of the sum total of countless rays that have bounced off mountains, boulders, crater walls, and other lunar features. The relative phase of these components rapidly fluctuates because of libration, so any signal returning from the moon is "fluttery" and distorted. Amateur-radio moonbounce generally requires the following: A sensitive receiver with a narrowband filter A transmitter capable of operating on at least one amateur band above 144 MHz, and capable of producing 1500 watts of continuous radio-frequency output
53 An antenna with high directivity and gain, capable of being rotated in both the azimuth and elevation planes A location in which the moon can be seen without obstruction for extended periods A location in which humanmade radio noise is minimal Neighbors who will tolerate the presence of a large antenna and the proximity of a high-power radio transmitter A neighborhood without ordinances or covenants prohibiting large antennas and/or high-power radio transmitters Operating skill and patience
EMS Enhanced Messaging Service (EMS) is an adaptation of the Short Message Service (SMS) that allows users to send and receive ring tones and operator logos, as well as combinations of simple media to and from EMS-compliant handsets. Because EMS is based on SMS, it can use SMS Centers (SMSCs) the same way that SMS does. EMS works on all Global System for Mobile communications (GSM) networks (widely used in Europe and increasingly available elsewhere). If a message is sent to a phone that is not EMS-capable, the recipient will still receive the text portion of the message. EMS users can integrate text, melodies, pictures, sounds, and animations to enhance the expressive power of messages that are limited by the display constraints of mobile devices. Message senders can use images, sounds, and animation they download from an online library or create images and sounds directly on the phone. EMS is an open standard developed by the Third Generation Partnership Project (3GPP), a mobile telecommunications standards collaborative. The standard is being actively promoted by Alcatel, Ericsson, Motorola, and Siemens. Nokia is promoting a similar standard, Multimedia Messaging Service (MMS).
Enhanced Data GSM Environment Pl. See EGSM
Enhanced Messaging Service Pl. see EMS
Enhanced Specialized Mobile Radio Pl. see ESMR
ESMR ESMR (Enhanced Specialized Mobile Radio) is a wireless communication system in which numerous mobile/portable transceivers are linked in a network of repeaters. Each repeater has a range of approximately 5 to 10 miles. Operating frequencies are in the UHF (ultra-high-frequency) range, that is, between approximately 300 MHz and 3 GHz. Usually, the working band is near 900 MHz. ESMR can function like its fundamentally simpler cousin, SMR, but it can also offer features similar to those of a cellular telephone network. The PTT (push-to-talk), half-duplex mode can be used; in this case the operation resembles
54 communications between old style two-way radios. full-duplex mode can also be used, so either party can listen and talk at the same time. Interconnection with the telephone networks is commonly done. In addition to voice communication, an ESMR system can offer paging, wireless fax, and data transmission. ESMR systems use digital radio transmission. Spread-spectrum modes, such as frequency hopping, are common. In a well-designed ESMR system, connection is almost instantaneous, compared with the typical 15 to 20 seconds required to dial and set up a call in a public cellular network. The coverage of an ESMR system depends on the geographical distribution and needs of the users. Some systems are confined to single municipalities; others cover selected groups of metro areas; others operate over entire states or regions of a country. Examples of ESMR networks include Ericsson's EDACS (Enhanced Digital Access Communications System), Motorola's IDEN (Integrated Dispatch Enhanced Network), and the Nextel System.
EPOP Ethernet point-of-presence (EPOP) is a technology developed by Level 3 Communications that provides widespread access to broadband networks. As a large network increases its bandwidth, it can include large and expanding groups of subscribers. This trend, largely the result of the depolyment of fiber optic infrastructure, is expected to continue. Thus, wide-area networks (WANs) are taking on some of the characteristics previously unique to local area networks (LANs). The opposite is also true; LANs are becoming increasingly complex, resembling WANs in miniature. Level 3 EPOP provides high-speed Internet access to non-Level 3 facilies spread over vast geographical distances. High-speed Ethernet (up to 100 Mbps) and gigabit Ethernet (up to 1 Gbps) are available.
EPOC EPOC is an operating system designed for small, portable computer-telephones with wireless access to phone and other information services. EPOC is based on an earlier operating system from Psion, the first major manufacturer of personal digital assistants (PDAs). The name derived from the company's belief that the world is entering "a new epoch of personal convenience." To earlier systems, EPOC adds wireless communication and an architecture for adding application programs. Psion declared its first version of EPOC to be an open operating system and licensed it to other equipment makers. Psion then formed a new company with Ericsson, Nokia, and later Motorola called Symbian, which now licenses EPOC and continues to develop it. For portable equipment manufacturers, EPOC is an alternative to Microsoft's Windows CE. (The popular Palm PDA uses its own proprietary operating system, PalmOS.). Symbian refers to the class of hardware EPOC serves as "wireless information devices." EPOC is a 32-bit, multitasking operating system that supports a pen-based graphical user interface (GUI). It is written in the C++ programming language using an object-oriented programming design. The code is very compact so that it can fit on a small ROM chip. In addition to basic services, the operating system comes with an "application suite," that includes a word processor, e-mail handler, spreadsheet program, a scheduling application, general purpose database, sketch program, world clock, voice
55 recorder, spell checker, calculator, communication programs, and a Web browser. EPOC can be scaled from relatively large configurations for a fully-functional handheld computer to small configurations for embedded systems programming applications. Although EPOC can be ported to other microprocessors, Symbian's preferred platform is the Advanced RISC Machines (ARM) architecture. Symbian considers ARM the best platform in terms of millions of instructions per second (MIPS) per watt and per dollar cost. Symbian provides development kits for C++, for OPL (a BASIC-like language), and for Java. Programmers write programs at a PC and use an emulator to test them.
EPG An electronic program guide (EPG) is an application used with digital set-top boxes and newer television sets to list current and scheduled programs that are or will be available on each channel and a short summary or commentary for each program. EPG is the electronic equivalent of a printed television program guide. An EPG is accessed using a remote control device. Menus are provided that allow the user to view a list of programs scheduled for the next few hours up to the next seven days. A typical EPG includes options to set parental controls, order pay-per-view programming, search for programs based on theme or category, and set up a VCR to record programs. Each digital television (DTV) provider offers its own user interface and content for its EPG.
European Global Navigation Satellite System Pl. see Galileo
Erlang The erlang is a unit of traffic density in a telecommunications system. One erlang is the equivalent of one call (including call attempts and holding time) in a specific channel for 3600 seconds in an hour. The 3600 seconds need not be, and generally are not, in a contiguous block. In digital telecommunications, the voice signals are compressed. This makes it possible for one channel to carry numerous calls simultaneously by means of multiplexing. In theory, there are many ways in which a channel can carry a certain number of erlangs. For example, a traffic density of 3 erlangs can consist of three simultaneous calls, each lasting for an hour (a total of 10,800 seconds); it can consist of six calls, each of which are allocated 30 minutes (1800 seconds) of time during the hour; it might consist of 180 calls, each of which occupy one minute (60 seconds) of time during an hour. Smaller units of traffic density are sometimes used. The hundred or centum call second or CCS is the equivalent of one call for 100 seconds out of an hour. A traffic density of 1 CCS is equal to 1/36 erlang. An erlang can be applied to the group of lines in a telephone trunk line or to the traffic in a telephone call center. The term is named after the Danish telephone engineer, A. K. Erlang, the originator of queueing theory. Erlang B is a calculation for any one of these three factors if you know or predict the other two: Busy Hour Traffic (BHT), or the number of hours of call traffic during the busiest hour of operation
56 Blocking, or the percentage of calls that are blocked because not enough lines are available Lines, or the number of lines in a trunk group
An extended version of Erlang B lets you add the factor of how many people who are blocked retry their calls immediately. Erlang C is a calculation for how many call agents (answerers) you'll need in a call center that has a given number of calls per hour, a given average duration of call, and an acceptable level of delay in answering the call.
The Erlang programming language is not the same thing as the erlang, a unit of traffic density.
Erbium Amplifier Pl. see EDFA
Erbium-doped fiber amplifier Pl. see EDFA
Earth-Moon-Earth Pl. see EME
Ethernet Ethernet is the most widely-installed local area network (LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless LANs. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol. Fast Ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards. Gigabit Ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second). 10-Gigabit Ethernet provides up to 10 billion bits per second.
EtherExpress EtherExpress is a technology from Intel that is used in network server adapters (devices that attach the server to the network cable) for Ethernet-based local area networks (LANs). The EtherExpress adapter has a built-in i960 processor (an Intel processor with 32-bit RISC-based architecture) that allows it to offload work from the server's processor, improving network efficiency by reducing demands on server resources. Adapters that offload processor work are known as intelligent adapters. Network adapter performance is usually assessed by comparing its data throughput (the rate at which data can be sent and received, measured in megabits per second or in frames per second) against the host CPU utilization, a measurement of the proportion of time the server's processor has to spend on
57 network-related tasks. EtherExpress technology enhances adapter performance both by increasing throughput and by lowering CPU utilization, which increases the amount of time the server has for other requests. EtherExpress server adapters are designed to take advantage of a number of advanced server technologies, such as: Adapter Fault Tolerance (AFT) A method of protecting vital network links through the use of multiple adapters and redundant links which take over automatically if the first link fails Adaptive Load Balancing (ALB) which allows as many as four adapters to handle outgoing server traffic, enabling scalable network bandwidth increases (up to 400 Mbps Fast EtherChannel (FEC) A technology from Cisco Systems that balances incoming and outgoing traffic to increase throughput PCI Hot Plug A technology from Compaq that makes it possible for a failed adapter to be hot swapped (replaced without powering down the server), allowing the network to function normally throughout the procedure.
EtherExpress server adapters include the EtherExpress PRO/100+ Server Adapter, the EtherExpress PRO/100 Intelligent Server Adapter, and the EtherExpress PRO/1000 Gigabit Server Adapter.
Ethernet point-of-presence Pl. see EPOP
EWSD EWSD (Electronic Worldwide Switch Digital, or, in German, Elektronisches WaehlSystem [Digital]) is a widely-installed telephonic switch system developed by Siemens. Siemens says that EWSD performs switching for over 160 million lines in more than 100 countries. EWSD is a modular system in which some switches in the system can be installed in a telephone company's centrex facility and other switches can be located at the customer. Important features of EWSD include the following: Advanced Intelligent Network (AIN) 0.1 and 0.2 capabilities allow switching services to be added at Service Control Points, meaning that new services can be added and performed by the switching system without the customer having to buy new equipment. In addition to AIN capabilities, EWSD provides ISDN, CLASS, SS7, and centrex services. digital and analog lines can be combined in the same line groups, allowing full interoperability between digital and analog terminals. Carriers using EWSD can provide Automatic Call Distributor services for customers with call centers. EWSD provides both Bellcore AIN 0.2 and Global System for Mobile (GSM) communications for personal communications services.
58 Line types supported include single or two-party analog, coin, TR08, and ISDN Basic Rate Interface. Unidirectional Digital Subscriber Line (DSL) support is planned. Any line type can be added by simply changing the line card.
E-ZPass E-ZPass is a toll collection system in the northeastern U.S. that uses RFID technology to allow a driver to pass through a tollbooth and pay the toll without stopping the vehicle. Typically, the driver pre-pays a month of access tolls and is issued a transponder (a tag about the size of a deck of cards) that gets mounted on the inside of the windshield. When approaching a toll, the E-ZPass driver passes through a special booth that has an antenna. The antenna emits a radio frequency (RF) field that activates the driver's transponder, which in turn, sends back information about the driver's account to the antenna. The toll is then deducted from the driver's pre-paid account. E-Z Pass significantly cuts down on labor costs for toll collection and keeps the lines of traffic moving faster. Mobil Corporation uses a similar technology to enable drivers to purchase gasoline by using a transponder that attaches to the driver's key ring. Both E-ZPass and Mobil are experimenting with licensing their transponder payment systems to non-competitive vendors, such as McDonald's, who wish to move traffic through their fast food take- out windows more efficiently.
Easy Pass Pl. see E-ZPass
Extremely Low Frequency Pl. see ELF
Evernet The term Evernet has been used to describe the convergence of wireless, broadband, and Internet telephony technologies that will result in the ability to be continuously connected to the Web anywhere using virtually any information device. Considered the next generation of Internet access, the Evernet assumes the emergence of an amount of bandwidth that would enable millions of homes to access the Web through inexpensive cable modem, DSL, or wireless connections. The "Evernet" can also be considered to include common household appliances and home and office networks that include devices that control the environment; such networks require an "always on" capability. In addition, portable devices that can connect quickly and easily without wires to other devices (see Bluetooth) might also be considered part of the Evernet.
59 F F Failover Failover is a backup operational mode in which the functions of a system component (such as a processor, server, network, or database, for example) are assumed by secondary system components when the primary component becomes unavailable through either failure or scheduled down time. Used to make systems more fault-tolerant, failover is typically an integral part of mission-critical systems that must be constantly available. The procedure involves automatically offloading tasks to a standby system component so that the procedure is as seamless as possible to the end user. Failover can apply to any aspect of a system: within an personal computer, for example, failover might be a mechanism to protect against a failed processor; within a network, failover can apply to any network component or system of components, such as a connection path, storage device, or Web server. Originally, stored data was connected to servers in very basic configurations: either point-to-point or cross-coupled. In such an environment, the failure (or even maintenance) of a single server frequently made data access impossible for a large number of users until the server was back online. More recent developments, such as the storage area network (SAN), make any-to-any connectivity possible among servers and data storage systems. In general, storage networks use many paths - each consisting of complete sets of all the components involved - between the server and the system. A failed path can result from the failure of any individual component of a path. Multiple connection paths, each with redundant components, are used to help ensure that the connection is still viable even if one (or more) paths fail. The capacity for automatic failover means that normal functions can be maintained despite the inevitable interruptions caused by problems with equipment.
FasTrak Pl. see E-Zpass
Fax Sometimes called "telecopying," a fax is the telephonic transmission of scanned-in printed material (text or images), usually to a telephone number associated with a printer or other output device. The original document is scanned with a fax machine, which treats the contents (text or images) as a single fixed graphic image, converting it into a bitmap. In this digital form, the information is transmitted as electrical signals through the telephone system. The receiving fax machine reconverts the coded image and prints a paper copy of the document. Almost all modems manufactured today are capable of sending and receiving fax data. Fax/modem software generates fax signals directly from disk files or the screen. Even if a document is text only, it is treated by the computer as a scanned image and is transmitted to the receiver as a bitmap. Faxing a message online works well if the recipient wants only to read the message. However, if the document requires
60 editing, it must be converted into ASCII text by an OCR (optical character recognition) program, or it must be retyped manually into the computer. A more efficient method of sending documents that require modification is through the e- mail system. E-mail files are already ASCII text so they can be edited immediately in any text editor or word processing program. The Internet now provides a new and cheaper way to send faxes in some cases. A number of free and commercial companies provide arrangements for using the Internet rather than the public telephone system for most or part of the path to the fax point. Some services also provide the ability to broadcast a fax to multiple addresses.
FC/IP Fibre Channel over IP (FCIP or FC/IP, also known as Fibre Channel tunneling or storage tunneling) is an Internet Protocol (IP)-based storage networking technology developed by the Internet Engineering Task Force (IETF). FCIP mechanisms enable the transmission of Fibre Channel (FC) information by tunneling data between storage area network (SAN) facilities over IP networks; this capacity facilitates data sharing over a geographically distributed enterprise. One of two main approaches to storage data transmission over IP networks, FCIP is among the key technologies expected to help bring about rapid development of the storage area network market by increasing the capabilities and performance of storage data transmission. FCIP Versus iSCSI The other method, iSCSI, generates SCSI codes from user requests and encapsulates the data into IP packets for transmission over an Ethernet connection. Intended to link geographically distributed SANs, FCIP can only be used in conjunction with Fibre Channel technology; in comparison, iSCSI can run over existing Ethernet networks. SAN connectivity, through methods such as FCIP and iSCSI, offers benefits over the traditional point-to-point connections of earlier data storage systems, such as higher performance, availability, and fault-tolerance. A number of vendors, including Cisco, Nortel, and Lucent have introduced FCIP- based products (such as switches and routers). A hybrid technology called Internet Fibre Channel Protocol (iFCP) is an adaptation of FCIP that is used to move Fibre Channel data over IP networks using the iSCSI protocols.
FDDI FDDI (Fiber Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber optic lines in a local area network (LAN) that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users. FDDI is frequently used on the backbone for a wide area network (WAN). An FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles). FDDI is a product of American National Standards Committee X3-T9 and conforms to the Open Systems Interconnection (OSI) model of functional layering. It can be used to interconnect LANs using other
61 protocols. FDDI-II is a version of FDDI that adds the capability to add circuit- switched service to the network so that voice signals can also be handled. Work is underway to connect FDDI networks to the developing Synchronous Optical Network (SONET).
FDM Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is assigned a different frequency (subchannel) within the main channel. A typical analog Internet connection via a twisted pair telephone line requires approximately three kilohertz (3 kHz) of bandwidth for accurate and reliable data transfer. Twisted-pair lines are common in households and small businesses. But major telephone cables, operating between large businesses, government agencies, and municipalities, are capable of much larger bandwidths. Suppose a long-distance cable is available with a bandwidth allotment of three megahertz (3 MHz). This is 3,000 kHz, so in theory, it is possible to place 1,000 signals, each 3 kHz wide, into the long-distance channel. The circuit that does this is known as a multiplexer. It accepts the input from each individual end user, and generates a signal on a different frequency for each of the inputs. This results in a high-bandwidth, complex signal containing data from all the end users. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high- bandwidth cable. When FDM is used in a communications network, each input signal is sent and received at maximum speed at all times. This is its chief asset. However, if many signals must be sent along a single long-distance line, the necessary bandwidth is large, and careful engineering is required to ensure that the system will perform properly. In some systems, a different scheme, known as time- division multiplexing, is used instead.
Feature Group D Pl. FG-D
Feed Line In a wireless communications or broadcasting antenna system, the feed line connects the antenna to the receiver, transmitter, or transceiver. The line transfers radio-frequency (RF) energy from a transmitter to an antenna, and/or from an antenna to a receiver, but, if operating properly, does not radiate or intercept energy itself. There are three types of antenna feed lines, also called RF transmission lines, commonly used in wireless systems. Coaxial line, also called coaxial cable, consists of a wire conductor surrounded by a tubular, braided metallic shield. The conductor is kept at the center of the shield by a dielectric, which is usually solid or foamed polyethylene. The shield is connected to RF ground, while the center conductor carries the signal. The shield, as its name implies, prevents the electromagnetic field (EM field) inside the cable from escaping, and also prevents EM energy from entering the cable from outside. Coaxial cables are used at frequencies below approximately 1 gigahertz. Parallel-
62 wire line consists of two wires running alongside each other. At each point along the line, the RF current in the two wires are always equal in magnitude but opposite in direction. The two wires are spaced close together in terms of the EM wavelength. Because of this, the EM fields from the two wires practically cancel out each other in the region outside the line. This prevents the line from radiating RF energy. In receiving systems, EM fields from the external environment induce RF currents that flow in the same direction in each conductor. The receiver circuitry cancels out RF currents that flow in the same direction in both conductors, while responding to RF currents that flow in opposite directions. This prevents external EM fields from affecting the line. Parallel-wire line is rarely employed in commercial installations, but a prefabricated form, called TV ribbon, is sometimes used with television receivers in fringe areas for reception of channels 2 through 13. Another type of two-wire line, known as window line, ladder line, or open wire, is popular among amateur radio operators and shortwave listeners. A waveguide is a hollow, metallic tube or pipe with a circular or rectangular cross section. The diameter of the waveguide is comparable to the wavelength of the EM field. The EM field travels along the inside of the waveguide in a manner somewhat analogous to the way sound waves propagate down a narrow tunnel. The metal structure prevents EM fields inside the waveguide from escaping, and also prevents external EM fields from penetrating to the interior. Waveguides are used at microwave frequencies, that is, at 1 GHz and above. Because the currents in a parallel-wire line always exactly cancel or balance each other, this type of line constitutes a balanced feed line. Such lines work best with antenna systems that are bilaterally symmetrical; an example is the dipole antenna. Coaxial cables and waveguides are unbalanced feed lines. This type of line will work satisfactorily with antennas that are not symmetrical. With the use of a transformer called a balun (contraction of the words "balanced" and "unbalanced"), coaxial cables and waveguides can be used with symmetrical antennas.
FG-D FG-D (Feature Group D) is a type of telecommunication trunk used to provide "equal access" capability from telecommunication carriers and central offices (where the switching equipment is located and customer lines are connected and terminated) to the access tandem. Feature Groups (FGs) categorize telco products according to services and features. The most commonly mentioned feature groups are FG-A, FG-B, and FG-D (FG-C is used almost exclusively by AT&T). FG-D provides higher quality services than the other Feature Groups. It is sometimes known as equal access because it guarantees that all carriers are processed equally. The FG-D protocol defines interconnection rules between a local exchange carrier (LEC) and an inter-exchange carrier (IEC) such as MCI or Sprint. FG-D services route inter-LATA calls to the IEC point of termination (POT), route calls with a carrier access code (CAC) to the user's carrier, and pass information to the carrier. Information passed includes the caller's number, through automatic number identification (ANI). Because of its ability to pass on caller information, FG- D is proposed for use with the 911 service.
FH-CDMA
63 Frequency hopping is one of two basic modulation techniques used in spread spectrum signal transmission. It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications. It also is known as frequency- hopping code division multiple access (FH-CDMA). Spread spectrum modulation techniques have become more common in recent years. Spread spectrum enables a signal to be transmitted across a frequency band that is much wider than the minimum bandwidth required by the information signal. The transmitter "spreads" the energy, originally concentrated in narrowband, across a number of frequency band channels on a wider electromagnetic spectrum. Benefits include improved privacy, decreased narrowband interference, and increased signal capacity. In an FH-CDMA system, a transmitter "hops" between available frequencies according to a specified algorithm, which can be either random or preplanned. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter. A short burst of data is transmitted on a narrowband. Then, the transmitter tunes to another frequency and transmits again. The receiver thus is capable of hopping its frequency over a given bandwidth several times a second, transmitting on one frequency for a certain period of time, then hopping to another frequency and transmitting again. Frequency hopping requires a much wider bandwidth than is needed to transmit the same information using only one carrier frequency. The spread spectrum approach that is an alternative to FH-CDMA is direct sequence code division multiple access (DS-CDMA), which chops the data into small pieces and spreads them across the frequency domain. FH-CDMA devices use less power and are generally cheaper, but the performance of DS-CDMA systems is usually better and more reliable. The biggest advantage of frequency hopping lies in the coexistence of several access points in the same area, something not possible with direct sequence. Certain rules govern how frequency-hopping devices are used. In North America, the Industrial, Scientific, and Medial (ISM) waveband is divided into 75 hopping channels, with power transmission not to exceed 1 watt on each channel. These restrictions ensure that a single device does not consume too much bandwidth or linger too long on a single frequency. The Federal Communications Commission (Fcc) has amended rules to allow frequency hopping spread spectrum systems in the unregulated 2.4 GHz band. The rule change is designed to allow wider bandwidths, thus enabling Internet devices to operate at higher speeds and fostering development of wireless LANs and wireless cable modems. Movie star Hedy Lamarr is generally credited as co-originator of the idea of spread spectrum transmission. She and her pianist were issued a patent for the technique during World War II. They discovered the technique using a player piano to control the frequency hops, and envisioned it as a way to provide secure communications during wartime. The pair never made any money off the invention and their patent eventually expired. Sylvania introduced a similar concept in the 1950s and coined the term "spread spectrum."
Fiber To The Curb Pl. see FTTC
64 Fiber Distributed Data Interface Pl. see FDDI
Fibre Channel over IP Pl. see FCIP
Fibre Channel over TCP/IP Pl. See FCIP
Fibre Channel Tunneling Pl. See FCIP
Filter In computer programming, a filter is a program or section of code that is designed to examine each input or output request for certain qualifying criteria and then process or forward it accordingly. This term was used in UNIX systems and is now used in other operating systems. A filter is "pass-through" code that takes input data, makes some specific decision about it and possible transformation of it, and passes it on to another program in a kind of pipeline. Usually, a filter does no input/output operation on its own. Filters are sometimes used to remove or insert headers or control characters in data. In Windows operating systems, using Microsoft's Internet Server Application Programming Interface (ISAPI), you can write a filter (in the form of a dynamic link library or DLL file) that the operating system gives control each time there is a Hypertext Transport Control (HTTP) request. Such a filter might log certain or all requests or encrypt data or take some other selective action. In telecommunications, a filter is a device that selectively sorts signals and passes through a desired range of signals while suppressing the others. This kind of filter is used to suppress noise or to separate signals into bandwidth channels. In Photoshop and other graphic applications, a filter is a particular effect that can be applied to an image or part of an image. Filters can be fairly simple effects used to mimic traditional photographic filters (which are pieces of colored glass or gelatine placed over the lens to absorb specific wavelengths of light) or they can be complex programs used to create painterly effects.
Fixed Wireless Fixed wireless refers to the operation of wireless devices or systems in fixed locations such as homes and offices. Fixed wireless devices usually derive their electrical power from the utility mains, unlike mobile wireless or portable wireless which tend to be battery-powered. Although mobile and portable systems can be used in fixed locations, efficiency and bandwidth are compromised compared with fixed systems. Mobile or portable, battery-powered wireless systems can serve as emergency backups for fixed systems in case of a power blackout or natural disaster. The technology for wireless connection to the Internet is as old as the Net iteself. Amateur radio operators began "patching" into telephone lines with fixed, mobile, and portable two-way voice radios in the middle of the 20th Century. A wireless modem works something like an amateur-radio "phone patch," except faster. High-end fixed wireless employs broadband modems that bypass the
65 telephone system and offer Internet access hundreds of times faster than twisted- pair hard-wired connections or cell-phone modems. Some of the most important assets of fixed wireless are as follows. Subscribers can be added or moved (to a certain extent) without modifying the infrastructure. Subscribers in remote areas can be brought into a network without the need for stringing new cables or optical fibers across the countryside. Broad bandwidth is possible because there are no wires or cables to introduce reactance into the connection (reactance limits bandwidth by preventing signals higher than a certain frequency from efficiently propagating). As the number of subscribers increases, the connection cost per subscriber goes down.
FM Frequency modulation (FM) is a method of impressing data onto an alternating- current (AC) wave by varying the instantaneous frequency of the wave. This scheme can be used with analog or digital data. In analog FM, the frequency of the AC signal wave, also called the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier frequencies. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz (kHz, where 1 kHz = 1000 hertz or alternating cycles per second) above and below the frequency of the carrier with no modulation. In wideband FM, used in wireless broadcasting, the instantaneous frequency varies by up to several megahertz (MHz, where 1 MHz = 1,000,000 Hz). When the instantaneous input wave has positive polarity, the carrier frequency shifts in one direction; when the instantaneous input wave has negative polarity, the carrier frequency shifts in the opposite direcetion. At every instant in time, the extent of carrier-frequency shift (the deviation) is directly proportional to the extent to which the signal amplitude is positive or negative. In digital FM, the carrier frequency shifts abruptly, rather than varying continuously. The number of possible carrier frequency states is usually a power of 2. If there are only two possible frequency states, the mode is called frequency-shift keying (FSK). In more complex modes, there can be four, eight, or more different frequency states. Each specific carrier frequency represents a specific digital input data state. Frequency modulation is similar in practice to phase modulation (PM). When the instantaneous frequency of a carrier is varied, the instantaneous phase changes as well. The converse also holds: When the instantaneous phase is varied, the instantaneous frequency changes. But FM and PM are not exactly equivalent, especially in analog applications. When an FM receiver is used to demodulate a PM signal, or when an FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between frequency and phase variations is not linear; that is, frequency and phase do not vary in direct proportion.
Fractional T-1 A fractional T-1 or T-3 line is a T-1 or T-3 digital phone line in the North American T-carrier system that is leased to a customer at a fraction of its data-carrying capacity and at a correspondingly lower cost. A T-1 line contains 24 channels, each
66 with a data transfer capacity of 64 Kbps. The customer can rent some number of the 24 channels. The transmission method and speed of transfer remain the same. Overhead bits and framing are still used, but the unrented channels simply contain no data. T-3 lines (which offer 672 64 Kbps channels) are also sometimes offered as a fractional service. T-1 and fractional T-1 service are sometimes advertised as "point-to-point" service (from the customer to the service provider).
Frame In telecommunications, a frame is data that is transmitted between network points as a unit complete with addressing and necessary protocol control information. A frame is usually transmitted serial bit by bit and contains a header field and a trailer field that "frame" the data. (Some control frames contain no data.) Here is a simple representation of a frame, based on the frame used in the frame relay access standard:
------Header------Trailer------Flag (01111110) Address Information (data) Frame check Flag (01111110) field field (0-4096 bytes) sequence
In the figure above, the flag and address fields constitute the header. The frame check sequence and second flag fields constitute the trailer. The information or data in the frame may contain another encapsulated frame that is used in a higher- level or different protocol. In fact, a frame relay frame typically carries data that has been framed by an earlier protocol program. Following are some descriptions of frames in different technologies: In time-division multiplexing (TDM), a frame is a complete cycle of events within the time division period. In film and video recording and playback, a frame is a single image in a sequence of images that are recorded and played back. In computer video display technology, a frame is the image that is sent to the display image rendering devices. It is continuously updated or refreshed from a frame buffer, a highly accessible part of video RAM. In artificial intelligence (AI) applications, a frame is a set of data with information about a particular object, process, or image. An example is the iris-print visual recognition system used to identify users of certain bank automated teller machines. This system compares the frame of data for a potential user with the frames in its database of authorized users.
Frame Relay Frame Relay is a simplified form of Packet Switching, similar in principle to X.25, in which synchronous frames of data are routed to different destinations depending on header information.
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The biggest difference between Frame Relay and X.25 is that X.25 guarantees data integrity and network managed flow control at the cost of some network delays. Frame Relay switches packets end to end much faster, but there is no guarantee of data integrity at all. As line speeds have increased from speeds below 64kbps to T1/E1 and beyond, the delays inherent in the store-and-forward mechanisms of X.25 become intolerable. At the same time, improvements in digital transmission techniques have reduced line errors to the extent that node-to-node error correction throughout the network is no longer necessary. The vast majority of Frame Relay traffic consists of TCP/IP or other protocols that provide their own flow control and error correction mechanisms. Much of this traffic is fed into the Internet, another packet switched network without any built in error control. Because Frame Relay does not 'care' whether the frame it is switching is error free or not, a Frame Relay node can start switching traffic out onto a new line as soon as it has read the first two bytes of addressing information at the beginning of the frame. Thus a frame of data can travel end to end, passing through several switches, and still arrive at its destination with only a few bytes delay. These delays are small enough that network latency under Frame Relay is not noticeably different from direct leased line connections. So the performance of a Frame Relay network is virtually identical to that of a leased line, but because most of the network is shared, costs are lower. Frame Format Frame Relay uses the synchronous HDLC frame format up to 4kbytes in length. Each frame starts and ends with a Flag character (7E Hex). The first 2 bytes of each frame following the flag contain the information required for multiplexing across the link. The last 2 bytes of the frame are always generated by a Cyclic Redundancy Check (CRC) of the rest of the bytes between the flags. The rest of the frame contains the user data. Virtual Circuits
68 Packets are routed through one or more Virtual Circuits known as Data Link Connection Identifiers (DLCIs). Most Virtual Circuits are Permanent Virtual Circuits or PVCs, which means that the network provider sets up all DLCI connections at subscription time. Switched Virtual Circuits (SVCs) are also part of the Frame Relay specification. They provide a link that only lasts only as long as the session. By having a system with several DLCIs configured, you can communicate simultaneously with several different sites simultaneously. Sangoma's WANPIPE™; and APIs support up to 250 DLCs per physical link, allowing servers to be used as substantial network hubs. Data Integrity There is none. The network delivers frames, whether the CRC check matches or not. It does not even necessarily deliver all frames, discarding frames whenever there is network congestion. Thus it is usual to run an upper layer protocol above Frame Relay that is capable of recovering from errors, such as TCP/IP, X.25 or IPX. In practice, however, the network delivers data quite reliably. Unlike the analog communication lines that were used in the past, modern digital lines have very low error rates. Very few frames are discarded by the network, particularly at this time when the networks are operating at well below design capacity. Flow Control and Information rates There is no true flow control on Frame Relay. The network simply discards frames it cannot deliver. However, the protocol does include features designed to control and minimize frame loss at the user level. When subscribed, the applicant will specify the line speed (e.g., 56kbps or T1) and also, typically, you will be asked to specify a Committed Information Rate (CIR) for each DLCI. This value specifies the maximum average data rate that the network undertakes to deliver under "normal conditions". If you send faster than the CIR on a given DLCI, the network will flag some frames with a Discard Eligibility (DE) bit. The network will do its best to deliver all packets but will discard any DE packets first if there is congestion. For example, your Frame Relay access may be a full T1 (1.54Mbps), but you may have subscribed to a CIR of only 512kbps. What happens is that the Access Node measures your average throughput over a time period, usually one second. If the average throughput is over 512kbps, then the 'extra' frames are marked with a DE bit, and will be discarded first. Note that all your data transmissions always occur at the line speed, in this case 1.54Mbps. Because this is synchronous communications the data is clocked out at a constant speed. The frames you are transmitting have idle gaps between them so that in one second, the total number of bits sent is a number smaller than 1.54Mbits, and that number is the Information Rate. Many inexpensive Frame Relay services are based on a CIR of zero. This means that every frame is a DE frame, and the network will throw any frame away when it needs to. Frame Relay provides indications that the network is becoming congested by means of the Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) bits in data frames. These are used to tell the application to slow down, hopefully before packets start to be discarded. It is a good idea never to subscribe to high CIR until you are absolutely sure that your data is being discarded. There are significant savings to be made in choosing a low or zero CIR, unless there is evidence of packet loss. Status Polling
69 The Frame Relay Customer Premises Equipment (CPE) polls the Access Node (switch) at set intervals to find out the status of the network and DLCI connections. A Link Integrity Verification (LIV) packet exchange takes place about every 10 seconds, which verifies that the connection is still good. It also provides information to the network that the CPE is active, and this status is reported at the other end. About every minute, a Full Status (FS) exchange occurs, which passes information on which DLCIs are configured and active. Until the first FS exchange has occurred, the CPE does not know which DLCIs are active, and so no data transfer can take place. There exist various standards for the Status Polling function. The oldest, the Link Management Interface (LMI), was a temporary standard adopted by manufacturers prior to the international standards bodies getting their standards out. The official ANSI T1.617 Annex D (known as ANSI or Annex D) standard is currently the one most used. The newer Q.933 standard also supports Switched Virtual Circuits. Uses of Frame Relay For companies with numerous distributed offices, Frame Relay provides a cost effective way of providing a secure private IP based network. While some companies use VPNs over the Internet for inter-company communications, that option does expose the organization to some serious security issues, not the least of which is keeping viruses and hackers out of perhaps hundreds of individual internet connections at the offices. In contrast, Frame Relay privacy is guaranteed by the nature of the network, backed up by legislation. Frame Relay is also used as a low cost carrier to replace the networks of leased lines previously used to connect ATM machines, POS terminals and other legacy devices to head office mainframes. These applications involve some kind of protocol conversion to spoof the equipment at both ends. Many Frame Relay connections are used for high-end Internet connections. Reliability Frame Relay is a mature technology that is well implemented worldwide. You can expect reliability as good as or exceeding that of point-to-point leased digital lines.
Frequency For an oscillating or varying current, frequency is the number of complete cycles per second in alternating current direction. The standard unit of frequency is the hertz, abbreviated Hz. If a current completes one cycle per second, then the frequency is 1 Hz; 60 cycles per second equals 60 Hz (the standard alternating- current utility frequency in some countries). Larger units of frequency include the kilohertz (kHz) representing thousands (1,000's) of cycles per second, the megahertz (MHz) representing millions (1,000,000's) of cycles per second, and the gigahertz (GHz) representing billions (1,000,000,000's) of cycles per second. Occasionally the terahertz (THz) is used; 1 THz = 1,000,000,000,000 cycles per second. Note that these prefixes represent specific powers of 10, in contrast to the prefixes for multiples of bytes, which represent specific powers of 2. Computer clock speed is generally specified in megahertz and, more recently, in gigahertz. Frequency is important in wireless communications, where the frequency of a signal is mathematically related to the wavelength. If f is the frequency of an
70 electromagnetic field in free space as measured in megahertz, and w is the wavelength as measured in meters, then w = 300/f and conversely f = 300/w
The following table summarizes characteristics of various types of frequencies.
Free- Abbr space Designation eviati Frequencies Users Wavelen on gths
33 km - 10 Time signals Very Low Frequency VLF 9 kHz - 30 kHz km Standard frequencies Fixed 30 kHz - 300 10 km - 1 Mobile Low Frequency LF kHz km Navigational systems Radio broadcasting 1 km - 100 Land maritime mobile Medium Frequency MF 300 kHz - 3 MHz m Radio broadcasting Fixed Mobile 100 m - 10 Aeronautical High Frequency HF 3 MHz - 30 MHz m Marine mobile Amateur radio Radio broadcasting Fixed Mobile Aeronautical 30 MHz - 300 Marine mobile Very High Frequency VHF 10 m - 1 m MHz Amateur radio Television Radio broadcasting Radio navigation Fixed Mobile Aeronautical Marine mobile 300 MHz - 3 1 m - 100 Amateur Radio Ultra High Frequency UHF GHz mm Television Radio Navigation & location Meteorological Space Communication Fixed Mobile Super High 100 mm - Radio navigation and SHF 3 GHz - 30 GHz Frequency 10 mm location Space and satellite communication Extremely High 30 GHz - 300 10 mm - 1 Amateur radio EHF Frequency GHz mm Satellite
71 Earth and space exploration
Frequency-Division Multiplexing Pl. see FDM
Frequency Hopping Frequency hopping is one of two basic modulation techniques used in spread spectrum signal transmission. It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications. It also is known as frequency- hopping code division multiple access (FH-CDMA). Spread spectrum modulation techniques have become more common in recent years. Spread spectrum enables a signal to be transmitted across a frequency band that is much wider than the minimum bandwidth required by the information signal. The transmitter "spreads" the energy, originally concentrated in narrowband, across a number of frequency band channels on a wider electromagnetic spectrum. Benefits include improved privacy, decreased narrowband interference, and increased signal capacity. In an FH-CDMA system, a transmitter "hops" between available frequencies according to a specified algorithm, which can be either random or preplanned. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter. A short burst of data is transmitted on a narrowband. Then, the transmitter tunes to another frequency and transmits again. The receiver thus is capable of hopping its frequency over a given bandwidth several times a second, transmitting on one frequency for a certain period of time, then hopping to another frequency and transmitting again. Frequency hopping requires a much wider bandwidth than is needed to transmit the same information using only one carrier frequency. The spread spectrum approach that is an alternative to FH-CDMA is direct sequence code division multiple access (DS-CDMA), which chops the data into small pieces and spreads them across the frequency domain. FH-CDMA devices use less power and are generally cheaper, but the performance of DS-CDMA systems is usually better and more reliable. The biggest advantage of frequency hopping lies in the coexistence of several access points in the same area, something not possible with direct sequence. Certain rules govern how frequency-hopping devices are used. In North America, the Industrial, Scientific, and Medial (ISM) waveband is divided into 75 hopping channels, with power transmission not to exceed 1 watt on each channel. These restrictions ensure that a single device does not consume too much bandwidth or linger too long on a single frequency. Movie star Hedy Lamarr is generally credited as co-originator of the idea of spread spectrum transmission. She and her pianist were issued a patent for the technique during World War II. They discovered the technique using a player piano to control the frequency hops, and envisioned it as a way to provide secure communications during wartime. The pair never made any money off the invention and their patent eventually expired. Sylvania introduced a similar concept in the 1950s and coined the term "spread spectrum.
Frequency Shift keying
72 Pl. see FSK
Free-space Optics Pl. see FSO
Free Space Photonics Pl. see FSO
FSK Frequency-shift keying (FSK) is a method of transmitting digital signals. The two binary states, logic 0 (low) and 1 (high), are each represented by an analog waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a wave at a different frequency. A modem converts the binary data from a computer to FSK for transmission over telephone lines, cables, optical fiber, or wireless media. The modem also converts incoming FSK signals to digital low and high states, which the computer can "understand."
The FSK mode was introduced for use with mechanical teleprinters in the mid- 1900s. The standard speed of those machines was 45 baud, equivalent to about 45 bits per second. When personal computers became common and networks came into being, this signaling speed was tedious. Transmission of large text documents and programs took hours; image transfer was unknown. During the 1970s, engineers began to develop modems that ran at faster speeds, and the quest for ever-greater bandwidth has continued ever since. Today, a standard telephone modem operates at thousands of bits per second. Cable and wireless modems work at more than 1,000,000 bps (one megabit per second or 1 Mbps), and optical fiber modems function at many Mbps. But the basic principle of FSK has not changed in more than half a century.
FSO Free-space optics (FSO), also called free-space photonics (FSP), refers to the transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain broadband communications. laser beams are generally used, although non-
73 lasing sources such as light-emitting diodes (LEDs) or IR-emitting diodes (IREDs) will serve the purpose. The theory of FSO is essentially the same as that for fiber optic transmission. The difference is that the energy beam is collimated and sent through clear air or space from the source to the destination, rather than guided through an optical fiber. If the energy source does not produce a sufficiently parallel beam to travel the required distance, collimation can be done with lenses. At the source, the visible or IR energy is modulated with the data to be transmitted. At the destination, the beam is intercepted by a photodetector, the data is extracted from the visible or IR beam (demodulated), and the resulting signal is amplified and sent to the hardware. FSO systems can function over distances of several kilometers. As long as there is a clear line of sight between the source and the destination, communication is theoretically possible. Even if there is no direct line of sight, strategically positioned mirrors can be used to reflect the energy. The beams can pass through glass windows with little or no attenuation (as long as the windows are kept clean!). Although FSO systems can be a good solution for some broadband networking needs, there are limitations. Most significant is the fact that rain, dust, snow, fog, or smog can block the transmission path and shut down the network.
FTTC "Fiber to the curb" (FTTC) refers to the installation and use of optical fiber cable directly to the curbs near homes or any business environment as a replacement for "plain old telephone service" (POTS). Think of removing all the telephone lines you see in your neighborhood and replacing them with optical fiber lines. Such wiring would give us extremely high bandwidth and make possible movies-on-demand and online multimedia presentations arriving without noticeable delay. The term "fiber to the curb" recognizes that optical fiber is already used for most of the long- distance part of your telephone calls and Internet use. Unfortunately, the last part - installing fiber to the curb - is the most expensive. For this reason, fiber to the curb is proceeding very slowly. Meanwhile, other less costly alternatives, such as Asymmetric Digital Subscriber Line on regular phone lines and satellite delivery, are likely to arrive much sooner in most homes. Fiber to the curb implies that coaxial cable or another medium might carry the signals the very short distance between the curb and the user inside the home or business. "Fiber to the building" (FTTB) refers to installing optical fiber from the telephone company central office to a specific building such as a business or apartment house. "Fiber to the neighborhood" (FTTN) refers to installing it generally to all curbs or buildings in a neighborhood. Hybrid Fiber Coax (HFC) is an example of a distribution concept in which optical fiber is used as the backbone medium in a given environment and coaxial cable is used between the backbone and individual users (such as those in a small corporation or a college environment).
74 G G Galileo Galileo is the informal name for the European Global Navigation Satellite System (GNSS), a system that will offer users anywhere in the world "near pinpoint" geographic positioning when it becomes fully operational by 2008. Designed to be interoperable with the other two such systems, the United States' Global Positioning System (GPS) and Russia's Global Orbiting Navigation Satellite System (GLONASS), Galileo will enable a user to take a position from any combination of satellites with a single receiver. Both GLONASS and GPS are run by the defense departments of their respective countries. Galileo will be civilian-operated. The Galileo system, which consists of 30 satellites orbiting the earth at a height of 15,000 miles, is expected to pinpoint a geographical position to within a single meter. Because the service's availability will be guaranteed in almost any circumstance, the system will be ideal for applications in which precision and reliability are critical, such as air traffic management (among many possible examples). Galileo will also perform global search and rescue (SAR) functions, based on the Cospas-Sarsat SAR system. Each satellite will have its own transponder, which will transfer distress signals from a user's transmitter to a rescue co-ordination center. Once the rescue operation is launched, the system will transmit a signal to the user, to notify them that help is on the way. When fully deployed, the Galileo system will use 27 operational satellites and three spares for failover redundancy. The satellites will be positioned in three circular Medium Earth Orbit (MEO) planes 15,000 miles up, at an inclination of 56 degrees relative to the equatorial plane. Two Galileo Control Centers (GCC) will be located in Europe; these centers will receive data from a global network of twenty Galileo Sensor Stations (GSS). GSS data will allow the control centers to synchronize the time signals of satellites with the ground station clocks, and to calculate data about system integrity. Five S-Band (2.0-4.0 GHz) and 10 C-Band (4.0-8.0 GHz) uplink stations around the globe will manage the flow of data between the satellites and the control centers. Galileo is named for the Italian astronomer, mathematician, and physicist, Galileo Galilei. In the 16th and 17th centuries, Galileo applied mathematics to the study of physical science. This new approach, coupled with discoveries through the telescope, revolutionized both astronomy and the study of motion. Galileo is also the name of an unmanned American spacecraft sent to explore Jupiter and its moons in the mid-to-late 1990s.
Gateway A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (end- point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes. The computers that control traffic within your company's network or at your local Internet service provider (ISP) are gateway
75 nodes. In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.
Gbps Gbps stands for billions of bits per second and is a measure of bandwidth on a digital data transmission medium such as optical fiber. With slower media and protocols, bandwidth may be in the Mbps (millions of bits or megabits per second) or the Kbps (thousands of bits or kilobits per second) range.
GBIC A gigabit interface converter (GBIC) is a transceiver that converts electric currents (digital highs and lows) to optical signals, and optical signals to digital electric currents. The GBIC is typically employed in fiber optic and Ethernet systems as an interface for high-speed networking. The data transfer rate is one gigabit per second (1 Gbps) or more. GBIC modules allow technicians to easily configure and upgrade electro-optical communications networks. The typical GBIC transceiver is a plug-in module that is hot-swappable (it can be removed and replaced without turning off the system). The devices are economical, because they eliminate the necessity for replacing entire boards at the system level. Upgrading can be done with any number of units at a time, from an individual module to all the modules in a system.
General Packet Radio Services Pl. see GPRS
Geostationary Satellite A geostationary satellite is a satellite that orbits the earth directly over the equator, approximately 22,000 miles up, remaining over the same spot on the earth's surface at all times, and staying fixed in the sky relative to any point on the surface. At 22,000 miles up, one complete trip around the earth (relative to the sun) takes 24 hours. Weather and environmental satellites are usually geostationary. So are some communications satellites. A single geostationary satellite can "see" approximately 40 percent of the earth's surface. Three such satellites, spaced at equal intervals (120 angular degrees apart), can provide coverage of the entire globe. A geostationary satellite can be accessed using a dish antenna aimed at the spot in the sky where the satellite hovers. The principal advantage of this type of satellite is the fact that earthbound dish antennas can be pointed and then left in position. Geostationary satellites have two significant limitations. First, because the orbital zone is a limited region around the earth, the number of satellites that can be maintained in such orbits is limited. Second, there is a latency problem. The distance that a signal must travel to and from such a satellite is approximately 45,000 to 46,000 miles (depending on the locations of the communicating stations). The speed of electromagnetic (EM) wave propagation in free space is about 186,000 miles per second. Thus, a delay of nearly 1/4 second is introduced for one round trip from the surface to the satellite and back to the surface. For two round
76 trips, such as is necessary in some satellite Internet connections and in two-way communications, this delay is doubled, to nearly 1/2 second. For some applications, this can be quite noticeable. The actual position of a geostationary satellite, as observed from any ground-based spot, varies slightly depending on the time of day, because an absolutely perfect geostationary orbit is impossible to attain. The region in the sky within which the satellite can always be found is called the box. Over time, as seen from the surface, a geostationary satellite appears to wander around in the box. This limits the useful directivity that earth-based antennas can have. If the antenna focuses its energy or response in a beam that is too narrow, the satellite will sometimes stray outside the main lobe, causing a reduction in signal strength. Geostationary satellites are also affected by solar fade, an increase in received noise that occurs when the sun passes behind the satellite in the sky as seen from the earth's surface. This effect usually lasts only a few minutes per episode, takes place only once per 24 hours, and is a problem only within a few days of the equinoxes in late March and late September. In order to remain fixed in the sky as "seen" from the surface, a satellite must orbit directly over the earth's equator. Otherwise it will appear to oscillate from north to south (and also a tiny bit from east to west) in an elongated figure-8. Such a satellite is geosynchronous, but not geostationary.
Gigabit Interface Converter Pl. see GBIC
Gigaflop As a measure of computer speed, a gigaflop is a billion floating-point operations per second (FLOPS). In computers, FLOPS are floating-point operations per second. Floating-point is, according to IBM, "a method of encoding real numbers within the limits of finite precision available on computers." Using floating-point encoding, extremely long numbers can be handled relatively easily. A floating-point number is expressed as a basic number or mantissa, an exponent, and a number base or radix (which is often assumed). The number base is usually ten but may also be 2. Floating-point operations require computers with floating-point registers. The computation of floating-point numbers is often required in scientific or real-time processing applications and FLOPS is a common measure for any computer that runs these applications. In larger computers and parallel processing, computer operations can be measured in megaflops, gigaflops, and teraflops. Some computer scientists have at least begun to think about petaflops.
G.lite G.Lite is the informal name for what is now a standard way to install Asymmetric Digital Subscriber Line (ADSL) service. Also known as Universal ADSL, G.Lite makes it possible to have Internet connections to home and business computers at up to 1.5 Mbps (millions of bits per second) over regular phone lines. Even at the lowest downstream rate generally offered of 384 Kbps (thousands of bits per second), G.Lite is about seven times faster than regular phone service with a V.90 modem and three times faster than an Integrated Services Digital Network (ISDN)
77 connection. Upstream speeds from the computer are at up to 128 Kbps. (Theoretical speeds for ADSL are much higher, but the data rates given here are what is realistically expected.) With G.Lite, your computer's analog-to-digital modem is replaced with an "ADSL modem." and the transmission from the phone company is digital rather than the analog tranmission of "plain old telephone service." G.Lite is also known as "splitterless DSL" because, unlike other DSL technologies, it does not require that a technician come to install a splitter, a device that separates voice from data signals, at the home or business (sometimes referred to as "the truck roll"). The G.Lite standard was developed by the Universal ADSL Working Group, whose members include major phone companies in the U.S. and globally, including Ameritech, Bell Atlantic, BellSouth, GTE, MCI, USWest, Sprint, SBC Communications, Deutsche Telekom, France Telecom, British Telecommunications, Singapore Telecom, and Nippon Telegraph and Telephone. Microsoft, Intel, and Compaq are also represented in the Working Group. The telephone companies and ADSL are competing with the cable TV companies and the cable modems to capture the market for fast Internet access. While phone companies conceded the early lead to the cable TV companies, most industry experts believe that G.lite and ADSL will eventually become the dominant technology for most homes and businesses. The G.Lite standard is officially known as G.992.2. For more information, see DSL.
G.99.2 Pl. See G.lite
Global Positioning System Pl. see GPS
GPRS General Packet Radio Services (GPRS) is a packet-based wireless communication service that promises data rates from 56 up to 114 Kbps and continuous connection to the Internet for mobile phone and computer users. The higher data rates will allow users to take part in video conferences and interact with multimedia Web sites and similar applications using mobile handheld devices as well as notebook computers. GPRS is based on Global System for Mobile (GSM) communication and will complement existing services such circuit-switched cellular phone connections and the Short Message Service (SMS). In theory, GPRS packet-based service should cost users less than circuit-switched services since communication channels are being used on a shared-use, as-packets-are-needed basis rather than dedicated only to one user at a time. It should also be easier to make applications available to mobile users because the faster data rate means that middleware currently needed to adapt applications to the slower speed of wireless systems will no longer be needed. As GPRS becomes available, mobile users of a virtual private network (VPN) will be able to access the private network continuously rather than through a dial-up connection. GPRS will also complement Bluetooth, a standard for replacing wired connections between devices with wireless radio connections. In addition to the Internet Protocol (IP), GPRS supports X.25, a packet-based protocol
78 that is used mainly in Europe. GPRS is an evolutionary step toward Enhanced Data GSM Environment (EDGE) and Universal Mobile Telephone Service (UMTS).
Globalnaya Navigatsionnay Sputnikovaya Sistema Pl. see GLONASS
GLONASS GLONASS (for Globalnaya Navigatsionnay Sputnikovaya Sistema), the Russian Federation's Global Navigation Satellite System (GNSS), is the Russian version of a global positioning system. Similar to the United States' Global Positioning System (GPS), GLONASS is owned and operated by the military. GLONASS provides two separate levels of precision: deliberately degraded (for security purposes) signals for civilian users offer accuracy to within 100 meters, while its signals for military users offer accuracy of 10-20 meters. GLONASS was designed to operate a system of 24 satellites (21 operational, and three spares for failover redundancy) orbiting at a height of 19,140 kilometers in three circular planes, at an inclination of 64.8 degrees relevant to the equatorial plane. However, because of a lack of maintenance funding, fewer than half of the GLONASS satellites are likely to be in operation at any given time.
GNSS GNSS (Global Navigation Satellite System) is a satellite system that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the United States' Global Positioning System (GPS) and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). A third, Europe's Galileo, is slated to reach full operational capacity in 2008. Each of the GNSS systems employs a constellation of orbiting satellites working in conjunction with a network of ground stations. Satellite-based navigation systems use a version of triangulation to locate the user, through calculations involving information from a number of satellites. Each satellite transmits coded signals at precise intervals. The receiver converts signal information into position, velocity, and time estimates. Using this information, any receiver on or near the earth's surface can calculate the exact position of the transmitting satellite and the distance (from the transmission time delay) between it and the receiver. Coordinating current signal data from four or more satellites enables the receiver to determine its position. Depending on the particular technologies used, GNSS precision varies. For example, the United States Department of Defense originally used an intentional degradation (known as "Selective Availability," or "SA") of GPS signals to prevent potential military adversaries from using the positioning data. Because of SA, GPS accuracy was limited to a 100-meter range for civilian users, although military equipment enabled accuracy to within a single meter. In May 2000, a presidential order mandated that SA be discontinued. Without SA, all GPS receivers are potentially accurate to within 15 meters. When available, Galileo will provide position accuracy to within one meter.
GPS
79 The GPS (Global Positioning System) is a "constellation" of 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 100 to 10 meters for most equipment. Accuracy can be pinpointed to within one (1) meter with special military-approved equipment. GPS equipment is widely used in science and has now become sufficiently low-cost so that almost anyone can own a GPS receiver. The GPS is owned and operated by the U.S. Department of Defense but is available for general use around the world. How it works? 21 GPS satellites and three spare satellites are in orbit at 10,600 miles above the Earth. The satellites are spaced so that from any point on Earth, four satellites will be above the horizon. Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, it continually broadcasts its changing position and time. Once a day, each satellite checks its own sense of time and position with a ground station and makes any minor correction. On the ground, any GPS receiver contains a computer that "triangulates" its own position by getting bearings from three of the four satellites. The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within 100 meters. If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map. If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position. If you are moving, your receiver may also be able to calculate your speed and direction of travel and give you estimated times of arrival to specified destinations. The GPS is being used in science to provide data that has never been available before in the quantity and degree of accuracy that the GPS makes possible. Scientists are using the GPS to measure the movement of the arctic ice sheets, the Earth's tectonic plates, and volcanic activity. GPS receivers are becoming consumer products. In addition to their outdoor use (hiking, cross-country skiing, ballooning, flying, and sailing), receivers can be used in cars to relate the driver's location with traffic and weather information. Here are some Web locations that describe GPS receiver products:
Ground Plane Antenna A ground-plane antenna is a variant of the dipole antenna, designed for use with an unbalanced feed line such as coaxial cable. It resembles a coaxial antenna whose lower section consists of straight elements called radials instead of a hollow conducting cylinder. There are two or more radials, each measuring 1/4 wavelength. The main element can be any length, but it must be adjusted to function at and near a specific frequency. This adjustment is done using a tuning coil. The radials are connected to the outer conductor or shield of the feed line cable; the main element is connected to the center conductor.
80
The main element of a ground-plane antenna is almost always oriented vertically. This results in transmission of, and optimum response to, vertically polarized wireless signals. When the base of the antenna is placed at least 1/4 wavelength above the ground or other conducting surface, the radials behave as a near-perfect ground system for an electromagnetic field, and the antenna is highly efficient. It works equally well in all horizontal directions. Ground-plane antennas are favored at frequencies above approximately 10 MHz where the dimensions are manageable. This type of antenna is especially popular among Citizens Band radio operators for fixed-station use in the class-D band at 27 MHz.
GSM GSM (Global System for Mobile communication) is a digital mobile telephone system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1800 MHz frequency band. GSM is the de facto wireless telephone standard in Europe. GSM has over 120 million users worldwide and is available in 120 countries, according to the GSM MoU Association. Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their mobile phones when they travel to other countries. American Personal Communications (APC), a subsidiary of Sprint, is using GSM as the technology for a broadband personal communications service (PCS). The service will ultimately have more than 400 base stations for the palm-sized handsets that are being made by Ericsson, Motorola, and Nokia. The handsets include a phone, a text pager, and an answering machine. GSM together with other technologies is part of an evolution of wireless mobile telemmunication that includes High-Speed Circuit-Switched Data (HCSD), General Packet Radio System (GPRS), Enhanced Data GSM Environment (EDGE), and Universal Mobile Telecommunications Service (UMTS).
81
82 H H
Half-duplex Half-duplex data transmission means that data can be transmitted in both directions on a signal carrier, but not at the same time. For example, on a local area network using a technology that has half-duplex transmission, one workstation can send data on the line and then immediately receive data on the line from the same direction in which data was just transmitted. Like full-duplex transmission, half- duplex transmission implies a bidirectional line (one that can carry data in both directions).
Ham Radio Pl. see Amateur radio
Handie Talkie Pl. see HT
Handoff In an ideal cellular telephone network, each end user's telephone set or modem (the subscriber's hardware) is always within range of a base station. The region covered by each base station is known as its cell. The size and shape of each cell in a network depends on the nature of the terrain in the region, the number of base stations, and the transmit/receive range of each base station. In theory, the cells in a network overlap; for much of the time, a subscriber's hardware is within range of more than one base station. The network must decide, from moment to moment, which base station will handle the signals to and from each and every subscriber's hardware. Each time a mobile or portable cellular subscriber passes from one cell into another, the network automatically switches coverage responsibility from one base station to another. Each base-station transition, as well as the switching process or sequence itself, is called handoff. In a properly functioning network, handoff occurs smoothly, without gaps in communications and without confusion about which base station should be dealing with the subscriber. Subscribers to a network need not do anything to make handoff take place, nor should they have to think about the process or about which base station is dealing with the signals at any given moment.
Head-end Pl. see Head-end
Helical Antenna A helical antenna is a specialized antenna that emits and responds to electromagnetic fields with rotating (circular) polarization. These antennas are commonly used at earth-based stations in satellite communications systems. This
83 type of antenna is designed for use with an unbalanced feed line such as coaxial cable. The center conductor of the cable is connected to the helical element, and the shield of the cable is connected to the reflector.To the casual observer, a helical antenna appears as one or more "springs" or helixes mounted against a flat reflecting screen. The length of the helical element is one wavelength or greater. The reflector is a circular or square metal mesh or sheet whose cross dimension (diameter or edge) measures at least 3/4 wavelength. The helical element has a radius of 1/8 to 1/4 wavelength, and a pitch of 1/4 to 1/2 wavelength. The minimum dimensions depend on the lowest frequency at which the antenna is to be used. If the helix or reflector is too small (the frequency is too low), the efficiency is severely degraded. Maximum radiation and response occur along the axis of the helix. Helical antennas are commonly connected together in so-called bays of two, four, or occasionally more elements with a common reflector. The entire assembly can be rotated in the horizontal (azimuth) and vertical (elevation) planes, so the system can be aimed toward a particular satellite. If the satellite is not in a geostationary orbit, the azimuth and elevation rotators can be operated by a computerized robot that is programmed to follow the course of the satellite across the sky.
Hertz Hertz is a unit of frequency (of change in state or cycle in a sound wave, alternating current, or other cyclical waveform) of one cycle per second. It replaces the earlier term of "cycle per second (cps)." For example, in the United States, common house electrical supply is at 60 hertz (meaning the current changes direction or polarity 120 times, or 60 cycles, a second). (In Europe, line frequency is 50 hertz, or 50 cycles per second.) Broadcast transmission is at much higher frequency rates, usually expressed in kilohertz (KHz) or megahertz (MHz). In acoustic sound, the range of human hearing is from 0 Hz to roughly 20 KHz (depending on many factors, including age and how loud the drummer in your high school rock band played!). The pitch of Middle C on a piano is 263 Hz. Hertz is also used frequently when describing the individual bands of an audio equalizer. To make that Middle C louder, you could boost other frequencies to around 263 Hz with an equalizer. The unit of measure is named after Heinrich Hertz, German physicist. Megahertz A megahertz (MHz or sometimes Mhz) is a million cycles of electromagnetic currency alternation per second and is used as a unit of measure for the "clock speed" of computer microprocessor. In designing computer bus architectures, the microprocessor speed is considered together with the potential speed or amount of data that can come into the computer from I/O devices in order to optimize overall computer performance. The hertz as a unit of measure is named after Heinrich Hertz, German physicist.
Gigahertz The gigahertz, abbreviated GHz, is a unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one thousand million hertz (1,000,000,000 Hz). The gigahertz is used as an indicator of the frequency of ultra-
84 high-frequency (UHF) and microwave EM signals and also, in some computers, to express microprocessor clock speed.An EM signal having a frequency of 1 GHz has a wavelength of 300 millimeters, or a little less than a foot. An EM signal of 100 GHz has a wavelength of 3 millimeters, which is roughly 1/8 of an inch. Some radio transmissions are made at frequencies up to hundreds of gigahertz. Personal computer clock speeds are increasing month by month as the technology advances, and reached the 1 GHz point in March of 2000, with a processor from AMD, closely followed by a 1 GHz Pentium 3 from Intel. Other commonly-used units of frequency are the kHz, equal to 1,000 Hz or 0.000001 GHz, and the MHz, equal to 1,000,000 Hz or 0.001 GHz.
High Bit-Rate DSL Pl. see DSL
High-Speed Circuit-Switched Data Pl. see HSCSD
High-Speed Serial Interface Pl. see HSSI
HiperLAN HiperLAN is a set of wireless local area network (WLAN) communication standards primarily used in European countries. There are two specifications: HiperLAN/1 and HiperLAN/2. Both have been adopted by the European Telecommunications Standards Institute (ETSI). The HiperLAN standards provide features and capabilities similar to those of the IEEE 802.11 wireless local area network (LAN) standards, used in the U.S. and other adopting countries. HiperLAN/1 provides communications at up to 20 Mbps in the 5-GHz range of the radio frequency (RF) spectrum. HiperLAN/2 operates at up to 54 Mbps in the same RF band. HiperLAN/2 is compatible with 3G (third-generation) WLAN systems for sending and receiving data, images, and voice communications. HiperLAN/2 has the potential, and is intended, for implementation worldwide in conjunction with similar systems in the 5-GHz RF band.
HLR The Home Location Register (HLR) is the main database of permanent subscriber information for a mobile network. The HLR is an integral component of CDMA (code division multiple access), TDMA (time division multiple access), and GSM (Global System for Mobile communications) networks. Maintained by the subscriber's home carrier (or the network operator where the user initiated the call), the HLR contains pertinent user information, including address, account status, and preferences. The HLR interacts with the Mobile Switching Center (MSC), which is a switch used for call control and processing. The MSC also serves as a point-of-access to the Public Switched Telephone Network (PSTN - the fixed network). The third integral element is the Visiting Location Register (VLR), which maintains temporary user information (such as current location) to manage requests from subscribers who are out of the area covered by their home system. When a user initiates a call, the switching equipment determines whether or not the call is coming from the device's home
85 area. If the user is out of the home area, the area VLR sends out a request for information required to process the call. An MSC queries the HLR identified by the call for information, which it relays to the appropriate MSC, which in turn relays it to the VLR. The VLR sends routing information back to the MSC which allows it to find the station where the call originated, and, finally, the mobile device to connect. Communications between the elements are based on Signaling System (SS7) protocols and signaling.
Home Location Register Pl. see HLR
HomeRF HomeRF (for home radio frequency) is a home networking standard developed by Proxim Inc. that combines the 802.11b and Digital Enhanced Cordless Telecommunication (DECT) portable phone standards into a single system. HomeRF uses a frequency-hopping technique to deliver speeds of up to 1.6 Mbps over distances of up to 150 ft - too short a range for most business applications, but suitable for the home market that it was specifically developed for. HomeRF is one of two standards currently vying for the wireless home network market share. The other main contender, Wi-Fi uses a direct sequence spread spectrum (DSSS) transmission method to deliver speeds of up to 11 Mbps. HomeRF is said to have better mechanisms in place to deal with interference (from microwave ovens, for example) and to handle voice, video, and audio data better than Wi-Fi. Nevertheless, Wi-Fi is significantly faster than HomeRF - albeit more expensive as well. Wi-Fi products have already become fairly well established in corporate wide area networks (WANs), which tend to support the older standard for home networks, since consumers tend to prefer to use the same technologies in both home and work settings. Although industry support is split between the two technologies, a number of companies (such as IBM and Proxim itself) have begun to back both standards.
Horn Antenna A horn antenna is used for the transmission and reception of microwave signals. It derives its name from the characteristic flared appearance. The flared portion can be square, rectangular, or conical. The maximum radiation and response corresponds with the axis of the horn. In this respect, the antenna resembles an acoustic horn. It is usually fed with a waveguide.
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In order to function properly, a horn antenna must be a certain minimum size relative to the wavelength of the incoming or outgoing electromagnetic field. If the horn is too small or the wavelength is too large (the frequency is too low), the antenna will not work efficiently. Horn antennas are commonly used as the active element in a dish antenna. The horn is pointed toward the center of the dish reflector. The use of a horn, rather than a dipole antenna or any other type of antenna, at the focal point of the dish minimizes loss of energy (leakage) around the edges of the dish reflector. It also minimizes the response of the antenna to unwanted signals not in the favored direction of the dish. Horn antennas are used all by themselves in short-range radar systems, particularly those used by law- enforcement personnel to measure the speeds of approaching or retreating vehicles.
Hoot and Holler In telecommunications, a hoot-n-holler is a dedicated "always on" connection used for two-way business-to-business voice communication. Hoot-n-holler networks evolved from a type of crude point-to-point plain old telephone system (POTS) used by small businesses with large inventories in the mid-1900's. A plumbing supply company in the 1950's, for instance, might use a full-duplex, transmit-and-receive device commonly called a "squawk box" or "shout down" to allow the front desk person to have two-way communication with the warehouse supervisor over a dedicated open phone line without having to pick up a receiver or dial a phone. Hoot-n-holler found a home at brokerage firms in the 1960's, where it became more sophisticated and grew into the speakerphone and conference-call technology many businesses use today. Hoot-n-holler is still used extensively in the financial community to share market updates and trading orders and is also used at news agencies, weather bureaus, transportation providers, and in manufacturing work environments. According to Cisco Systems, some larger financial firms budget 2-3 million dollars a year just for distribution of their hoot-n-holler feeds to remote branch offices. Voice over IP (VoIP) hoot-n-holler is slowly gaining popularity and is being promoted as a cost-effective solution for "party-line" communication because it still allows users in a hoot-n-holler network to talk simultaneously if they want to, but also allows any idle bandwidth to be reclaimed and used by data applications.
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Hoot-n-holler Pl. see hoot and holler
HotSync HotSync is the registered trade name for a sophisticated method of linking between a Palm handheld computer and a more substantial notebook, desktop, or other computer. Such a link can be done using a so-called HotSync cable, or using a wireless connection. If the portable computer and the larger machine are in the same room, the link can use infrared radiation (IR). Direct linking between computers eliminates the need for cumbersome disks or tapes when data must be copied from one hard drive to another.? But the versatility of the scheme goes beyond simple data transfer.? In a network, direct linking allows each portable computer to take advantage of the resources of the whole system, and also allows communication between or among end users.
HSCSD High-Speed Circuit-Switched Data (HSCSD) is circuit-switched wireless data transmission for mobile users at data rates up to 38.4 Kbps, four times faster than the standard data rates of the Global System for Mobile (GSM) communication standard in 1999. HSCSD is comparable to the speed of many computer modems that communicate with today's fixed telephone networks. HSCSD is an evolutionary technology on the way to Universal Mobile Telecommunications Service (UMTS).
HSSI High-Speed Serial Interface (HSSI) is a short-distance communications interface that is commonly used to interconnect routing and switching devices on local area networks (LANs) with the higher-speed lines of a wide area network (WAN). HSSI is used between devices that are within fifty feet of each other and achieves data rates up to 52 Mbps. Typically, HSSI is used to connect a LAN router to a T-3 line. HSSI can be used to interconnect devices on token ring and Ethernet LANs with devices that operate at Synchronous Optical Network (SONET) OC-1 speeds or on T-3 lines. HSSI is also used for host-to-host linking, image processing, and disaster recovery applications. Like ISDN and DSL, HSSI operates at the physical layer of a network, using the standard Open Systems Interconnection (OSI) model. The electrical connection uses a 50-PIN connector. The HSSI transmission technology uses differential emitter-coupled logic (ECL). (ECL is a circuit design in which two transistor emitters are connected to a resistor that is switched between the emitters, producing high bit rates.) HSSI uses gapped timing. Gapped timing allows a Data Communications Equipment (DCE) device to control the flow of data being transmitted from a Data Terminating Equipment (DTE) device such as a terminal or computer by adjusting the clock speed or deleting clock impulses. For diagnosing problems, HSSI offers four loopback tests. The first loopback tests the cable by looping the signal back after it reaches the DTE port. The second and third loopbacks test the line ports of the local DCE and the remote DTE. The fourth tests the DTE's DCE port. HSSI requires two control signals ("DTE available" and "DCE available") before the data circuit is valid. The HSSI cable uses the same number of
88 pins and wires as a SCSI-2 cable, but uses the HSSI electrical interface. It is not recommended to use a SCSI-2 cable with an HSSI interface.
HT A handie talkie, often referred to by its abbreviation, HT, is a handheld, portable two-way radio transceiver. This type of radio is sometmes called a "walkie talkie" or a "handheld." Handie talkies are popular among amateur radio operators, especially on their VHF and UHF bands at 144 and 432 MHz. Handie talkies are widely used by security personnel, military personnel, and police officers. Most HTs are used in conjunction with repeaters for extended range. Some HTs are designed for the 27-MHz Citizens Band (CB) radio service. A typical HT is a rectangular box about the size and weight of an old-fashioned telephone handset. The antenna protrudes from the top end, and consists of a coiled-up element encased in rubber and wound around a flexible rod. This type of antenna, known as a "rubber duck," is not particularly efficient, but is convenient and rugged. Volume and squelch controls are usually placed next to the antenna. The frequency control knob or buttons are on the top end or the front. A speaker/microphone is contained within the box, as is a battery power supply. A small display shows the frequency or channel, received signal strength, and relative RF output power. The transmitter produces RF power output ranging from about 100 milliwatts (100 mW) to five watts (5 W), depending on the service and the intended use. Rechargeable nickel- cadmium (NiCd), nickel-metal-hydride (NiMH), or lithium (Li) batteries (see battery) are employed as the power supply for a typical HT, with a nominal voltage of 12 to 14 volts DC. Most HTs can be operated while recharging, which is usually done using an adapter connecting the HT to a 117-volt AC utility outlet. Some HTs have adapters that allow recharging from automotive batteries.
Hub In general, a hub is the central part of a wheel where the spokes come together. The term is familiar to frequent fliers who travel through airport "hubs" to make connecting flights from one point to another. In data communications, a hub is a place of convergence where data arrives from one or more directions and is forwarded out in one or more other directions. A hub usually includes a switch of some kind. (And a product that is called a "switch" could usually be considered a hub as well.) The distinction seems to be that the hub is the place where data comes together and the switch is what determines how and where data is forwarded from the place where data comes together. Regarded in its switching aspects, a hub can also include a router. In describing network topologies, a hub topology consists of a backbone (main circuit) to which a number of outgoing lines can be attached ("dropped"), each providing one or more connection port for device to attach to. For Internet users not connected to a local area network, this is the general topology used by your access provider. Other common network topologies are the bus network and the ring network. (Either of these could possibly feed into a hub network, using a bridge.) As a network product, a hub may include a group of modem cards for dial-in users, a gateway card for connections to a local area network (for example, an Ethernet or a token ring), and a connection to a line (the main line in this example).
89 I I iDEN iDEN (Integrated Digital Enhanced Network) is a wireless technology from Motorola combining the capabilities of a digital cellular telephone, two-way radio, alphanumeric pager, and data/fax modem in a single network. iDEN operates in the 800 MHz, 900MHz, and 1.5 GHz bands and is based on time division multiple access (TDMA) and GSM architecture. It uses Motorola's Vector Sum Excited Linear Predictors (VSELP) vocoder for voice compression and QAM modulation to deliver 64 Kbps over a 25 KHz channel. iDEN is designed to give the mobile user quick access to information without having to carry around several devices. In early 2001, a new iDEN handset is scheduled for release that will tackle added business needs like logistics management and alerts. Currently, iDEN systems work in more than a dozen countries.
IDSL Pl. See DSL
IMT-2000 direct spread Pl. see WDCDMA
In-band signaling In the public switched telephone network, (public switched telephone network), in-band signaling is the exchange of signaling (call control) information on the same channel that the telephone call itself is using. Today, most long-distance communication uses out-of-band signaling as specified in various Signaling System 7 (Signaling System 7) standards.
Infrared Wireless IR wireless is the use of wireless technology in devices or systems that convey data through infrared (IR) radiation. Infrared is electromagnetic energy at a wavelength or wavelengths somewhat longer than those of red light. The shortest- wavelength IR borders visible red in the electromagnetic radiation spectrum; the longest-wavelength IR borders radio waves. Some engineers consider IR technology to be a sub-specialty of optical technology. The hardware is similar, and the two forms of energy behave in much the same way. But strictly speaking, "optical" refers to visible electromagnetic radiation, while "infrared" is invisible to the unaided eye. To compound the confusion, IR is sometimes called "infrared light". IR wireless is used for short- and medium-range communications and control. Some systems operate in line-of-sight mode; this means that there must be a visually unobstructed straight line through space between the transmitter (source) and receiver (destination). Other systems operate in diffuse mode, also called scatter mode. This type of system can function when the source and destination are not directly visible to each other. An example is a television remote-
90 control box. The box does not have to be pointed directly at the set, although the box must be in the same room as the set, or just outside the room with the door open. IR wireless technology is used in intrusion detectors; home-entertainment control units; robot control systems; medium-range, line-of-sight laser communications; cordless microphones, headsets, modems, and printers and other peripherals. Unlike radio-frequency (RF) wireless links, IR wireless cannot pass through walls. Therefore, IR communications or control is generally not possible between different rooms in a house, or between different houses in a neighborhood (unless they have facing windows). This might seem like a disadvantage, but IR wireless is more private than RF wireless. Some IR wireless schemes offer a level of security comparable to that of hard-wired systems. It is difficult, for example, to eavesdrop on a well-engineered, line-of-sight, IR laser communications link.
Integrated Digital Enhanced Network Pl. see iDEN
Intelligent Network Intelligent Network (IN) is a telephone network architecture originated by Bell Communications Research (Bellcore) in which the service logic for a call is located separately from the switching facilities, allowing services to be added or changed without having to redesign switching equipment. According to Bell Atlantic, IN is a "service-specific" architecture. That is, a certain portion of a dialed phone number, such as 800 or 900, triggers a request for a specific service. A later version of IN called Advanced Intelligent Network (AIN) introduces the idea of a "service- independent" architecture in which a given part of a telephone number can interpreted differently by different services depending on factors such as time of day, caller identity, and type of call. AIN makes it easy to add new services without having to install new phone equipment. Bellcore called its network IN/1. It included this model: The customer's telephone The switching system (starting with the switch a call is handled by first, usually at a telephone company central office (CO) A database called a service control point (SCP) that defines the possible services and their logic A service management system (SMS)
Interexchange Carrier Pl. see IXC
International callback Pl. see Callback
International Private Leased Circuit Pl. see IPLC
Integrated Services Digital Network Pl. see ISDN
91 Inverse Multiplexing Inverse multiplexing speeds up data transmission by dividing a data stream into multiple concurrent streams that are transmitted at the same time across separate channels (such as a T-1 or E-1 lines) and are then reconstructed at the other end back into the original data stream. Just the reverse of ordinary multiplexing, which combines multiple signals into a single signal, inverse multiplexing is a technique commonly used where data in a high-speed local area network (LAN) flows back and forth into a wide area network (WAN) across the "bottleneck" of a slower line such as a T-1 (1.544 Mbps). By using multiple T-1 lines, the data stream can be load-balanced across all of the lines at the same time. As a general rule, inverse multiplexing across up to 8 T-1 lines (or E-1 lines in Europe and elsewhere) is said to be less expensive than the cost of renting a T-3 (45 Mbps) line (or E-3 line in Europe and elsewhere). Inverse multiplexing is also sometimes used in combination with frame relay and asynchronous transfer mode (ATM) transmission. Inverse multiplexing is sometimes called inverse muxing or imuxing. i-Mode i-Mode is the packet-based service for mobile phones offered by Japan's leader in wireless technology, NTT DoCoMo. Unlike most of the key players in the wireless arena, i-Mode eschews the Wireless Application Protocol (WAP) and uses a simplified version of HTML, Compact Wireless Markup Language (CWML) instead of WAP's Wireless Markup Language (WML). NTT DoCoMo has said that eventually it will support WAP and WML, but the company has not said exactly when this will happen. First introduced in 1999, i-Mode was the world's first smart phone for Web browsing. The i-Mode wireless data service offers color and video over many phones. Its mobile computing service enables users to do telephone banking, make airline reservations, conduct stock transactions, send and receive e-mail, and have access to the Internet. As of early 2000, i-Mode had an estimated 5.6 million users
IPLC An IPLC (international private leased circuit) is a point-to-point private line used by an organization to communicate between offices that are geographically dispersed throughout the world. An IPLC can be used for Internet access, business data exchange, video conferencing, and any other form of telecommunication. To simplify IPLC ordering and billing, a concept called One Stop Shopping (OSS) was developed. OSS allows an organization to place a single order with a single carrier for two private leased circuits for two offices in two different countries. In the past, an organization had to contact each carrier in each country to order the two circuits, which included two separate invoices. OSS consolidates the billing for both circuits into a single invoice, handles all currency issues, and allows the organization to report all problems from either circuit to one carrier.
IR Wireless Pl. see infrared wireless
ISDN ISDN (Integrated Services Digital Network) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire as well as over other
92 media. Home and business users who install an ISDN adapter (in place of a modem) can see highly-graphic Web pages arriving very quickly (up to 128 Kbps). ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company in most urban areas in the United States and Europe. There are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channels. Each B-channel carries data, voice, and other services. Each D-channel carries control and signaling information. The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D- channel. Thus, a Basic Rate user can have up to 128 Kbps service. The Primary Rate consists of 23 B-channels and one 64 Kpbs D-channel in the United States or 30 B-channels and 1 D-channel in Europe. Integrated Services Digital Network in concept is the integration of both analog or voice data together with digital data over the same network. Although the ISDN you can install is integrating these on a medium designed for analog transmission, broadband ISDN (BISDN) will extend the integration of both services throughout the rest of the end-to-end path using fiber optic and radio media. Broadband ISDN will encompass frame relay service for high-speed data that can be sent in large bursts, the Fiber Distributed-Data Interface (FDDI), and the Synchronous Opical Network (SONET). BISDN will support transmission from 2 Mbps up to much higher, but as yet unspecified, rates.
Isochronous In information technology, isochronous (from the Greek "equal" and "time"; pronounced "eye-SAH-krun-us") pertains to processes that require timing coordination to be successful, such as voice and digital video transmission. A sound or picture going from a peripheral computer device or across a network into a computer or television set needs to arrive at close to the same rate of data flow as the source. In feeding digital image data from a peripheral device (such as a video camera) to a display mechanism within a computer, isochronous data transfer ensures that data flows continously and at a steady rate in close timing with the ability of the display mechanism to receive and display the image data. (FireWire, the IEEE 1394 High Performance Serial Bus, includes an isochronous interface.) Isochronous can be distinguished from asynchronous, which pertains to processes that proceed independently of each other until a dependent process has to "interrupt" the other process, and synchronous, which pertains to processes in which one process has to wait on the completion of an event in another process before continuing.
IXC An interexchange carrier (IXC) is a telephone company that provides connections between local exchanges in different geographic areas. IXCs provide interLATA service as described in the Telecommunications Act of 1996. They're commonly referred to as "long-distance carriers." IXCs include AT&T, MCI, Sprint, and others
93 J J J2ME J2ME (Java 2 Platform, Micro Edition) is a technology that allows programmers to use the Java programming language and related tools to develop programs for mobile wireless information devices such as cellular phones and personal digital assistants (PDAs). J2ME consists of programming specifications and a special virtual machine, the K Virtual Machine, that allows a J2ME-encoded program to run in the mobile device. There are two programming specifications: Connected, Limited Device Configuration (CLDC) and the Mobile Information Device Profile (MIDP). CLDC lays out the application program interface (API) and virtual machine features needed to support mobile devices. MIDP adds to the CLDC the user interface, networking, and messaging details needed to interface with mobile devices. MIDP includes the idea of a midlet, a small Java application similar to an applet but one that conforms with CLDC and MIDP and is intended for mobile devices. Devices with systems that exploit J2ME are already available and are expected to become even more available in the next few years.
Java 2 Platform Micro Edition Pl. see J2ME
Jitter Jitter is the deviation in or displacement of some aspect of the pulses in a high- frequency digital signal. As the name suggests, jitter can be thought of as shaky pulses. The deviation can be in terms of amplitude, phase timing, or the width of the signal pulse. Another definition is that it is "the period frequency displacement of the signal from its ideal location." Among the causes of jitter are electromagnetic interference (EMI) and crosstalk with other signals. Jitter can cause a display monitor to flicker; affect the ability of the processor in a personal computer to perform as intended; introduce clicks or other undesired effects in audio signals, and loss of transmitted data between network devices. The amount of allowable jitter depends greatly on the application.
94 K K Keitai Keitai (pronounced k-tie) is a Japanese term that refers, in general, to cellular telephone sets, particularly the handheld variety, and their associated hardware, programming, and services. The term has acquired cultural significance (even a fad status) in Japan, especially among young people. Examples of keitai include portable cellular telephones with the capability to transmit and receive e-mail, surf the Web, and play interactive games online. Global Positioning System (GPS) devices and other wireless navigational systems in cars are also examples of keitai.
Key station Commonly used by a company within its private automatic branch exchange (PABX) telephone system, a keyphone (abbreviated as K/P, sometimes called a key station) is a telephone with the extra buttons and the intelligence to allow incoming calls to be transferred to other extensions. Extra lights, either on the phone console or in the handset, indicate the status of lines and extensions; extra keys facilitate connections between extensions and external lines. analog, digital, and Internet- enabled versions of keyphones are available.
Keyphone Pl. see Key station
Kbps In the U.S., Kbps stands for kilobits per second (thousands of bits per second) and is a measure of bandwidth (the amount of data that can flow in a given time) on a data transmission medium. Higher bandwidths are more conveniently expressed in megabits per second (Mbps, or millions of bits per second) and in gigabits per second (Gbps, or billions of bits per second). In international English outside the U.S., the equivalent usage is "kbps" or "kbits s-1."
95 L L Latency In a network, latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another. In some usages (for example, AT&T), latency is measured by sending a packet that is returned to the sender and the round-trip time is considered the latency. The latency assumption seems to be that data should be transmitted instantly between one point and another (that is, with no delay at all). The contributors to network latency include: Propagation: This is simply the time it takes for a packet to travel between one place and another at the speed of light. Transmission: The medium itself (whether optical fiber, wireless, or some other) introduces some delay. The size of the packet introduces delay in a round trip since a larger packet will take longer to receive and return than a short one. Router and other processing: Each gateway node takes time to examine and possibly change the header in a packet (for example, changing the hop count in the time-to-live field). Other computer and storage delays: Within networks at each end of the journey, a packet may be subject to storage and hard disk access delays at intermediate devices such as switches and bridges. (In backbone statistics, however, this kind of latency is probably not considered.)
In a computer system, latency is often used to mean any delay or waiting that increases real or perceived response time beyond the response time desired. Specific contributors to computer latency include mismatches in data speed between the microprocessor and input/output devices and inadequate data buffers. Within a computer, latency can be removed or "hidden" by such techniques as prefetching (anticipating the need for data input requests) and multithreading, or using parallelism across multiple execution threads. In 3D simulation, in describing a helmet that provides stereoscopic vision and head tracking, latency is the time between the computer detecting head motion to the time it displays the appropriate image.
LAN A local area network (LAN) is a group of computers and associated devices that share a common communications line and typically share the resources of a single processor or server within a small geographic area (for example, within an office building). Usually, the server has applications and data storage that are shared in common by multiple computer users. A local area network may serve as few as two or three users (for example, in a home network) or many as thousands of users (for example, in an FDDI network).
96 The main local area network technologies are: Ethernet token ring ARCNET FDDI
Last Mile Technology Last-mile technology is any telecommunications technology, such as wireless radio, that carries signals from the broad telecommunication along the relatively short distance (hence, the "last mile") to and from the home or business. Or to put it another way: the infrastructure at the neighborhood level. In many communities, last-mile technology represents a major remaining challenge to high-bandwidth applications such as on-demand television, fast Internet access, and Web pages full of multimedia effects. Today, in addition to "plain old telephone (dial-up) service", last-mile technologies to deliver voice, data, and TV can include: ISDN, a somewhat faster technology than regular phone service Digital Subscriber Line (DSL) over existing telephone twisted pair lines Cable and the cable modem for data, using the same installed coaxial cable that already is used for television Wireless, including services such as DirecTV Less frequently because of the installation expense, optical fiber and its transmission technologies
LATA LATA (local access and transport area) is a term in the U.S. for a geographic area covered by one or more local telephone companies, which are legally referred to as local exchange carriers (LECs). A connection between two local exchanges within the LATA is referred to as intraLATA. A connection between a carrier in one LATA to a carrier in another LATA is referred to as interLATA. InterLATA is long-distance service. The current rules for permitting a company to provide intraLATA or interLATA service (or both) are based on the Telecommunications Act of 1996.
Layer 1-7 Pl. see OS!
LBS Location-based services (LBS) are services that exploit knowledge about where an information device user is located. For example, the user of a wireless-connected smartphone could be shown ads specific to the region the user is traveling in. Location-based services exploit any of several technologies for knowing where a network user is geographically located. One is the Global Positioning System (GPS), based on a collection of 24 Navstar satellites developed originally for the U.S. Department of Defense. A land-based GPS receiver uses these satellites to determine its location, within 50 meters to 100 meters. A location-based service could require that each of its users have a mobile device that contains a GPS receiver. A second approach is E911, an initiative of the Federal Communications
97 Commission (Fcc) that requires wireless carriers to pinpoint a caller's telephone number to emergency dispatchers. E911 also ensures that carriers will be able to provide call locations from wireless phones. E911 is the most widely used location- based service in the U.S. Another obstacle to immediate widespread adoption of LBS are concerns about privacy and unsolicited wireless advertising. The Cellular Telecommunications and Internet Association (CTIA) is asking the FCC to create specific rules about wireless location privacy. The CTIA proposal says a technical solution must include notice, consent, and security - and be technology-neutral.
Leased Line A leased line is a telephone line that has been leased for private use. In some contexts, it's called a dedicated line. A leased line is usually contrasted with a switched line or dial-up line. Typically, large companies rent leased lines from the telephone message carriers (such as AT&T) to interconnect different geographic locations in their company. The alternative is to buy and maintain their own private lines or, increasingly perhaps, to use the public switched lines with secure message protocols. (This is called tunneling.)
LEC LEC (local exchange carrier) is the term for a public telephone company in the U.S. that provides local service. Some of the largest LECs are the Bell operating companies (BOCs) which were grouped into holding companies known collectively as the regional Bell operating companies (RBOCs) when the Bell System was broken up by a 1983 consent decree. In addition to the Bell companies, there are a number of independent LECs, such as GTE. LEC companies are also sometimes referred to as "telcos." A "local exchange" is the local "central office" of an LEC. Lines from homes and businesses terminate at a local exchange. Local exchanges connect to other local exchanges within a local access and transport area (LATA) or to interexchange carriers (IXCs) such as long-distance carriers AT&T, MCI, and Sprint.
LEO Pl. see Satellite
Line Information Database A line information database (LIDB) is a database maintained by the local telephone company that contains subscriber information, such as a service profile, name and address, and credit card validation information
LLC In the Open Systems Interconnection (OSI) model of communication, the Logical Link Control layer is one of two sublayers of the Data-Link layer and is concerned with managing traffic (flow and error control) over the physical medium. The Logical Link Control layer identifies a line protocol, such as SDLC, NetBIOS, or NetWare, and may also assign sequence numbers to frames and track acknowledgements. The other Data-Link sublayer is the Media Access Control layer.
98 LMDS LMDS (Local Multipoint Distribution System) is a system for broadband microwave wireless transmission direct from a local antenna to homes and businesses within a line-of-sight radius, a solution to the so-called last-mile technology problem of economically bringing high-bandwidth services to users. LMDS is an alternative to installing optical fiber all the way to the user or to adapting cable TV for broadband Internet service. Depending on the implementation, LMDS offers a bandwidth of up to 1.5 Gbps downstream to users and 200 Mbps upstream from the user. A more typical data rate is 38 Mbps downstream. Some services offer both downstream and upstream service (symmetrical service); others offer downstream only (asymmetrical service) with upstream being obtained using wire connections. In addition to the investment by service providers for transmitters, users need to install transceivers costing about $125-225. However, the cost of installing LMDS is considered far lower than installing fiber optic cable or upgrading cable TV systems. The first markets for LMDS are seen as: High-speed data transmission for businesses Interactive television and streaming multimedia from Web sites Voice service (usually as a supplement to other services)
Because LMDS requires a more expensive and possibly larger transceiver than can conveniently be packaged in a handheld device, LMDS is not viewed as a replacement for or alternative to mobile wireless technologies such as cellular and GSM. On the other hand, LMDS offers much higher data rates because of its use of a higher range of frequencies with their wider bandwidths. In general, LMDS is for fixed locations and offers higher data rates; cellular digital such as GSM is for mobile users at lower data rates (although these will increase with technologies leading up to UMTS). LMDS uses the range of electromagnetic radiation spectrum in the vicinity of 28 GHz, with the allocated range differing slightly between the U.S., Canada, and other countries. In Europe, ETSI sponsors an equivalent technology. In Canada, it is called Local Multipoint Communication Service (LMCS). Like cellular telephone technologies, LMDS is point-to-multipoint. It is viewed as a future component in the convergence of data and telephony services. Ericsson's LMDS system uses either Ethernet, ATM, or T-carrier system/E-carrier system network interfaces at the user end. ATM allows the user to select and pay for varying qualities of service.
LNP Local Number Portability (LNP) is the ability of a telephone customer in the U.S. to retain their local phone number if they switch to another local telephone service provider. The Telecommunications Act of 1996 required that the local exchange carriers (LECs) in the 100 largest metropolitan markets provide this capability by the end of 1998. The idea is that by removing the personal inconvenience of having to get a new phone number when changing service providers, competition among providers will be increased. LNP is one of the prices that local carriers must pay in order to be allowed to compete as well in the long-distance market. LNP is made possible by the Location Routing Number (LRN). In the future, phone number portability may be extended so that customers can retain their phone number when
99 they move to another locality. LNPs and LRNs are supervised by the Number Portability Administration Center, operated by Lockheed Martin under the appointment of the Federal Communications Commission (FCC). When a customer moves their local service to a competitive local exchange carrier (CLEC), a new LRN is assigned to the telephone number being ported. Each local exchange and long distance carrier needs to know what that new LRN is so when someone in an another area dials the number being ported, the carrier knows what LRN to route to. This is accomplished through Local Service Management System (LSMS) databases distributed among the exchange carriers. The NPAC updates all of these databases with the newly assigned LRN. Thus, when the call is made from another area, that carrier refers to its LSMS database to obtain the current LRN for the number dialed.
Location Based Services Pl. see LBS
Local Area Network Pl. see LAN
Local Access And Transport Area Pl. see LATA
Local Exchange Carrier Pl. see LEC
Local Loop In telephony, a local loop is the wired connection from a telephone company's central office in a locality to its customers' telephones at homes and businesses. This connection is usually on a pair of copper wires called twisted pair. The system was originally designed for voice transmission only using analog transmission technology on a single voice channel. Today, your computer's modem makes the conversion between analog signals and digital signals. With Integrated Services Digital Network (ISDN) or Digital Subscriber Line (DSL), the local loop can carry digital signals directly and at a much higher bandwidth than they do for voice only.
Local Number Portability Pl. see LNP
Location Routing Number Pl. see LRN
Long-Distance Carrier A long-distance carrier is a telephone company that provides connections between local exchanges in different geographic areas. Referred to in the U.S. as interexchange carriers (IXCs), long-distance carriers provides interlocal access and transport area (interLATA) service as described in the Telecommunications Act of 1996. Long-distance carriers include AT&T, MCI, Sprint, and others.
Long-Haul Optics
100 Long-haul optics refers to the transmission of visible light signals over optical fiber cable for great distances, especially without or with minimal use of repeaters. Normally, repeaters are necessary at intervals in a length of fiber optic cable to keep the signal quality from deteriorating to the point of non-usability. In long-haul optical systems, the goal is to minimize the number of repeaters per unit distance, and ideally, to render repeaters unnecessary. The main challenge facing developers of long-haul optics (and also the so-called ultra-long-haul or ULH optics) involves the loss inherent in the materials used in fiber optic cable. To some extent this loss can be overcome by increasing the brilliance of the optical signal at the input end, but this brute-force approach has limited practicality. Repeaters can be used to boost the signals at intervals along the transmission route, but this is costly, and the maintenance of such repeaters is difficult in underground or undersea fiber optic cable runs. This has resulted in a quest for the development of super- transparent substances that transmit energy at optical wavelengths with exceptionally low loss. An indirect contributor to fiber optic cable loss is the fact that all transparent materials transmit energy at slightly different speeds, depending on the wavelength. This is the same effect that causes a prism to split white visible light into its constituent colors, and takes place because the index of refraction is dependent on the wavelength. This is observed as a "smearing-out-over-time" effect in long fiber optic cable runs, unless the energy for each signal can be kept within a narrow range of wavelengths. Engineers have developed schemes such as wave-division multiplexing (WDM) and dense wave-division multiplexing (DWDM) in an attempt to minimize this problem.
Loopback In telephone systems, a loopback is a test signal sent to a network destination that is returned as received to the originator. The returned signal may help diagnose a problem. Sending a loopback test to each telephone system piece of equipment in succession, one at a time, is a technique for isolating a problem. (The loopback can be compared to the Internet's ping utility, which lets you send a message out to a host computer on the Internet. The ping echo tells you whether or not the host computer is available and the time the signal took to return.) If you are an ISDN user with more than one B-channel, you can do a loopback test from your computer.
Loopback test A loopback test is a test in which a signal in sent from a communications device and returned (looped back) to it as a way to determine whether the device is working right or as a way to pin down a failing node in a network. One type of loopback test is performed using a special plug, called a wrap plug, that is inserted in a port on a communications device. The effect of a wrap plug is to cause transmitted (output) data to be returned as received (input) data, simulating a complete communications circuit using a single computer. In telephone systems, the signal sent as part of a loopback test is referred to as a loopback.
LRN In the U.S., a Location Routing Number (LRN) is a 10-digit number in a database called a Service Control Point (SCP) that identifies a switching port for a local telephone exchange. LRN is a technique for providing Local Number Portability
101 (LNP). Using LRN, when a phone number is dialed, the local telephone exchange queries a routing database, usually the SCP, for the LRN associated with the subscriber. The LRN removes the need for the public telephone number to identify the local exchange carrier. If a subscriber changes to another telephone service provider, the current telephone number can be retained. Only the LRN needs to be changed. In addition to supporting service provider phone number portability, an LRN also supports the possibility of two other types of number portability: service portability (for example, ordinary service to ISDN) and geographic portability. LRN is an alternative to the current NPA-NXX format described in the North American Telephone Numbering System (NATNS). LNPs and LRNs are supervised by the Number Portability Administration Center, operated by Lockheed Martin under the appointment of the Federal Communications Commission (FCC).
LZW compression LZW compression is the compression of a file into a smaller file using a table- based lookup algorithm invented by Abraham Lempel, Jacob Ziv, and Terry Welch. Two commonly-used file formats in which LZV compression is used are the GIF image format served from Web sites and the TIFF image format. LZW compression is also suitable for compressing text files. A particular LZW compression algorithm takes each input sequence of bits of a given length (for example, 12 bits) and creates an entry in a table (sometimes called a "dictionary" or "codebook") for that particular bit pattern, consisting of the pattern itself and a shorter code. As input is read, any pattern that has been read before results in the substitution of the shorter code, effectively compressing the total amount of input to something smaller. Unlike earlier approaches, known as LZ77 and LZ78, the LZW algorithm does include the look-up table of codes as part of the compressed file. The decoding program that uncompresses the file is able to build the table itself by using the algorithm as it processes the encoded input.
102 M M Master In computer networking, master/slave is a model for a communication protocol in which one device or process (known as the master) controls one or more other devices or processes (known as slaves). Once the master/slave relationship is established, the direction of control is always from the master to the slave(s). Other communication protocol models include the client/server model, in which a server program responds to requests from a client program, and the peer-to-peer model, in which either of the two devices involved can initiate a communication session.
M-payment M-payment (mobile payment) is a point-of-sale payment made through a mobile device, such as a cellular telephone, a smartphone, or a personal digital assistant (PDA). Using m-payment, a person with a wireless device could pay for items in a store or settle a restaurant bill without interacting with any staff member. So, for example, if a restaurant patron wanted to pay quickly and leave the restaurant on time to get to an appointment, the bill could be paid directly from the table - without waiting for a server to bring the check. The patron would simply connect to the cash register with a wireless device, punch in the table number and bank personal identification number (PIN), and authorize payment. According to Orange Mobile Payment (a Danish company), the entire transaction should take no more than 10 seconds. The earliest m-payment trials were based on the wide area network (WAN) used for cellular phones. That meant, however, that users had to pay cell phone charges to make a payment, and also had to punch in long sequences of digits each time. Other technologies tested enable less cumbersome procedures. Palm and Verifone will use infrared (IR) data transmission for their initial trials. Among the other technologies being used are Bluetooth, WiFi, and RFID, a short- range transmission system. Public key infrastructure (PKI) encryption - considered to be necessary for secure m-commerce in general - is currently being incorporated into digital wireless networks and into an increasing number of wireless devices, a trend that is likely to increase consumer confidence in m-payment's security. M- payment is already being used in some parts of the world, including Europe and Asia. One small complication hindering wide-spread acceptance of m-payment is the distinction that credit card companies make between transactions where the card is physically present at the point of sale and those where it is absent - for example, when you use your credit card for transactions over the telephone or your computer's Internet connection. For payments in what are considered "card not present" situations, credit card companies charge the merchant a higher transaction fee. Whether m-payment would qualify as a "card present" situation or not has not yet been determined; that decision may depend on the degree of confidence credit card companies have in the security of m-payment.
Master/Slave
103 Pl. see master
MCM Multi-carrier modulation (MCM) is a method of transmitting data by splitting it into several components, and sending each of these components over separate carrier signals. The individual carriers have narrow bandwidth, but the composite signal can have broad bandwidth. The advantages of MCM include relative immunity to fading caused by transmission over more than one path at a time (multipath fading), less susceptibility than single-carrier systems to interference caused by impulse noise, and enhanced immunity to inter-symbol interference. Limitations include difficulty in synchronizing the carriers under marginal conditions, and a relatively strict requirement that amplification be linear. MCM was first used in analog military communications in the 1950s. Recently, MCM has attracted attention as a means of enhancing the bandwidth of digital communications over media with physical limitations. The scheme is used in some audio broadcast services. The technology lends itself to digital television, and is used as a method of obtaining high data speeds in asymmetric digital subscriber line (ADSL) systems. MCM is also used in wireless local area networks (WLANs). Also see orthogonal frequency-division multiplexing (OFDM), frequency-division multiplexing (FDM), and time-division multiplexing (TDM).
MDI/MDIX MDI/MDIX is a type of Ethernet port connection using twisted pair cabling. The MDI (for medium dependent interface) is the component of the media attachment unit (MAU) that provides the physical and electrical connection to the cabling medium. An MDIX (for MDI crossover) is a version of MDI that enables connection between like devices. MDI ports connect to MDIX ports via straight-through twisted pair cabling; both MDI-to-MDI and MDIX-to-MDIX connections use crossover twisted pair cabling.
MDIX Pl. see MDI/MDIX
Mean Opinion Score Pl. see MOS
Media Access Control layer In the Open Systems Interconnection (OSI) model of communication, the Media Access Control layer is one of two sublayers of the Data Link Control layer and is concerned with sharing the physical connection to the network among several computers. Each computer has its own unique MAC address. Ethernet is an example of a protocol that works at the Media Access Control layer level. The other Data Link Control sublayer is the Logical Link Control layer.
Medium Earth Orbit Satellite Pl. see MEO
MEO
104 A medium earth orbit (MEO) satellite is one with an orbit within the range from a few hundred miles to a few thousand miles above the earth's surface. Satellites of this type orbit higher than low earth orbit (LEO) satellites, but lower than geostationary satellites. The orbital periods of MEO satellites range from about two to 12 hours. Some MEO satellites orbit in near perfect circles, and therefore have constant altitude and travel at a constant speed. Other MEO satellites revolve in elongated orbits. The perigee (lowest altitude) of an elliptical-orbit satellite is much less than its apogee (greatest altitude). The orbital speed is much greater near perigee than near apogee. As seen from a point on the surface, a satellite in an elongated orbit crosses the sky in just a few minutes when it is near perigee, as compared to several hours when it is near apogee. Elliptical-orbit satellites are easiest to access near apogee, because the earth-based antenna orientation does not have to be changed often, and the satellite is above the horizon for a fairly long time. A fleet of several MEO satellites, with orbits properly coordinated, can provide global wireless communication coverage. Because MEO satellites are closer to the earth than geostationary satellites, earth-based transmitters with relatively low power and modest-sized antennas can access the system. Because MEO satellites orbit at higher altitudes than LEO satellites, the useful footprint (coverage area on the earth's surface) is greater for each satellite. Thus a global-coverage fleet of MEO satellites can have fewer members than a global-coverage fleet of LEO satellites.
MHP Multimedia Home Platform (MHP) is a digital video broadcasting (DVB) standard intended to combine digital television (DTV) with the Internet and the World Wide Web. The result will make high-definition television in an interactive, multi-purpose communication network available to consumers and businesses. The MHP, when fully implemented, will function through cable, fiber optic, and (including satellite) media. Users will be able to browse the Web, watch (and in some cases participate in) television programs, shop online, and enjoy broadband Internet access. The service will be available in three levels: enhanced broadcast, interactive broadcast, and Internet. The typical MHP installation will employ a so-called set-top box and a television receiver. The standard will also allow the use of a personal computer in place of the television receiver. The MHP standard will be downward- compatible. This will ensure that existing hardware will continue to function as new services are added.
MMDS Multichannel Multipoint Distribution Service (MMDS) is a broadcasting and communications service that operates in the ultra-high-frequency (UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless cable. It was conceived as a substitute for conventional cable television (TV). However, it also has applications in telephone/fax and data communications. In MMDS, a medium-power transmitter is located with an omindirectional broadcast antenna at or near the highest topographical point in the intended coverage area. The workable radius can reach up to 70 miles in flat terrain (significantly less in hilly or mountainous areas). Each subscriber is equipped with a small antenna, along
105 with a converter that can be placed next to, or on top of, a conventional TV set. There is a monthly fee, similar to that for satellite TV service. The MMDS frequency band has room for several dozen analog or digital video channels, along with narrowband channels that can be used by subscribers to transmit signals to the network. The narrowband channels were originally intended for use in an educational setting (so-called wireless classrooms). The educational application has enjoyed some success, but conventional TV viewers prefer satellite TV services, which have more channels. Because of recent deregulation that allows cable TV companies to provide telephone and Internet services, along with the development of digital technologies that make efficient use of available bandwidth, MMDS has considerable future potential. An MMDS network can provide high- speed Internet access, telephone/fax, and TV together, without the constraints of cable connections.
Mobile Payment Pl. see m-payment
Mobile Satellite Services Pl. see MSS
Mobitex Mobitex is a wireless network architecture that specifies a framework for the fixed equipment necessary to support all the wireless terminals in a packet-switched, radio-based communication system. The three major components of a Mobitex network are the radio base station, the MX switch, and the network management center (NCC). Mobitex was developed in 1984 by Eritel, an Ericsson subsidiary, for the Swedish Telecommunication Administration. In a Mobitex network, a radio base station, with one or more switches (called MX switches), serves as the transmitter for each single cell (area of coverage) of up to 30 km. The base stations, among them, provide an area of coverage and determine the network capacity. Users of wireless devices, such as mobile phones and personal digital assistants (PDAs), communicate through the base station nearest to them and can move freely from one cell to another. The use of packet-switching technology for data transmission is less expensive than circuit-switching, which uses a dedicated path for each transmission. Mobitex packets (called MPAKs) are limited to 512 bytes of data. Each packet contains information about its origin and destination, size, type, and sequence within a transmission to ensure that it reaches its destination intact. Because packets can be sent on any route and in any order, they make more efficient use of channel capacity, supporting up to 50 times as many users per channel as a circuit-switched network. At the destination, packets are reorganized into the original transmission format. MX switches control communication routes to and from base stations and between wireless and fixed devices. Switches may be organized hierarchically into groupings of regional and area switches, all connected by fixed links. The MX switches also act as a gateway to other networks. A single network management center (NCC) takes care of maintenance and operations such as configuration and subscriber administration and billing. Currently at least twenty-eight Mobitex networks are in operation in twenty-two countries, mostly operating at either 80, 400, or 900 megahertz (MHz). In the US, Mobitex networks
106 generally operate at 900 MHz, while European networks usually operate at 400 MHz. The Mobitex Operators Association (MOA) controls Mobitex specifications; Ericsson manufactures the infrastructure components.
Modulation Modulation is the addition of information to an electronic or optical signal carrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals. One can think of blanket waving as a form of modulation used in smoke signal transmission (the carrier being a steady stream of smoke). Morse code, invented for telegraphy and still used in amateur radio, uses a binary (two-state) digital code similar to the code used by modern computers. For most of radio and telecommunication today, the carrier is alternating current (AC) in a given range of frequencies. Common modulation methods include: Amplitude Modulation (AM) In which the voltage applied to the signal is varied over time Frequency Modulation (FM) In which the frequency of the carrier signal is transmitted is varied in small but meaningful amounts Phase modulation (PM) In which the natural flow of the alternating current waveform is delayed temporarily. These are sometimes known as continuous wave modulation methods to distinguish them from pulse code modulation (PCM), which is used to encode both digital and analog information in a binary way. Radio and television broadcast stations typically use AM or FM. Most two-way radios use FM, although some employ a mode known as single sideband (SSB). More complex forms of modulation are Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). Optical signals are modulated by applying an electromagnetic current to vary the intensity of the laser beam. Modem Modulation and Demodulation Any computer with an online or Internet connection includes a modem. This term is derived by combining the first three letters of the words modulator and demodulator. In a modem, the modulation process involves the conversion of the digital computer signals (high and low, or logic 1 and 0 states) to analog audio- frequency (AF) tones. Digital highs are converted to a tone having a certain constant pitch; digital lows are converted to a tone having a different constant pitch. These states alternate so rapidly that, if you listen to the output of a computer modem, it sounds like a hiss or roar. The demodulation process converts the audio tones back into digital signals that a computer can understand. directly. Multiplexing More information can be conveyed in a given amount of time by dividing the bandwidth of a signal carrier so that more than one modulated signal is sent on the same carrier. Known as multiplexing, the carrier is referred to as a channel and each separate signal carried on it is called a subchannel. The device that puts the separate signals on the carrier and takes them off of received transmissions is a multiplexer. Common types of multiplexing include frequency-division multiplexing (FDM) and time-division multiplexing (TDM). FDM is usually used for analog
107 communication and divides the main frequency of the carrier into separate subchannels, each with its own frequency band within the overall bandwidth. TDM is used for digital communication and divides the main signal into time-slots, with each time-slot carrying a separate signal.
Moonbounce Pl. see EME
Morphis Morphis is a Java-based open source wireless transcoding platform from Kargo, Inc., a wireless technology company based in New York City. The new application development platform will enable delivery of customized text and graphics to users of wireless devices, such as mobile phones, personal digital assistants (PDAs), and handheld computers. Morphis provides a framework for the transformation of binary, plain text, and text markup content to suitable formats for wireless applications. It can also process images, as well as Hypertext Markup Language (HTML), Wireless Markup Language (WML), and Wireless Abstract for XML, (WAX), an integral part of Morphis. According to Kargo's Morphis product description, the platform supplies a framework that allows users to retrieve, translate, and convert any electronic document. Morphis' Extensible Markup Language (XML) processing framework uses Simple API for XML (SAX) event processing and XSL Transformations (XSLT). A single document may undergo multiple translations and filters; complex logic can be provided by writing XSLT extensions. Multiple complex transformations are performed through a pipeline SAX processing engine. Kargo claims that Morphis will allow content providers to develop both current and future wireless applications without requiring they use specific standards or technologies.
MOS In voice communications, particularly Internet telephony, the mean opinion score (MOS) provides a numerical measure of the quality of human speech at the destination end of the circuit. The scheme uses subjective tests (opinionated scores) that are mathematically averaged to obtain a quantitative indicator of the system performance. Compressor/decompressor (codec) systems and digital signal processing (DSP) are commonly used in voice communications because they conserve bandwidth. But they also degrade voice fidelity. The best codecs provide the most bandwidth conservation while producing the least degradation of the signal. Bandwidth can be measured using laboratory instruments, but voice quality requires human interpretation. To determine MOS, a number of listeners rate the quality of test sentences read aloud over the communications circuit by male and female speakers. A listener gives each sentence a rating as follows: (1) bad; (2) poor; (3) fair; (4) good; (5) excellent. The MOS is the arithmetic mean of all the individual scores, and can range from 1 (worst) to 5 (best).
MSS Mobile satellite services (MSS) refers to networks of communications satellites intended for use with mobile and portable wireless telephones. There are three major types: AMSS (aeronautical MSS), LMSS (land MSS), and MMSS (maritime
108 MSS). A telephone connection using MSS is similar to a cellular telephone link, except the repeaters are in orbit around the earth, rather than on the surface. MSS repeaters can be placed on geostationary, medium earth orbit (MEO), or low earth orbit (LEO) satellites. Provided there are enough satellites in the system, and provided they are properly spaced around the globe, an MSS can link any two wireless telephone sets at any time, no matter where in the world they are located. MSS systems are interconnected with land-based cellular networks. As an example of how MSS can work, consider telephones in commercial airliners. These sets usually link into the standard cellular system. This allows communication as long as the aircraft is on a line of sight with at least one land-based cellular repeater. Coverage is essentially continuous over most developed countries. But converage is spotty over less well-developed regions, and is nonexistent at most points over the oceans. Using an MSS network, the aircraft can establish a connection from any location, no matter how remote.
Multi-Carrier Modulation Pl. see MCM
Multimedia Home Platform Pl. see MHM
Multichannel Multipoint Distribution Service Pl. see MMDS
109 N N Nagle's algorithm Named for its creator, John Nagle, the Nagle algorithm is used to automatically concatenate a number of small buffer messages; this process (called nagling) increases the efficiency of a network application system by decreasing the number of packets that must be sent. Nagle's algorithm, defined in 1984 as Ford Aerospace and Communications Corporation Congestion Control in IP/TCP Internetworks (IETF RFC 896) was originally designed to relieve congestion for a private TCP/IP network operated by Ford, but has since been broadly deployed. Nagle's document specified a means of dealing with what he called the small packet problem, created when an application generates data one byte at a time, causing the network to be overloaded with packets (a situation often referred to as send-side silly window syndrome). A single character - one byte of data - originating from a keyboard could result in the transmission of a 41 byte packet consisting of one byte of useful information and 40 bytes of header data. This situation translates into 4000% overhead, which was considered to be acceptable for a lightly loaded network such as the Advanced Research Projects Agency Network (ARPANET - the precursor of the Internet operating at that time), but not so for a heavily loaded network such as Ford's, where it could necessitate retransmissions, cause lost packets, and hamper propagation speed through excessive congestion in switching nodes and gateways. throughput could be hampered to the extent that connections were aborted. Nagle's algorithm - usually implemented through the insertion of two lines of code into a TCP program - instructs the sender to buffer (store) data if any unacknowledged data is outstanding. Any data sent subsequently is held until the outstanding data is acknowledged (ACKed) or until there is a full packet's worth of data to send. Although Nagle's algorithm addressed problems that were being experienced within Ford's network, the same problems were beginning to be experienced by ARPANet. Nagling has been broadly implemented across networks, including the Internet, and is generally performed by default - although it is sometimes considered to be undesirable in highly interactive environments, such as some client/server situations. In such cases, nagling may be turned off through use of the TCP_NODELAY sockets option.
Nagling Pl. see Nagle's algorithm
Narrowband Generally, narrowband describes telecommunication that carries voice information in a narrow band of frequencies. More specifically, the term has been used to describe a specific frequency range set aside by the U.S. Fcc for mobile or radio services, including paging systems, from 50 cps to 64 Kbps. The term is usually
110 contrasted with wideband or broadband. In telecommunication, a band - sometimes called a frequency band - is a specific range of frequencies in the radio frequency (RF) spectrum, which is divided among ranges from very low frequencies (vlf) to extremely high frequencies (ehf). Each band has a defined upper and lower frequency limit. Because two radio transmitters sharing the same frequency band cause mutual interference, band usage is regulated. International use of the radio spectrum is regulated by the International Telecommunication Union (ITU). Domestic use of the radio spectrum is regulated by national agencies such as the Federal Communications Commission (Fcc) in the U.S. Regulatory organizations assign each transmission source a band of operation, a transmitter radiation pattern, and a maximum transmitter power.
Network In information technology, a network is a series of points or nodes interconnected by communication paths. Networks can interconnect with other networks and contain subnetworks. The most common topology or general configurations of networks include the bus, star, and token ring topologies. Networks can also be characterized in terms of spatial distance as local area networks (LAN), metropolitan area networks (MAN), and wide area networks (WAN). A given network can also be characterized by the type of data transmission technology in use on it (for example, a TCP/IP or Systems Network Architecture network); by whether it carries voice, data, or both kinds of signals; by who can use the network (public or private); by the usual nature of its connections (dial-up or switched, dedicated or nonswitched, or virtual connections); and by the types of physical links (for example, optical fiber, coaxial cable, and Unshielded Twisted Pair). Large telephone networks and networks using their infrastructure (such as the Internet) have sharing and exchange arrangements with other companies so that larger networks are created.
Network Operations Center Pl. see NOC
Network Terminating Unit Pl. see NT1
Network Terminator 1 Pl. see network terminating Unit
Network Terminating Unit Using the Integrated Services Digital Network (ISDN) Basic Rate Interchange (Basic Rate Interface in ISDN) service, an NT1 (network terminating unit 1) is a device that accepts a two-wire signal from the phone company and converts it to a four-wire signal that sends and receives to and from devices within the home or business. In the U.K. and some other countries, the NT1 is located at the telephone company's central office. In the U.S., the NT1 is a separate box at the home or business or it can be integrated into one device. If it is a separate box, up to eight devices, such as telephones and computers, can be attached to it. If the NT1 is built into one device, then only that one device can be served by the line coming in
111 from the phone company. Additional devices would require one or more additional lines. The ISDN Basic Rate Interface (Basic Rate Interface in ISDN) is the most common service offered by ISDN providers. BRI supports two separate 64 Kbps B channels and one 16 Kbps D channel. The phone company replaces your conventional analog circuit with a BRI circuit. The NT1 provides the entry termination at your home or business that is required by the phone company. You must have one NT1 for each ISDN line. Some ISDN equipment such as an ISDN terminal adapter (the ISDN equivalent of a modem) may already have a built-in NT1. To find out if your ISDN device has a built-in NT1, check to see if it is designed to connect directly to the public ISDN network. If it is, it does not require a separate NT1. Most U.S. ISDN devices have the NT1 built-in. If you wish to have more than one ISDN device per line, consider purchasing a separate NT1 box with multiple jacks. Some NT1 boxes have built-in analog conversion so you can use both ISDN and analog equipment on the same line. Your ISDN provider may occasionally "talk" to your NT1 box to perform routine testing and maintenance.
NOC A network operations center (NOC) is a place from which a telecommunications network is supervised, monitored, and maintained. Large enterprises with large networks as well as large network service providers typically have a network operations center, a room containing visualizations of the network or networks that are being monitored, workstations at which the detailed status of the network can be seen, and the necessary software to manage the networks. The network operations center is the focal point for network troubleshooting, software distribution and updating, router and domain name management, performance monitoring, and coordination with affiliated networks.
Node In a network, a node is a connection point, either a redistribution point or an end point for data transmissions. In general, a node has programmed or engineered capability to recognize and process or forward transmissions to other nodes.
Noise Noise is unwanted electrical or electromagnetic energy that degrades the quality of signals and data. Noise occurs in digital and analog systems, and can affect files and communications of all types, including text, programs, images, audio, and telemetry.In a hard-wired circuit such as a telephone-line-based Internet hookup, external noise is picked up from appliances in the vicinity, from electrical transformers, from the atmosphere, and even from outer space. Normally this noise is of little or no consequence. However, during severe thunderstorms, or in locations were many electrical appliances are in use, external noise can affect communications. In an Internet hookup it slows down the data transfer rate, because the system must adjust its speed to match conditions on the line. In a voice telephone conversation, noise rarely sounds like anything other than a faint hissing or rushing. Noise is a more significant problem in wireless systems than in hard-wired systems. In general, noise originating from outside the system is inversely proportional to the frequency, and directly proportional to the wavelength. At a low frequency such as 300 kHz, atmospheric and electrical noise are much
112 more severe than at a high frequency like 300 megahertz. Noise generated inside wireless receivers, known as internal noise, is less dependent on frequency. Engineers are more concerned about internal noise at high frequencies than at low frequencies, because the less external noise there is, the more significant the internal noise becomes. Communications engineers are constantly striving to develop better ways to deal with noise. The traditional method has been to minimize the signal bandwidth to the greatest possible extent. The less spectrum space a signal occupies, the less noise is passed through the receiving circuitry. However, reducing the bandwidth limits the maximum speed of the data that can be delivered. Another, more recently developed scheme for minimizing the effects of noise is called digital signal processing (digital signal processing). Using fiber optics, a technology far less susceptible to noise, is another approach.
NT1 Using the Integrated Services Digital Network (ISDN) Basic Rate Interchange (Basic Rate Interface in ISDN) service, an NT1 (network terminating unit 1) is a device that accepts a two-wire signal from the phone company and converts it to a four-wire signal that sends and receives to and from devices within the home or business. In the U.K. and some other countries, the NT1 is located at the telephone company's central office. In the U.S., the NT1 is a separate box at the home or business or it can be integrated into one device. If it is a separate box, up to eight devices, such as telephones and computers, can be attached to it. If the NT1 is built into one device, then only that one device can be served by the line coming in from the phone company. Additional devices would require one or more additional lines. The ISDN Basic Rate Interface (Basic Rate Interface in ISDN) is the most common service offered by ISDN providers. BRI supports two separate 64 Kbps B channels and one 16 Kbps D channel. The phone company replaces your conventional analog circuit with a BRI circuit. The NT1 provides the entry termination at your home or business that is required by the phone company. You must have one NT1 for each ISDN line. Some ISDN equipment such as an ISDN terminal adapter (the ISDN equivalent of a modem) may already have a built-in NT1. To find out if your ISDN device has a built-in NT1, check to see if it is designed to connect directly to the public ISDN network. If it is, it does not require a separate NT1. Most U.S. ISDN devices have the NT1 built-in. If you wish to have more than one ISDN device per line, consider purchasing a separate NT1 box with multiple jacks. Some NT1 boxes have built-in analog conversion so you can use both ISDN and analog equipment on the same line. Your ISDN provider may occasionally "talk" to your NT1 box to perform routine testing and maintenance.
Nyquist Theorem The Nyquist Theorem, also known as the sampling theorem, is a principle that engineers follow in the digitization of analog signals. For analog-to-digital conversion (ADC) to result in a faithful reproduction of the signal, slices, called samples, of the analog waveform must be taken frequently. The number of samples per second is called the sampling rate or sampling frequency. Any analog signal consists of components at various frequencies. The simplest case is the sine wave, in which all the signal energy is concentrated at one frequency. In practice,
113 analog signals usually have complex waveforms, with components at many frequencies. The highest frequency component in an analog signal determines the bandwidth of that signal. The higher the frequency, the greater the bandwidth, if all other factors are held constant. Suppose the highest frequency component, in hertz, for a given analog signal is fmax. According to the Nyquist Theorem, the sampling rate must be at least 2fmax, or twice the highest analog frequency component. The sampling in an analog-to-digital converter is actuated by a pulse generator (clock). If the sampling rate is less than 2fmax, some of the highest frequency components in the analog input signal will not be correctly represented in the digitized output. When such a digital signal is converted back to analog form by a digital-to-analog converter, false frequency components appear that were not in the original analog signal. This undesirable condition is a form of distortion called aliasing.
114 O O OC levels The Synchronous Optical Network (SONET) includes a set of signal rate multiples for transmitting digital signals on optical fiber. The base rate (OC-1) is 51.84 Mbps. Certain multiples of the base rate are provided as shown in the following table. Asynchronous transfer mode (ATM) makes use of some of the Optical Carrier levels.
Optical Carrier Level Data Rate OC-1 51.84 Mbps OC-3 155.52 Mbps OC-12 622.08 Mbps OC-24 1.244 Gbps OC-48 2.488 Gbps OC-192 10 Gbps OC-256 13.271 Gbps OC-768 40 Gbps
OC-1-3 Pl. see OC Levels
OC-12 Pl. see OC Levels
OC-192 Pl. see OC Levels
OC-256 Pl. see OC Levels
OC-48 Pl. see OC Levels
OC-768 Pl. see OC Levels
OC-x Pl. see OC Levels
OFDM Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. The technology was first conceived in the 1960s and 1970s during research into minimizing interference among channels near each other in
115 frequency. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels. OFDM is used in European digital audio broadcast services. The technology lends itself to digital television, and is being considered as a method of obtaining high-speed digital data transmission over conventional telephone lines. It is also used in wireless local area networks.
One Armed Router A one-armed router is a router that routes traffic between virtual local area networks (VLANs). A one-armed router operates on the 80/20 rule, which states that 80% of traffic in a network remains within a virtual local area network and doesn't need routing service. The other 20% of network traffic is between VLANs and goes through the one-armed router. Because the one-armed router takes care of the more intensive traffic between VLANs, it frees the primary data path in a network for inter-VLAN traffic. In order for a one-armed router to be beneficial, the VLAN must be configured to the 80/20 rule. One disadvantage of using the one-armed router structure is that it represents a single point of failure in a network. Another disadvantage is it can develop into a bottleneck if there are large amounts of traffic between VLANs.
Operational Support System Pl. see OSS
Optical Fiber Cable Fiber optic cable a medium of voice and data transmission, functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. The light source can either be a light-emitting diode (LED)) or a laser. The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ones and zeros of the original signal. Even laser light shining through a fiber optic cable is subject to loss of strength, primarily through dispersion and scattering of the light, within the cable itself. The faster the laser fluctuates, the greater the risk of dispersion. Light strengtheners, called repeaters, may be necessary to refresh the signal in certain applications. Although fiber optic cable is still more expensive than other types of cable, it's favored for today's high-speed data communications because it eliminates the problems of twisted-pair cable, such as near-end crosstalk (NEXT), electromagnetic interference (EIVII), and security breaches. Some 10 billion digital bits can be transmitted per second along an optical fiber link in a commercial network, enough to carry tens of thousands of telephone calls. Hair-thin fibers consist of two concentric layers of high-purity silica glass the core and the cladding, which are enclosed by a protective sheath. Light rays modulated into digital pulses with a laser or a light-emitting diode move along the core without penetrating the cladding. The light stays confined to the core because the cladding has a lower refractive index—a measure of its ability to bend light. Refinements in optical fibers, along with the development of new lasers and diodes, may one day allow
116 commercial fiber-optic networks to carry trillions of bits of data per second. Total internal refection confines light within optical fibers (similar to looking down a mirror made in the shape of a long paper towel tube). Because the cladding has a lower refractive index, light rays reflect back into the core if they encounter the cladding at a shallow angle (red lines). A ray that exceeds a certain "critical" angle escapes from the fiber (yellow line). Fiber optic cable consists of a core, cladding, coating, strengthening fibers, and cable jacket (see above).
Core -This is the physical medium that transports optical data signals from an attached light source to a receiving device. The core is a single continuous strand of glass or plastic that's measured (in microns) by the size of its outer diameter. The larger the core, the more light the cable can carry. All fiber optic cable is sized according to its core diameter. The three sizes most commonly available are 50-, 62.5-, and 1 00-micron Gable. Cladding -This is a thin layer that surrounds the fiber core and serves as a boundary that contains the light waves and causes the refraction, enabling data to travel throughout the length of the fiber segment. Coating -This is a layer of plastic that surrounds the core and cladding to reinforce the fiber core, help absorb shocks, and provide extra protection against excessive cable bends. These buffer coatings are measured in microns (p) and can range from 250 p to 900 p. Strengthening fibers -These components help protect the core against crushing forces and excessive tension during installation. The materials can range from Kevlat4 to wire strands to gel-filled sleeves. Cable jacket -This is the outer layer of any cable. Most fiber optic cables have an orange jacket, although some may be black or yellow.
There are three types of fiber optic cable: single mode, multimode and plastic optical fiber (POF). Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns. (One micron is 1/250th the width of a human hair.)
117 Multimode cable is made of multiple strands of glass fibers, with a combined diameter in the 50-to-100 micron range. Each fiber in a multimode cable is capable of carrying a different signal independent from those on the other fibers in the cable bundle. POF is a newer plastic-based cable which promises performance similar to single mode cable, but at a lower cost. Single-mode or Multimode? Single-mode fiber gives a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode fiber-typically 5 to 10 microns. Only a single lightwave can be transmitted at a given time. Single-Mode Fiber has a narrow core (eight microns or less), and the index of refraction between the core and the cladding changes less than it does for multimode fibers. Light thus travels parallel to the axis, creating little pulse dispersion. Telephone and cable television networks install millions of kilometers of this fiber every year. The small core and single lightwave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.
Multimode fiber gives high bandwidth at high speeds over long distances. Lightwaves are dispersed into numerous paths, or modes, as they travel through the cable's core. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet [914.4 ml), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission. Step-Index Multimode Fiber has a large core, up to 100 microns in diameter. As a result, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternative pathways cause the different groupings of light rays, referred to as modes, to arrive separately at a receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits bandwidth that is, the amount of information that can be sent. Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope, for instance.
Basic Cable Design Two basic cable designs are: Loose-tube cable, used in the majority of outside- plant installations in North America, and tight-buffered cable, primarily used inside buildings. Loose-Tube Cable: In a loose-tube cable design, color-coded plastic buffer tubes house and protect optical fibers. A gel filling compound impedes water penetration. Excess fiber length (relative to buffer tube length) insulates fibers from stresses of installation and environmental loading. Buffer tubes are stranded around a dielectric or steel central member, which serves as an anti- buckling element. The cable core, typically surrounded by aramid yarn, is the primary tensile strength member. The outer polyethylene jacket is extruded over the core. If armoring is required, a corrugated steel tape is formed around
118 a single jacketed cable with an additional jacket extruded over the armor. Loose-tube cables typically are used for outside-plant installation in aerial, duct and direct-buried applications. The modular design of loose-tube cables typically holds up to 12 fibers per buffer tube with a maximum per cable fiber count of more than 200 fibers. Loose-tube cables can be all-dielectric or optionally armored. The modular buffer-tube design permits easy drop-off of groups of fibers at intermediate points, without interfering with other protected buffer tubes being routed to other locations. The loose-tube design also helps in the identification and administration of fibers in the system. As with loose- tube cables, optical specifications for tight-buffered cables also should include the maximum performance of all fibers over the operating temperature range and life of the cable. Averages should not be acceptable.
Tight-Buffered Cable: With tight-buffered cable designs, the buffering material is in direct contat with the fiber. This design is suited for "jumper cables" which connect outside plant cables to terminal equipment, and also for linking various devices in a premises network. Multi-fiber, tight-buffered cables often are used for intra-building, risers, general building and plenum applications. The tight- buffered design provides a rugged cable structure to protect individual fibers during handling, routing and connectorization. Yarn strength members keep the tensile load away from the fiber. Single-fiber tight-buffered cables are used as pigtails, patch cords and jumpers to terminate loose-tube cables directly into opto-electronic transmitters, receivers and other active and passive components. Multi-fiber tight-buffered cables also are available and are used primarily for alternative routing and handling flexibility and ease within buildings. Fiber Optic Cable Advantages Over Copper: Speed: Fiber optic networks operate at high speeds - up into the gigabits Bandwidth: large carrying capacity Distance: Signals can be transmitted further without needing to be "refreshed" or strengthened. Resistance: Greater resistance to electromagnetic noise such as radios, motors or other nearby cables. Maintenance: Fiber optic cables costs much less to maintain.
Optical Amplifier Pl. see EDFA
Optical Carrier levels (OCx) Pl. see OC Levels
Orthogonal Frequency-Division Multiplexing Pl. see OFDM
OSS An operational support system (OSS) is a set of programs that help a communications service provider monitor, control, analyze, and manage problems
119 with a telephone or computer network. As the traditional voice telephone system converges with packet-oriented Internet traffic (including Voice over IP), broadband applications such as teleconferencing, and DSL, more sophisticated systems are needed for such activities as ordering and keeping track of network components (including IP addresses), tracking usage, billing, and reporting.
OSI OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network. Its purpose is to guide product implementors so that their products will consistently work with other products. The reference model defines seven layers of functions that take place at each end of a communication. Although OSI is not always strictly adhered to in terms of keeping related functions together in a well-defined layer, many if not most products involved in telecommunication make an attempt to describe themselves in relation to the OSI model. It is also valuable as a single reference view of communication that furnishes everyone a common ground for education and discussion. Developed by representatives of major computer and telecommunication companies beginning in 1983, OSI was originally intended to be a detailed specification of interfaces. Instead, the committee decided to establish a common reference model for which others could develop detailed interfaces, that in turn could become standards. OSI was officially adopted as an international standard by the International Organization of Standards (ISO). Currently, it is Recommendation X.200 of the ITU-TS. The main idea in OSI is that the process of communication between two end points in a telecommunication network can be divided into layers, with each layer adding its own set of special, related functions. Each communicating user or program is at a computer equipped with these seven layers of function. So, in a given message between users, there will be a flow of data through each layer at one end down through the layers in that computer and, at the other end, when the message arrives, another flow of data up through the layers in the receiving computer and ultimately to the end user or program. The actual programming and hardware that furnishes these seven layers of function is usually a combination of the computer operating system, applications (such as your Web browser), TCP/IP or alternative transport and network protocols, and the software and hardware that enable you to put a signal on one of the lines attached to your computer. OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers (up to the network layer) are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host. The seven layers are: Layer 7: The application layer...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.)
120 Layer 6: The presentation layer...This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer. Layer 5: The session layer...This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination. Layer 4: The transport layer...This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer. Layer 3: The network layer...This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding. Layer 3 refers to the Network layer of the commonly-referenced multilayered communication model, Open Systems Interconnection (OSI). The Network layer is concerned with knowing the address of the neighboring nodes in the network, selecting routes and quality of service, and recognizing and forwarding to the Transport layer incoming messages for local host domains. A router is a layer 3 device, although some newer switches also perform layer 3 functions. The Internet Protocol (IP) address is a layer 3 address.
Layer 2: The data-link layer...This layer provides synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management. Layer 1: The physical layer...This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.
OSI illustrated Open Systems Interconnection (OSI) is a standard reference model for communication between two end users in a network. It is used in developing products and understanding networks. This figure shows where commonly-used Internet products and services fit within the model. Also see the notes below the figure.
121
Notes: The OSI Reference Model describes seven layers of related functions that are needed at each end when a message is sent from one party to another party in a network. An existing network product or program can be described in part by where it fits into this layered structure. For example, TCP/IP is usually packaged with other Internet programs as a suite of products that support communication over the Internet. This suite includes the File Transfer Protocol (FTP), Telnet, the Hypertext Transfer Protocol (HTTP), e-mail protocols, and sometimes others. Although TCP fits well into the Transport layer of OSI and IP into the Network layer, the other programs fit rather loosely (but not neatly within a layer) into the Session, Presentation, and Application layers. In this figure, we include only Internet-related programs in the Network and higher layers. OSI can also be applied to other network environments. A number of boxes under the Application and the Presentation layers do not fit as neatly into these layers as they are shown. A set of communication products that conformed fully to the OSI reference model would fit neatly into each layer.
OTA Over The Air (OTA) (or Over-The-Air) is a standard for the transmission and reception of application-related information in a wireless communications system. The standard is supported by Nokia, SmartTrust, and others. OTA is commonly used in conjunction with the Short Messaging Service (SMS), which allows the transfer of small text files even while using a mobile phone for more conventional
122 purposes. In addition to short messages and small graphics, such files can contain instructions for subscription activation, banking transactions, ringtones, and Wireless Access Protocol (WAP) settings. OTA messages can be encrypted to ensure user privacy and data security.
Optical Fiber Optical fiber (or "fiber optic") refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic wire or fiber. Optical fiber carries much more information than conventional copper wire and is in general not subject to electromagnetic interference and the need to retransmit signals. Most telephone company long-distance lines are now of optical fiber. Transmission on optical fiber wire requires repeater at distance intervals. The glass fiber requires more protection within an outer cable than copper. For these reasons and because the installation of any new wiring is labor-intensive, few communities yet have optical fiber wires or cables from the phone company's branch office to local customers (known as local loop). single mode fiber fiber is used for longer distances; multimode fiber fiber is used for shorter distances Single Mode Fiber In optical fiber technology, single mode fiber is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier and is used for long- distance signal transmission. For short distances, multimode fiber is used. Single mode fiber has a much smaller core than multimode fiber. Multimode Fiber In optical fiber technology, multimode fiber is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Multimode fiber transmission is used for relatively short distances because the modes tend to disperse over longer lengths (this is called modal dispersion) . For longer distances, single mode fiber (sometimes called monomode) fiber is used. Multimode fiber has a larger core than single mode.
Over the Air Pl. see OTA
Out-Of-Band Signaling Out-of-band signaling is telecommunication signaling (exchange of information in order to control a telephone call) that is done on a channel that is dedicated for the purpose and separate from the channels used for the telephone call. Out-of-band signaling is used in Signaling System 7 (SS7), the latest standard for the signaling that controls the world's phone calls.
123 P P PACS Personal Access Communications System (PACS) is a type of wireless telephone network compatible with telephone sets, answering machines, fax machines, and computers. A PACS can be used like a local area network (LAN) with voice capability and can be part of a larger network or can be connected into the telephone system. A typical PACS resembles a cellular telephone network in miniature. It contains numerous radio port control units (RCPUs), each of which is the equivalent of a cellular repeater, but with a shorter communications range, linking subscriber sets within a radius of a few hundred feet. RPCUs are located on utility poles, atop buildings, and in other unobtrusive places that offer good coverage for several hundred feet in all directions. RPCU transmitter power is limited to 800 milliwatts. The operating frequency is in the UHF (ultra-high- frequency) radio range at 1.9 GHz. The subscriber sets in a PACS can be fixed, mobile, or portable. Voice subscriber sets use 32 Kbps or 64 Kbps digital speech coding. Computer modems can be supported at speeds of up to 28.8 kbps or 57.6 kbps, respectively. Transmitter output power is limited to 200 milliwatts, but is often much less, on the order of a few tens of milliwatts. This low power level minimizes the likelihood of electromagnetic interference (EMI) to other electronic devices that might be located near the subscriber set.
Pager A pager is a small telecommunications device that receives (and, in some cases, transmits) alert signals and/or short messages. This type of device is convenient for people expecting telephone calls, but who are not near a telephone set to make or return calls immediately. A typical one-way pager fits easily in a shirt pocket; some are as small as a wristwatch. A miniature, short-range wireless receiver captures a message, usually accompanied by a beep. (This is why the device is also known as a beeper). The simplest one-way pagers display the return-call telephone number of the person who sent the message. Alternatively, a code can be displayed that indicates which of several designated parties is requesting a return phone call. Sophisticated one-way pagers can display short text messages. Until recently, pagers were designed as receive-only devices. There are at least two reasons for this. First, if two-way communication is needed, cell phones are available for that purpose. Second, it is difficult to engineer an efficient wireless transmitter that can fit inside a tiny package and provide enough signal range to reach repeaters from all points within the coverage zone. Despite the engineering challenge, a two-way pager, also called a two-way messaging device or two-way interactive system, has been developed. This system employs large numbers of repeaters, allowing low- power wireless transmitters with subminiature antennas to reach at least one repeater from any location within the coverage area. A typical unit is about the size of a pocket calculator and has a built-in, miniature keyboard and a liquid crystal
124 display (LCD) screen that can display several lines of text and/or simple graphics. A two-way pager is a pager that allows you to send data as well as receive it. In some cases, a two-way pager can serve as an alternative to a cellular telephone. At least one manufacturer, Paging Network, lets you record a message and have an answering service on the machine. Apple Computer, Hewlett-Packard, MobileMedia Corporation, Motorola, AirTouch Communications, Casio Computer, Mobile Telecommunication Technologies (Mtel), and Sharp are among companies that manufacture two-pagers or offer a two-way paging service.
Palm Palm is the trade name for a popular personal digital assistant (PDA), a form of handheld device that that is also known as a palmtop computer. Originally the Palm, which is used mainly for personal organization, wireless e-mail, note-taking, and electronic games, was called the PalmPilot. It was introduced in 1996 by Palm Computing, Inc. The Palm is about the same size as a stenographer's tablet. Data appears on an liquid crystal display screen. One of the outstanding features of the Palm is its user-friendly method of data entry. A writing device, called a stylus, can be pointed at icons on the display to select items for entry. The stylus can also be used to enter alphanumeric data (words and numbers) by manually scrawling them across the face of the display. This is known as Graffiti. The Palm can not only be used to originate, store, and process data on its own, but it can download data from a desktop or notebook computer or from the Internet, process it, and then upload the new data back.
PCS PCS (personal communications services) is a wireless phone service somewhat similar to cellular telephone service but emphasizing personal service and extended mobility. It's sometimes referred to as digital cellular (although cellular systems can also be digital). Like cellular, PCS is for mobile users and requires a number of antennas to blanket an area of coverage. As a user moves around, the user's phone signal is picked up by the nearest antenna and then forwarded to a base station that connects to the wired network. The phone itself is slightly smaller than a cellular phone. According to Sprint, PCS is now available to 230 million people. The "personal" in PCS distinguishes this service from cellular by emphasizing that, unlike cellular, which was designed for car phone use with transmitters emphazing coverage of highways and roads, PCS is designed for greater user mobility. It generally requires more cell transmitters for coverage, but has the advantage of fewer blind spots. Technically, cellular systems in the United States operate in the 824-849 megahertz (MHz) frequency bands; PCS operates in the1850-1990 MHz bands. Several technologies are used for PCS in the United States, including Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Global System for Mobile (GSM) communication. GSM is more commonly used in Europe and elsewhere.
Peering Peering is the arrangement of traffic exchange between Internet service providers (ISPs). Larger ISPs with their own backbone networks agree to allow traffic from
125 other large ISPs in exchange for traffic on their backbones. They also exchange traffic with smaller ISPs so that they can reach regional end points. Essentially, this is how a number of individual network owners put the Internet together. To do this, network owners and access providers, the ISPs, work out agreements that describe the terms and conditions to which both are subject. Bilateral peering is an agreement between two parties. Multilateral peering is an agreement between more than two parties. Peering requires the exchange and updating of router information between the peered ISPs, typically using the Border Gateway Protocol (BGP). Peering parties interconnect at network focal points such as the network access points (NAP) in the United States and at regional switching points. Initially, peering arrangements did not include an exchange of money. More recently, however, some larger ISPs have charged smaller ISPs for peering. Each major ISP generally develops a peering policy that states the terms and conditions under which it will peer with other networks for various types of traffic. Private peering is peering between parties that are bypassing part of the public backbone network through which most Internet traffic passes. In a regional area, some ISPs exchange local peering arrangements instead of or in addition to peering with a backbone ISP. In some cases, peering charges include transit charges, or the actual line access charge to the larger network. Properly speaking, peering is simply the agreement to interconnect and exchange routing information.
Perigee Pl. see Satellite
Personal Access Communications System Pl. see PACS
Personal Communications Services Pl. see PCS
Pervasive Computing Pervasive computing is the trend towards increasingly ubiquitous (another name for the movement is ubiquitous computing), connected computing devices in the environment, a trend being brought about by a convergence of advanced electronic - and particularly, wireless - technologies and the Internet. Pervasive computing devices are not personal computers as we tend to think of them, but very tiny - even invisible - devices, either mobile or embedded in almost any type of object imaginable, including cars, tools, appliances, clothing and various consumer goods - all communicating through increasingly interconnected networks. According to Dan Russell, director of the User Sciences and Experience Group at IBM's Almaden Research Center, by 2010 computing will have become so naturalized within the environment that people will not even realize that they are using computers. Russell and other researchers expect that in the future smart devices all around us will maintain current information about their locations, the contexts in which they are being used, and relevant data about the users. The goal of researchers is to create a system that is pervasively and unobtrusively embedded in the environment, completely connected, intuitive, effortlessly portable, and constantly available. Among the emerging technologies expected to prevail in the
126 pervasive computing environment of the future are wearable computers, smart homes and smart buildings. Among the myriad of tools expected to support these are: application-specific integrated circuitry (ASIC); speech recognition; gesture recognition; system on a chip (SoC); perceptive interfaces; smart matter; flexible transistors; reconfigurable processors; field programmable logic gates (FPLG); and microelectromechanical systems (MEMS). A number of leading technological organizations are exploring pervasive computing. Xerox's Palo Alto Research Center (PARC), for example, has been working on pervasive computing applications since the 1980s. Although new technologies are emerging, the most crucial objective is not, necessarily, to develop new technologies. IBM's project Planet Blue, for example, is largely focused on finding ways to integrate existing technologies with a wireless infrastructure. Carnegie Mellon University's Human Computer Interaction Institute (HCII) is working on similar research in their Project Aura, whose stated goal is "to provide each user with an invisible halo of computing and information services that persists regardless of location." The Massachusetts Institute of Technology (MIT) has a project called Oxygen. MIT named their project after that substance because they envision a future of ubiquitous computing devices as freely available and easily accessible as oxygen is today.
PDA PDA (personal digital assistant) is a term for any small mobile hand-held device that provides computing and information storage and retrieval capabilities for personal or business use, often for keeping schedule calendars and address book information handy. The term handheld is a synonym. Many people use the name of one of the popular PDA products as a generic term. These include Hewlett- Packard's Palmtop and 3Com's PalmPilot. Most PDAs have a small keyboard. Some PDAs have an electronically sensitive pad on which handwritng can be received. Apple's Newton, which has been withdrawn from the market, was the first widely-sold PDA that accepted handwriting. Typical uses include schedule and address book storage and retrieval and note-entering. However, many applications have been written for PDAs. Increasingly, PDAs are combined with telephones and paging systems. Some PDAs offer a variation of the Microsoft Windows operating system called Windows CE. Other products have their own or another operating system
Phantom Dialing On a computer using a dial-up connection, phantom (meaning ghost) dialing is a term used to describe what occurs when a computer's auto-connect feature has been enabled and the computer attempts to dial out and establish an Internet connection on its own. In mobile wireless communication, phantom dialing is a term used to describe what occurs when a user unintentionally presses a pre- programmed auto-dial number on their cellular telephone keypad and unintentionally initiates a phone call. In the United States, phantom dialing is a problem for 911 emergency centers, especially since many cell phones are configured to dial 911 (the emergency center) automatically when either a "9" or a "1" is pressed. When emergency services receives a phone call, the operator must,
127 by law, remain on the phone long enough to determine whether or not the call is an emergency. If the operator listens and determines that the call is probably a result of phantom dialing, they may terminate the call, but must dial back the caller and verbally confirm that there is no emergency. Operators across the United States report thousands of such calls daily, and say that phantom dialers are almost always unaware they have made a call. Users report that they may have dropped the phone, sat on it, put it in their pocket, or otherwise jostled it, hitting auto-dial and inadvertently initiating the call. Phantom dialing can be prevented by using the cell phone's keyguard, a feature that locks the keypad, or by disabling the auto-dial feature.
Phase Lock Loop Pl. see PLL
PLL A phase-locked loop (PLL) is an electronic circuit with a voltage- or current-driven oscillator that is constantly adjusted to match in phase (and thus lock on) the frequency of an input signal. In addition to stabilizing a particular communications channel (keeping it set to a particular frequency), a PLL can be used to generate a signal, modulate or demodulate a signal, reconstitute a signal with less noise, or multiply or divide a frequency. PLLs are frequently used in wireless communication, particularly where signals are carried using frequency modulation (FM) or phase modulation (PM). PLLs can also be used in amplitude modulation (AM). PLLs are more commonly used for digital data transmission, but can also be designed for analog information. Phase-locked loop devices are more commonly manufactured as integrated circuits (ICs) although discrete circuits are used for microwave. A PLL consists of a voltage-controlled oscillator (VCO) that is tuned using a special semiconductor diode called a varactor. The VCO is initially tuned to a frequency close to the desired receiving or transmitting frequency. A circuit called a phase comparator causes the VCO to seek and lock onto the desired frequency, based on the output of a crystal-controlled reference oscillator. This works by means of a feedback scheme. If the VCO frequency departs from the selected crystal reference frequency, the phase comparator produces an error voltage that is applied to the varactor, bringing the VCO back to the reference frequency. The PLL, VCO, reference oscillator, and phase comparator together comprise a frequency synthesizer. Wireless equipment that uses this type of frequency control is said to be frequency-synthesized. Since a PLL requires a certain amount of time to lock on the frequency of an incoming signal, the intelligence on the signal (voice, video, or data) can be obtained directly from the waveform of the measured error voltage, which will reflect exactly the modulated information on the signal.
Portable Keyboard A portable keyboard (or handheld keyboard) is one that is designed to be used with wireless devices, such as personal digital assistants (PDAs) and smartphones. Earlier means of text entry for handheld devices (such as Palm's Graffiti) are usually thought inadequate for anything more than a few characters of text, precluding most e-mail, for example. The task of adapting a
128 keyboard for mobile use has received a great deal of attention. Because carrying a bulky keyboard would defeat the purpose of handheld devices, portable keyboard manufacturers have resorted to a number of different approaches to making them easier to carry around: Smaller keyboard: The simplest approach is to shrink the overall size of the keyboard while maintaining the traditional QWERTY layout. LandWare's GoType is an example of this type of keyboard. According to some reviews, the smaller keyboards take some time to adapt to. Car-friendly keyboard: One portable keyboard is specifically designed to be used while traveling by car (although the vendors do not advocate typing while actually driving): Revolve Design's RoadWriter (compatible with PalmIII, PalmVII, Symbol SPT1500, and TRG Pro handhelds), is said to be sturdy enough to be comfortably used in transit. Folding keyboard: Another approach is to make a full-size keyboard foldable. The Targus Stowaway (also called the Palm Portable) keyboard is a popular example of this type. According to some reviews, the Stowaway keyboard is similar to a laptop keyboard in use, although it must be used on a flat surface (instead of a lap). The keyboard folds to a size not much bigger than that of the mobile device. Rollable keyboard: Some companies, using new technologies such as flexible transistors and smart matter, are exploring potential future keyboards that will be made of plastic or fabric and can be rolled up or folded to fit in a pocket. An early version (that relies on more traditional technology) is the iNPACE Flexi-keyboard: a washable (waterproof) version. On a discussion at Slashdot, users with a propensity for spilling beer in their keyboards were, for the most part, happy with the Flexi-keyboard.
PPPoE PPPoE (Point-to-Point Protocol over Ethernet) is a specification for connecting multiple computer users on an Ethernet local area network to a remote site through common customer premises equipment, which is the telephone company's term for a modem and similar devices. PPPoE can be used to have an office or building-full of users share a common Digital Subscriber Line (DSL), cable modem, or wireless connection to the Internet. PPPoE combines the Point-to-Point Protocol (PPP), commonly used in dialup connections, with the Ethernet protocol, which supports multiple users in a local area network. The PPP protocol information is encapsulated within an Ethernet frame. PPPoE has the advantage that neither the telephone company nor the Internet service provider (ISP) needs to provide any special support. Unlike dialup connections, DSL and cable modem connections are "always on." Since a number of different users are sharing the same physical connection to the remote service provider, a way is needed to keep track of which user traffic should go to and which user should be billed. PPPoE provides for each user-remote site session to learn each other's network addresses (during an initial exchange called "discovery"). Once a session is established between an individual user and the remote site (for example, an Internet service provider), the session can be monitored for billing purposes. Many apartment houses, hotels, and
129 corporations are now providing shared Internet access over DSL lines using Ethernet and PPPoE.
Predictive Dialer A predictive dialer is a telephone control system that automatically calls a list of telephone numbers in sequence and screens out no-answers, busy signals, answering machines, and disconnected numbers while predicting at what point a human caller will be able to handle the next call. Predictive dialers are commonly used for telemarketing, surveys, appointment confirmation, payment collection, and service follow-ups. Sellers of predictive dialer systems claim that they greatly increase caller productivity. The phone calls you receive from "no one there" are often predictive dialer calls in which a manual caller isn't ready yet. Not to be confused with an automatic dialer, a predictive dialer is programmed to predict when a human caller is available to pick up a call, A somewhat related system is the lead generator, which dials a list of telephone numbers and, when a live voice answers, delivers a recorded message.
PRI In the Integrated Services Digital Network (ISDN), there are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B- channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a Basic Rate Interface user can have up to 128 Kbps service. The Primary Rate Interface consists of 23 B-channels and one 64 Kpbs D-channel using a T-1 line or 30 B- channels and 1 D-channel using an E1 line. Thus, a Primary Rate Interface user on a T-1 line can have up to 1.544 Mbps service or up to 2.048 Mbps service on an E1 line. PRI uses the Q.931 protocol over the D-channel.
The Primary Rate Interface channels are carried on a T-carrier system line (in the U.S., Canada, and Japan) or an E-carrier line (in other countries) and are typically used by medium to large enterprises. The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as videoconferences. The Primary Rate user is hooked up directly to the telephone company central office.
Primary Rate Interface Pl. see PRI
Promiscuous Mode In a network, promiscuous mode allows a network device to intercept and read each network packet that arrives in its entirety. This mode of operation is sometimes given to a network snoop server that captures and saves all packets for analysis (for example, for monitoring network usage). In an Ethernet local area network (LAN), promiscuous mode is a mode of operation in which every data packet transmitted can be received and read by a network adapter. Promiscuous mode must be supported by each network adapter as well as by the input/output
130 driver in the host operating system. Promiscuous mode is often used to monitor network activity. Promiscuous mode is the opposite of non-promiscuous mode. When a data packet is transmitted in non-promiscuous mode, all the LAN devices "listen to" the data to determine if the network address included in the data packet is theirs. If it isn't, the data packet is passed onto the next LAN device until the device with the correct network address is reached. That device then receives and reads the data.
Protocol In information technology, a protocol (pronounced PROH-tuh-cahl, from the Greek protocollon, which was a leaf of paper glued to a manuscript volume, describing its contents) is the special set of rules that end points in a telecommunication connection use when they communicate. Protocols exist at several levels in a telecommunication connection. There are hardware telephone protocols. There are protocols between each of several functional layers and the corresponding layers at the other end of a communication. Both end points must recognize and observe a protocol. Protocols are often described in an industry or international standard. On the Internet, there are the TCP/IP protocols, consisting of: Transmission Control Protocol (TCP), which uses a set of rules to exchange messages with other Internet points at the information packet level. Internet Protocol (IP), which uses a set of rules to send and receive messages at the Internet address level. Additional protocols that are usually packaged with a TCP/IP suite, including the Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP), each with defined sets of rules to use with corresponding programs elsewhere on the Internet.
Probe In telecommunications generally, a probe is an action taken or an object used for the purpose of learning something about the state of the network. For example, an empty message can be sent simply to see whether the destination actually exists. ping is a common utility for sending such a probe. A probe is a program or other device inserted at a key juncture in a network for the purpose of monitoring or collecting data about network activity. Relative to computer security in a network, a probe is an attempt to gain access to a computer and its files through a known or probable weak point in the computer system. In semiconductor testing, a probe card is a microchip placed in a circuit in order to test its signals.
Protocol In information technology, a protocol (pronounced PROH-tuh-cahl, from the Greek protocollon, which was a leaf of paper glued to a manuscript volume, describing its contents) is the special set of rules that end points in a telecommunication connection use when they communicate. Protocols exist at several levels in a telecommunication connection. There are hardware telephone protocols. There are protocols between each of several functional layers and the corresponding layers at
131 the other end of a communication. Both end points must recognize and observe a protocol. Protocols are often described in an industry or international standard. On the Internet, there are the TCP/IP protocols, consisting of: Transmission Control Protocol (TCP), which uses a set of rules to exchange messages with other Internet points at the information packet level. Internet Protocol (IP), which uses a set of rules to send and receive messages at the Internet address level. Additional protocols that are usually packaged with a TCP/IP suite, including the Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP), each with defined sets of rules to use with corresponding programs elsewhere on the Internet.
Provision In general, provisioning means "providing". In telecommunications terminology, provisioning means providing a product or service, such as wiring or bandwidth. The term has a number of varied meanings when used in telecommunications: Providing telecommunications service to a user, including everything necessary to set up the service, such as equipment, wiring, and transmission. Used as a synonym for configuring, as in "Telecommunications lines must be correctly provisioned to work with the customer's equipment and enabled for various options the customer has chosen." In a traditional telecommunications environment, there are three separate types of provisioning: circuit provisioning, service provisioning, and switch provisioning. In a wireless environment, provisioning refers to service activation and involves programming various network databases with the customer's information. In a slightly different sense, network provisioning systems are intermediary systems that are used to provide customer services, log transactions, carry out requests, and update files. Provisioning is the fourth step of the telecommunications sequence called OAM&P: Operations, Administration, Maintenance, and Provisioning.
PSTN PSTN (public switched telephone network) is the world's collection of interconnected voice-oriented public telephone networks, both commercial and government-owned. It's also referred to as the Plain Old Telephone Service (POTS). It's the aggregation of circuit-switching telephone networks that has evolved from the days of Alexander Graham Bell ("Doctor Watson, come here!"). Today, it is almost entirely digital in technology except for the final link from the central (local) telephone office to the user. In relation to the Internet, the PSTN actually furnishes much of the Internet's long-distance infrastructure. Because Internet service providers ISPs pay the long-distance providers for access to their infrastructure and share the circuits among many users through packet-switching, Internet users avoid having to pay usage tolls to anyone other than their ISPs
132 Q
Q QAM Quadrature Amplitude Modulation (QAM) - a method of combining two amplitude- modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. QAM is used with pulse amplitude modulation (PAM) in digital systems. In a QAM signal, there are two carriers, each having the same frequency but differing in phase by 90 degrees (one quarter of a cycle, from which the term quadrature arises). One signal is called the I signal, and the other is called the Q signal. Mathematically, one of the signals can be represented by a sine wave, and the other by a cosine wave. The two modulated carriers are combined at the source for transmission. At the destination, the carriers are separated, the data is extracted from each, and then the data is combined into the original modulating information. CAP is very similar to QAM.
Quadbit A quadbit, sometimes called a nibble, is one of 16 possible four-bit combinations used in some communication signals. A signal may be encoded in quadbit (nibble) units rather than one bit at a time. According to Harry Newton, nibble interleaving or multiplexing takes a quadbit or nibble from a lower-speed channel as input for a multiplexed signal on a higher-speed channel. In the IEEE 1284 Parallel Port Interface standard, data can be sent in nibbles (a sequence of two four-bit units) across the line.
Quadrature Amplitude Modulation Pl. see QAM
Quad FastEthernet Pl. see QFE
QFE Quad FastEthernet (QFE) is a network interface card (NIC) manufactured by Sun Microsystems that is designed to enhance the bandwidth of a Peripheral Component Interconnect (PCI)-based server using Sun Microsystem's Solaris 8 or later operating environment. Speeds of up to 100 megabits per second (Mbps) are provided by converting PCI data streams into Fast Ethernet traffic. QFE cards are hot-swappable, minimizing downtime, and comply with the IEEE 802/3U Ethernet standard. A single card can work with up to four network interfaces at a time and provide support for multihoming.
133 R R Radio Frequency Pl. see RF
Rain Fade Rain fade is an interruption of wireless communication signals as a result of rain or snow droplets whose separation approximates the signal wavelengths. The phenomenon can affect satellite Internet connections as well as satellite television and other systems. Most satellite communication takes place in the microwave portion of the electromagnetic radiation spectrum. Signals at these wavelengths, typically on the order of a few inches, are affected by heavy concentrations of water droplets or ice crystals in the atmosphere. When the mean distance between water droplets or crystals is comparable to the wavelength of the electromagnetic signals, severe attenuation can occur. The observed effect is a degradation or loss of communications during heavy downpours, snow squalls, and blizzards. Rain fade usually does not last long. Once a heavy shower or squall has passed, normal communications returns. However, during tropical storms or severe winter storms at northern latitudes, fadeouts can persist for hours at a time. The phenomenon occurs with all types of satellite systems, including geostationary (GEO), low-earth- orbit (LEO), and medium-earth-orbit (MEO). It can also affect the Global Positioning System (GPS).
RCA connector An RCA connector is a plug and a jack designed for use with coaxial cable for frequencies ranging from the very lowest up to several megahertz. An RCA connector is sometimes known as a phono plug and jack. The male RCA plug consists of a central pin measuring approximately two millimeters (mm) in diameter, and an outer shell whose inside diameter is approximately six mm. The plug shell is slotted rather than threaded, to facilitate quick insertion to, and removal from, the female jack or receptacle. Contact is maintained by physical pressure between the slotted shell of the plug and the smooth cylindrical barrel of the jack. The plug shell is connected to the outer conductor, or shield, of the coaxial cable, normally at electrical ground. The center pin of the plug is connected to the cable center conductor, which carries the signal. In the jack, the barrel is grounded and the center hole is plated inside to conduct the signal.
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RCA connectors are suitable for audio-frequency (AF) applications. They are also used in radio-frequency (RF) systems at low and medium frequencies, and at power levels up to approximately 100 watts. At higher frequencies or higher power levels, larger connectors are necessary. RCA connectors are designed for cables with small outside diameters (less than 6 or 7 mm) and are intended for indoor use only.
Reliability Reliability is an attribute of any computer-related component (software, or hardware, or a network, for example) that consistently performs according to its specifications. It has long been considered one of three related attributes that must be considered when making, buying, or using a computer product or component. Reliability, availability, and serviceability - RAS, for short - are considered to be important aspects to design into any system. In theory, a reliable product is totally free of technical errors; in practice, however, vendors frequently express a product's reliability quotient as a percentage. Evolutionary products (those that have evolved through numerous versions over a significant period of time) are usually considered to become increasingly reliable, since it is assumed that bugs have been eliminated in earlier releases. For example, IBM's z/OS (an operating system for their S/390 server series), has a reputation for reliability because it evolved from a long line of earlier MVS and OS/390 operating system versions. The Institute of Electrical and Electronics Engineers (IEEE) sponsors an organization devoted to reliability in engineering, the IEEE Reliability Society (IEEE RS). The Reliability Society promotes industry-wide acceptance of a systematic approach to design that will help to ensure reliable products. To that end, they promote reliability not just in engineering, but in maintenance and analysis as well. The Society encourages collaborative effort and information sharing among its membership, which encompasses organizations and individuals involved in all areas of
135 engineering, including aerospace, transportation systems, medical electronics, computers, and communications.
Remote Access Remote access is the ability to get access to a computer or a network from a remote distance. In corporations, people at branch offices, telecommuters, and people who are travelling may need access to the corporation's network. Home users get access to the Internet through remote access to an Internet service provider (ISP). Dial-up connection through desktop, notebook, or handheld computer modem over regular telephone lines is a common method of remote access. Remote access is also possible using a dedicated line between a computer or a remote local area network and the "central" or main corporate local area network. A dedicated line is more expensive and less flexible but offers faster data rates. Integrated Services Digital Network (ISDN) is a common method of remote access from branch offices since it combines dial-up with faster data rates. wireless, cable modem, and Digital Subscriber Line (DSL) technologies offer other possibilities for remote access. A remote access server is the computer and associated software that is set up to handle users seeking access to network remotely. Sometimes called a communication server, a remote access server usually includes or is associated with a firewall server to ensure security and a router that can forward the remote access request to another part of the corporate network. A remote access server may include or work with a modem pool manager so that a small group of modems can be shared among a large number of intermittently present remote access users. A remote access server may also be used as part of a virtual private network (VPN).
Remote Access Server Pl. see remote access
Repeater In telecommunication networks, a repeater is a device that receives a signal on an electromagnetic or optical transmission medium, amplifies the signal, and then retransmits it along the next leg of the medium. Repeaters overcome the attenuation caused by free-space electromagnetic-field divergence or cable loss. A series of repeaters make possible the extension of a signal over a distance. Repeaters are used to interconnect segments in a local area network (LAN). They're also used to amplify and extend wide area network transmission on wire and wireless media. In his Telecom Dictionary, Harry Newton points out that, in addition to strengthening the signal, repeaters also remove the "noise" or unwanted aspects of the signal. According to Newton, repeaters can do this with digital signals because, unlike analog signals, the original signal, even if weak or distorted, can be clearly perceived and restored. With analog transmission, signals are restrengthened with amplifiers which unfortunately also amplify noise as well as information. Because digital signals depend on the presence or absence of voltage, they tend to dissipate more quickly than analog signals and need more frequent repeating. Whereas analog signal amplifiers are spaced at 18,000 meter intervals, digital signal repeaters are typically placed at 2,000 to 6,000 meter intervals. In a
136 cable system, a repeater can be simple, consisting of an amplifier circuit and a couple of signal transformers. The impedance of the cable must be matched to the input and output of the amplifier to optimize the efficiency of the amplifier. Impedance matching also minimizes reflection of signals along the cable. Such reflection can produce undesirable echo effects. In a wireless communications system, a repeater consists of a radio receiver, an amplifier, a transmitter, an isolator, and two antennas. The transmitter produces a signal on a frequency that differs from the received signal. This so-called frequency offset is necessary to prevent the strong transmitted signal from disabling the receiver. The isolator provides additional protection in this respect. A repeater, when strategically located on top of a high building or a mountain, can greatly enhance the performance of a wireless network by allowing communications over distances much greater than would be possible without it. In a fiber optic network, a repeater consists of a photocell, an amplifier, and a light-emitting diode (LED) or infrared-emitting diode (IRED) for each light or IR signal that requires amplification. Fiber optic repeaters operate at power levels much lower than wireless repeaters, and are also much simpler and cheaper. However, their design requires careful attention to ensure that internal circuit noise is minimized. A bus repeater links one computer bus to a bus in another computer chassis, essentially chaining one computer to another. Repeaters are commonly used by amateur and commercial radio operators to extend signals in the radio frequency range from one receiver to another. These consist of drop repeaters, similar to the cells in cellular radio, and hub repeaters, which receive and retransmit signals from and to a number of directions.
RF Radio frequency (abbreviated RF, rf, or r.f.) is a term that refers to alternating current (AC) having characteristics such that, if the current is input to an antenna, an electromagnetic (EM) field is generated suitable for wireless broadcasting and/or communications. These frequencies cover a significant portion of the electromagnetic radiation spectrum, extending from nine kilohertz (9 kHz), the lowest allocated wireless communications frequency (it's within the range of human hearing), to thousands of gigahertz (GHz). When an RF current is supplied to an antenna, it gives rise to an electromagnetic field that propagates through space. This field is sometimes called an RF field; in less technical jargon it is a "radio wave." Any RF field has a wavelength that is inversely proportional to the frequency. In the atmosphere or in outer space, if f is the frequency in megahertz and s is the wavelength in meters, then s = 300/f The frequency of an RF signal is inversely proportional to the wavelength of the EM field to which it corresponds. At 9 kHz, the free-space wavelength is approximately 33 kilometers (km) or 21 miles (mi). At the highest radio frequencies, the EM wavelengths measure approximately one millimeter (1 mm). As the frequency is increased beyond that of the RF spectrum, EM energy takes the form of infrared (IR), visible, ultraviolet (UV), X rays, and gamma rays. Many types of wireless devices make use of RF fields. Cordless and cellular telephone, radio and television broadcast stations, satellite communications systems, and two-way radio services all operate in the RF spectrum. Some wireless devices operate at IR or
137 visible-light frequencies, whose electromagnetic wavelengths are shorter than those of RF fields. Examples include most television-set remote-control boxes, some cordless computer keyboards and mice, and a few wireless hi-fi stereo headsets. The RF spectrum is divided into several ranges, or bands. With the exception of the lowest-frequency segment, each band represents an increase of frequency corresponding to an order of magnitude (power of 10). The table depicts the eight bands in the RF spectrum, showing frequency and bandwidth ranges. The SHF and EHF bands are often referred to as the microwave spectrum
RFID RFID (radio frequency identification) is a technology that incorporates the use of electromagnetic or electrostatic coupling in the radio frequency (RF) portion of the electromagnetic spectrum to uniquely identify an object, animal, or person. RFID is coming into increasing use in industry as an alternative to the bar code. The advantage of RFID is that it does not require direct contact or line-of-sight scanning. An RFID system consists of three components: an antenna and transceiver (often combined into one reader) and a transponder (the tag). The antenna uses radio frequency waves to transmit a signal that activates the transponder. When activated, the tag transmits data back to the antenna. The data is used to notify a programmable logic controller that an action should occur. The action could be as simple as raising an access gate or as complicated as interfacing with a database to carry out a monetary transaction. Low-frequency RFID systems (30 KHz to 500 KHz) have short transmission ranges (generally less than six feet). High-frequency RFID systems (850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz) offer longer transmission ranges (more than 90 feet). In general, the higher the frequency, the more expensive the system. RFID is sometimes called dedicated short range communication (DSRC).
Ring Back Ringback is an intermittent audio tone that a caller in a telephone system hears after dialing a number, when the distant end of the circuit is receiving a ringing signal. It can be generated by the servicing switch of either the called party or the calling party. It is not generated by the called instrument. Ringback is also known as the audible, the audible ringing tone, the ringback tone, and the ringtone. The presence of ringback does not necessarily mean a distant device has been actuated. For example, if the circuit and the distant phone line are in proper working order but no phone set or other device is connected at the called-party end, the calling party will hear ringback anyway. The absence of ringback generally indicates that the distant end of the circuit has not been contacted. The term ringback is sometimes used in reference to a ringing signal that a caller receives when a delayed automatic-calling system places a call to a distant device.
Ring Tone In the original usage, the ringtone (or ring tone) is a tone returned by receiving equipment that tells a caller that the phone at that end is ringing. (Somewhat confusingly, this meaning is also called ringback.) The tone is sent back in between the ring sequence at the receiving end. The pulsing rate is one on, two off from a 3- phase generator with each call using a single phase. The called and calling phones
138 would not necessarily use the same phase, so if you wanted to ring someone's phone (for example, to wake them up), you would need to hear it ringing for a full cycle to make sure that the phone actually rang at the other end. Mobile phone users use the term to mean the ring that the caller hears. The proliferation of cellular telephones in recent years has given rise to a wide variety of ringtones. These do not necessarily follow the intermittent ringdown signal. A contemporary ringtone might consist of several bars of a familiar musical tune, played by an audio oscillator through a small speaker. Such ringtones are popular because, in a crowd of people with many cellular phone sets, they make it easy to tell whose phone is calling out for attention.
RJ In the U. S., telephone jacks are also known as registered jacks, sometimes described as RJ-XX, and are a series of telephone connection interfaces (receptacle and plug) that are registered with the U.S. Federal Communications Commission (FCC). They derive from interfaces that were part of AT&T's Universal Service Order Codes (USOC) and were adopted as part of FCC regulations (specifically Part 68, Subpart F. Section 68.502). The term jack sometimes means both receptacle and plug and sometimes just the receptacle. RJ-11 The most common telephone jack is the RJ-11 jack, which can have six conductors but usually is implemented with four. The RJ-11 jack is likely to be the jack that your household or office phones are plugged into from the ordinary "untwisted" wire (sometimes called "gray satin" or "flat wire") people are most familiar with. In turn, the jacks connect to the "outside" longer wires known as twisted pair that connect to the telephone company central office or to a private branch exchange (PBX). The four wires are usually characterized as a red and green pair and a black and white pair. The red and green pair typically carry voice or data. On an outside phone company connection, the black and white pair may be used for low-voltage signals such as phone lights. On a PBX system, they may be used for other kinds of signaling. Your computer modem is usually connected to an RJ-11 jack. RJ-14 The RJ-14 is similar to the RJ-11, but the four wires are used for two phone lines. Typically, one set of wires (for one line) contains a red wire and a green wire. The other set contains a yellow and black wire. Each set carries one analog "conversation" (voice or data). RJ-45 The RJ-45 is a single-line jack for digital transmission over ordinary phone wire, either untwisted or twisted. The interface has eight pins or positions. For connecting a modem, printer, or a data PBX at a data rate up to 19.2 Kbps, you can use untwisted wire. For faster transmissions in which you're connecting to an Ethernet 10BASET network, you need to use twisted pair wire. (Untwisted is usually a flat wire like common household phone extension wire. Twisted is often round.) There are two varieties of RJ-45: keyed and unkeyed. Keyed has a small bump on its end and the female complements it. Both jack and plug must match.
Diagram of RJ Connectors
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Roaming Roaming service is the ability to get access to the Internet when away from home at the price of a local call or at a charge considerably less than the regular long- distance charges. For example, if you normally get access to the Internet from an access provider in Brooklyn, New York and are traveling to Hong Kong, you can call a designated access provider in Hong Kong. Instead of paying long distance charges to your local provider in Brooklyn, you pay the local phone connection charge in Hong Kong and possibly a modest additional charge for the service. Roaming service is made possible through Internet service providers (ISPs) who have cooperative agreements to grant each others' customers local access to the Internet. Special software allows cooperating ISPs to keep track of and calculate prearranged payments for usage differences. Here's how it works for the user: The Internet user must already subscribe to an ISP that offers roaming service arrangements. Assuming the ISP does, the user can determine a cooperating ISP in a city to which the user is travelling. In the travel location, the user can call the local ISP's designated phone number through the computer modem, entering information during login that will identify the user's home ISP.
140 The "foreign" ISP will contact the ISP and determine that the user is a valid user. The "foreign" ISP will grant the user access to the Internet. The user will be able to access e-mail from the home mail server.
The user will be charged at local phone rates. In addition, depending on the particular service arrangement, the home ISP may levy an additional hourly usage charge of several dollars an hour or a monthly charge in case the service is used during that month. A similar roaming service is provided by some cooperating cellular telephone telephone or personal digital assistant (PDA) service providers. If you are travelling and simply need to be able to exchange e-mail, you can consider getting a freemail membership (usually free) from HotMail, Rocketmail, or other freemail providers. Hotmail also offers POP3 server accounts for access to up to four e-mail accounts you may already have, assuming you remember the POP3 server name and your user IDs and passwords. If you subscribe to a somewhat global service such as AT&T's WorldNet or the IBM Global Network, you may already be able to access your account in certain cities through your provider's local point-of-presence (POP) on the Internet without having to pay for a long- distance call.
Roaming Service Pl. see roaming
Runt In networks, a runt is a packet that is too small. For example, the Ethernet protocol requires that each packet be at least 64 bytes long. In Ethernet, which operates on the idea that two parties can attempt to get use of the line at the same time and sometimes do, runts are usually the fragments of packet collisions. Runts can also be the result of bad wiring or electrical interference. Runts are recorded by programs that use the Remote Network Monitoring (RNM) standard information base for network adminstration. RMON calls them "undersize packets".
141 S S S interface In Integrated Services Digital Network (ISDN) service, an S interface is the electrical interface between a network terminating unit 1 (NT1) and up to eight addressable devices such as a computer or a telephone. Like the T interface (which usually connects the signal from the NT1 to a PBX, the S interface has four wires, allowing up to eight devices to be addressed. In Basic Rate Interface ISDN, the bits that flow from the central office through the NT1 are divided into two 64 Kbps channels (known as B, or bearer, channels) and one 16 Kbps channel (the D, for data or delta, channel) for control signals. The control signals allow a specific device to be addressed. The wires to the devices from the NT-1 are sometimes referred to as the S-bus. The following table summarizes the various ISDN electrical interfaces at different demarcation points or places in the traffic flow: Electrical Between what two points interface U interface Central office and NT1 T interface NT1 and NT2 devices (such as a PBX) NT1 or NT2 and ISDN devices (such as a telephone or S interface terminal adapter) Terminal adapter and non-ISDN devices (such as a R interface computer) Within the ISDN node at the central office; separates line V interface termination equipment from exchange termination equipment
S/N In analog and digital communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise. The ratio is usually measured in decibels (dB). If the incoming signal strength in microvolts is Vs, and the noise level, also in microvolts, is Vn, then the signal-to-noise ratio, S/N, in decibels is given by the formula
S/N = 20 log10(Vs/Vn)
If Vs = Vn, then S/N = 0. In this situation, the signal borders on unreadable, because the noise level severely competes with it. In digital communications, this will probably cause a reduction in data speed because of frequent errors that require the source (transmitting) computer or terminal to resend some packets of data. Ideally, Vs is greater than Vn, so S/N is positive. As an example, suppose that Vs = 10.0 microvolts and Vn = 1.00 microvolt. Then
S/N = 20 log10(10.0) = 20.0 dB
142 which results in the signal being clearly readable. If the signal is much weaker but still above the noise -- say 1.30 microvolts – then
S/N = 20 log10(1.30) = 2.28 dB which is a marginal situation. There might be some reduction in data speed under these conditions. If Vs is less than Vn, then S/N is negative. In this type of situation, reliable communication is generally not possible unless steps are taken to increase the signal level and/or decrease the noise level at the destination (receiving) computer or terminal. Communications engineers always strive to maximize the S/N ratio. Traditionally, this has been done by using the narrowest possible receiving-system bandwidth consistent with the data speed desired. However, there are other methods. In some cases, spread spectrum techniques can improve system performance. The S/N ratio can be increased by providing the source with a higher level of signal output power if necessary. In some high-level systems such as radio telescopes, internal noise is minimized by lowering the temperature of the receiving circuitry to near absolute zero (-273 degrees Celsius or -459 degrees Fahrenheit). In wireless systems, it is always important to optimize the performance of the transmitting and receiving antennas.
Satellite A satellite is a specialized wireless receiver/transmitter that is launched by a rocket and placed in orbit around the earth. There are hundreds of satellites currently in operation. They are used for such diverse purposes as weather forecasting, television broadcast, amateur radio communications, Internet communications, and the Global Positioning System, (GPS). The first artificial satellite, launched by Russia (then known as the Soviet Union) in the late 1950s, was about the size of a basketball. It did nothing but transmit a simple Morse code signal over and over. In contrast, modern satellites can receive and re-transmit thousands of signals simultaneously, from simple digital data to the most complex television programming. There are three types of communications satellite systems. They are categorized according to the type of orbit they follow. A geostationary satellite orbits the earth directly over the equator, approximately 22,000 miles up. At this altitude, one complete trip around the earth (relative to the sun) takes 24 hours. Thus, the satellite remains over the same spot on the earth's surface at all times, and stays fixed in the sky from any point on the surface from which it can be "seen." So-called weather satellites are usually of this type. You can view images from some of these satellites on the Internet via the Purdue Weather Processor. A single geostationary satellite can "see" approximately 40 percent of the earth's surface. Three such satellites, spaced at equal intervals (120 angular degrees apart), can provide coverage of the entire civilized world. A geostationary satellite can be accessed using a dish antenna aimed at the spot in the sky where the satellite hovers. A low- earth-orbit (LEO) satellite system employs a large fleet of "birds," each in a circular orbit at a constant altitude of a few hundred miles. The orbits take the satellites over, or nearly over, the geographic poles. Each revolution takes approximately 90 minutes to a few hours. The fleet is arranged in such a way that, from any point on
143 the surface at any time, at least one satellite is on a line of sight. The entire system operates in a manner similar to the way a cellular telephone functions. The main difference is that the transponders, or wireless receiver/transmitters, are moving rather than fixed, and are in space rather than on the earth. A well-designed LEO system makes it possible for anyone to access the Internet via wireless from any point on the planet, using an antenna no more sophisticated than old-fashioned television "rabbit ears". Some satellites revolve around the earth in elliptical orbits. These satellites move rapidly when they are near perigee, or their lowest altitude; they move slowly when they are near apogee, or their highest altitude. Such "birds" are used by amateur radio operators, and by some commercial and government services. They require directional antennas whose orientation must be constantly adjusted to follow the satellite's path across the sky. Perigee When a satellite follows a non-circular orbit around the earth, the satellite's path is an ellipse with the center of the earth at one focus. Such a satellite has variable altitude and variable orbital speed. The point of lowest altitude is called perigee. The term also applies to the minimum distance in kilometers or miles between the satellite and the center of the earth. (Perigee can be measured between the satellite and the earth's surface, although this is a less precise specification because the earth is not a perfect sphere. The difference is approximately 4,000 miles or 6,400 kilometers.)
At perigee, a satellite travels faster than at any other point in its orbit. When viewed from the earth's surface, a satellite at or near perigee traverses the sky at a rapid pace. In communications, perigee is the least desirable time to access a satellite. Although its proximity means that the signal path is short, the fact that the satellite is rapidly moving means that it is accessible for only a brief time. In addition, if a directional antenna is used at a ground-based station, it is difficult to track the satellite because the position of the antenna (azimuth and elevation) must be constantly and rapidly adjusted. One of the principal advantages of a geostationary satellite is the fact that it follows a circular orbit, so the orbital speed is constant. In addition, the satellite's synchronization with the earth's orbit means that the
144 antenna of an earth-based station can be pointed at a fixed spot in the sky, and no further adjustment of antenna orientation is needed. Apogee When a satellite follows a non-circular orbit around the earth, the satellite's path is an ellipse with the center of the earth at one focus. Such a satellite has variable altitude and variable orbital speed. The point of highest altitude is called apogee. The term also applies to the maximum distance in kilometers or miles between the satellite and the center of the earth. (Apogee can be measured between the satellite and the earth's surface, although this is a less precise specification because the earth is not a perfect sphere. The difference is approximately 4,000 miles or 6,400 kilometers.)
At apogee, a satellite travels more slowly than at any other point in its orbit. When viewed from the earth's surface, a satellite at or near apogee takes a long time to traverse the sky. In communications, apogee is the best time to access a satellite. Although its distance means that the signal path is long, the fact that the satellite is slowly moving means that it is accessible for a comparatively long time. In addition, if a directional antenna is used at a ground-based station, it is relatively easy to track the satellite because the position of the antenna (azimuth and elevation) need not be adjusted very often or rapidly. One of the principal advantages of a geostationary satellite is the fact that it follows a circular orbit, so the orbital speed is constant. In addition, the satellite's synchronization with the earth's orbit means that the antenna of an earth-based station can be pointed at a fixed spot in the sky, and no further adjustment of antenna orientation is needed.
Satellite Internet Connection A satellite Internet connection is an arrangement in which the upstream (outgoing) and the downstream (incoming) data are sent from, and arrive at, a computer through a satellite. Each subscriber's hardware includes a satellite dish antenna and a transceiver (transmitter/receiver) that operates in the microwave portion of the radio spectrum. In a two-way satellite Internet connection, the upstream data is usually sent at a slower speed than the downstream data arrives. Thus, the connection is asymmetric. A dish antenna, measuring about two feet high by three
145 feet wide by three feet deep, transmits and receives signals. Uplink speeds are nominally 50 to 150 Kbps for a subscriber using a single computer. The downlink occurs at speeds ranging from about 150 Kbps to more than 1200 Kbps, depending on factors such as Internet traffic, the capacity of the server, and the sizes of downloaded files. Satellite Internet systems are an excellent, although rather pricey, option for people in rural areas where Digital Subscriber Line (DSL) and cable modem connections are not available. A satellite installation can be used even where the most basic utilities are lacking, if there is a generator or battery power supply that can produce enough electricity to run a desktop computer system. The two-way satellite Internet option offers an always-on connection that bypasses the dial-up process. In this respect, the satellite system resembles a cable modem Internet connection. But this asset can also be a liability, unless a firewall is used to protect the computer against hack attempts. The nature of the satellite connection is good for Web browsing and for downloading of files. Because of long latency compared with purely land-based systems, interactive applications such as online gaming are not compatible with satellite networks. In a two-way geostationary-satellite Internet connection, a transaction requires two round trips between the earth's surface and transponders orbiting 22,300 miles above the equator. This occurs in addition to land-based data transfer between the earthbound satellite system hub and the accessed Internet sites. The speed in such a connection is theoretically at least 0.48 second (the time it takes an electromagnetic signal to make two round trips at 186,000 miles per second to and from a geostationary satellite), and in practice is somewhat longer. Satellite systems are also prone to rain fade (degradation during heavy precipitation) and occasional brief periods of solar interference in mid-March and late September, when the sun lines up with the satellite for a few minutes each day. Rain fade and solar interference affect all satellite links from time to time, not just Internet systems. This author recently had StarBand, a two-way satellite Internet service, installed at his rural home office. Bandwidth tests were conducted with the new system compared with a conventional telephone modem. The telephone connection provided actual bandwidth ranging from 10 to 15 Kbps. StarBand worked at 200 to 1350 Kbps; throughput seemed to depend mainly on the download file size. The fastest speeds were obtained with files of 50 KB (kilobytes) or less, typical of images and text contained in Web sites. Surprisingly, fast downloads were obtained even during times of maximum Internet traffic.
SDH SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission on optical media. It is the international equivalent of Synchronous Optical Network. Both technologies provide faster and less expensive network interconnection than traditional PDH (Plesiochronous Digital Hierarchy) equipment. In digital telephone transmission, "synchronous" means the bits from one call are carried within one transmission frame. "Plesiochronous" means "almost (but not) synchronous," or a call that must be extracted from more than one transmission frame. SDH uses the following Synchronous Transport Modules (STM) and rates: STM-1 (155 megabits per second), STM-4 (622 Mbps), STM-16 (2.5 gigabits per second), and STM-64 (10 Gbps).
146 SDMA Spatial division multiple access (SDMA) is a satellite communications mode that optimizes the use of radio spectrum and minimizes system cost by taking advantage of the directional properties of dish antennas. In SDMA, also known as SDM (spatial-division multiplex), satellite dish antennas transmit signals to numerous zones on the earth's surface. The antennas are highly directional, allowing duplicate frequencies to be used for multiple surface zones. Consider a scenario in which signals must be transmitted simultaneously by one satellite to mobile or portable wireless receivers in 20 different surface zones. In a conventional system, 20 channels and 20 antennas would be necessary to maintain channel separation. In SDMA, there can be far fewer channels than zones. If duplicate-channel zones are sufficiently separated, the 20 signals can be transmitted to earth using four or five channels. The narrow signal beams from the satellite antennas ensure that interference will not occur between zones using the same frequency. SDMA requires careful choice of zones for each transmitter, and also requires precise antenna alignment. A small error can result in failure of one or more channels, interference among channels, and/or confusion between surface coverage zones.
SDR Software-defined radio (SDR), sometimes shortened to software radio (SR), refers to wireless communication in which the transmitter modulation is generated or defined by a computer, and the receiver uses a computer to recover the signal intelligence. To select the desired modulation type, the proper programs must be run by microcomputers that control the transmitter and receiver. A typical voice SDR transmitter, might be used in mobile two-way radio or cellular telephone communication, consists of the following stages. Items with asterisks represent computer-controlled circuits whose parameters are determined by the programming (software). Microphone Audio amplifier Analog-to-digital converter (ADC) that converts the voice audio to ASCII data * Modulator that impresses the ASCII intelligence onto a radio-frequency (RF) carrier * Series of amplifiers that boosts the RF carrier to the power level necessary for transmission Transmitting antenna
A typical receiver designed to intercept the above-described voice SDR signal would employ the following stages, essentially reversing the transmitter's action. Again, items followed by asterisks represent programmable circuits. Receiving antenna superheterodyne system that boosts incoming RF signal strength and converts it to a constant frequency Demodulator that separates the ASCII intelligence from the RF carrier * Digital-to-analog converter (DAC) that generates a voice waveform from the ASCII data *
147 Audio amplifier Speaker, earphone, or headset
The most significant asset of SDR is versatility. Wireless systems employ protocols that vary from one service to another. Even in the same type of service, for example wireless fax, the protocol often differs from country to country. A single SDR set with an all-inclusive software repertoire can be used in any mode, anywhere in the world. Changing the service type, the mode, and/or the modulation protocol involves simply selecting and launching the requisite computer program, and making sure the batteries are adequately charged if portable operation is contemplated. The ultimate goal of SDR engineers is to provide a single radio transceiver capable of playing the roles of cordless telephone, cell phone, wireless fax, wireless e-mail system, pager, wireless videoconferencing unit, wireless Web browser, Global Positioning System (GPS) unit, and other functions still in the realm of science fiction, operable from any location on the surface of the earth, and perhaps in space as well.
Sequential Couleur avec Memoire SECAM (Sequential Couleur avec Memoire) is the television display technology that is standard in France, the countries of the former Soviet Union, and certain other countries. It is one of the three world TV standards together with National Television Standards Committee (United States) and Phase Alternation Line (Europe other than France). Like PAL, SECAM scans the cathode ray tube (CRT) horizontally 625 times to form the video image
Service Level Agreement Pl. see SLA
SFF Small form factor (SFF) refers to any of several physically compact connector designs that have been developed for use in fiber optic systems. They are about half the size of conventional connectors. Currently there are at least three designs: the LC by Lucent, the VF-45 by 3M, and the MT-RJ by Tyco. The main motivator for the development of SFF connectors is an ongoing demand for smaller components in network systems. Using SFF connectors, it is possible to get many more interfaces on a single card. In addition, the use of connectors, rather than direct soldering, increases the flexibility and versatility of network systems, and makes it easier and less expensive to maintain them. The fact that there are several SFF connector configurations allows the use of products from multiple sources. An argument has been made for standardization, but individual connector makers have so far resisted this idea, preferring instead to develop and promote their own connector designs. SFF connectors are used with transceivers called small-form-factor pluggable (SFP) modules. This design has been called a miniature gigabit interface converter (GBIC). Also see small-form-factor pluggable (SFP) and gigabit interface converter (GBIC).
SFP
148 Small form-factor pluggable (SFP) is a specification for a new generation of optical modular transceivers. The devices are designed for use with small form factor (SFF) connectors, and offer high speed and physical compactness. They are hot- swappable. SFP transceivers are expected to perform at data speeds of up to five gigabits per second (5 Gbps), and possibly higher. Because SFP modules can be easily interchanged, electro-optical or fiber optic networks can be upgraded and maintained more conveniently than has been the case with traditional soldered-in modules. Rather than replacing an entire circuit board containing several soldered- in modules, a single module can be removed and replaced for repair or upgrading. This can result in a substantial cost savings, both in maintenance and in upgrading efforts. Several companies have formed a consortium supporting the use of SFP transceivers to meet their common objectives of broad bandwidth, small physical size and mass, and ease of removal and replacement. Also see small form factor (SFF) and gigabit interface converter (GBIC).
Shielded Twisted Pair Pl. see STP
Short Message A short message is a brief text message sent to or from a mobile phone subscriber through the Short Message Service (SMS). The standard short message consists of up to 160 alphanumeric characters, although messages at least 50% longer can be sent using data compression. Developed as part of the Global System for Mobile communications (GSM) Phase 1 standard, a short message is exchanged between two mobile devices or between a nonmobile device and a mobile device (for example, a short message can be sent from a PC attached to the Internet to a mobile subscriber). Short messages are stored in and forwarded from a Short Message Service Center (SMSC) so that - unlike the user of a pager - the recipient can get messages that arrive when their mobile device is not turned on. SMS compression increases the amount of text that can be sent, and SMS concatenation enables short messages to be strung together into a longer one. Users of devices that are not SMS-enabled can send short messages using an alternate version known as Internet SMS. Instant messaging (IM) messages are also sometimes referred to as short messages. To make the most of a short message, people frequently use a shorthand typing mixture of letters and numerals known as Alphanumerish.
Short Message Service Pl. see SMS
Signal In electronics, a signal is an electric current or electromagnetic field used to convey data from one place to another. The simplest form of signal is a direct current (DC) that is switched on and off; this is the principle by which the early telegraph worked. More complex signals consist of an alternating-current (AC) or electromagnetic carrier that contains one or more data streams. Data is superimposed on a carrier current or wave by means of a process called modulation. Signal modulation can be done in either of two main ways: analog and
149 digital. In recent years, digital modulation has been getting more common, while analog modulation methods have been used less and less. There are still plenty of analog signals around, however, and they will probably never become totally extinct. Except for DC signals such as telegraph and baseband, all signal carriers have a definable frequency or frequencies. Signals also have a property called wavelength, which is inversely proportional to the frequency. In some information technology contexts, a signal is simply "that which is sent or received," thus including both the carrier and the data together. In telephony, a signal is special data that is used to set up or control communication. See signaling.
Signal Computing System Architecture Signal Computing System Architecture (SCSA) is an industry standard architectural framework for the hardware and software components in a computer-telephony integration system. The framework has three layers: Applications, which control the media portion of a call (play, record, voice recognition, and other applications) and the call flow. The Software Model, which provides programming interfaces for manipulating the media portion of a call. This portion of the framework includes the SCSA Telephony Applications Objects (TAO) Framework. It also includes the call control framework and possible interfaces such as Telephony Application Program Interface, TSAPI, and JTAPI. The Hardware Model, consisting of one of two bus protocols, ANSI/VITA SCBus or ECTF H.100 CT Bus. Both the Hardware Model and the Software Model layers can interface with intelligent switch, such as those used in the Advanced Intelligent Network (AIN), and with the Public Switched Telephone Network (PSTN) or the Internet.
Signal-to-noise ratio Pl see S/N
Signaling In telephony, signaling is the exchange of information between involved points in the network that sets up, controls, and terminates each telephone call. In in- band signaling, the signaling is on the same channel as the telephone call. In out- of-band signaling, signaling is on separate channels dedicated for the purpose.
Signaling System 7 Pl. see SS7
Silence Suppression In Voice over IP (VoIP), voice activation detection (VAD) is a software application that allows a data network carrying voice traffic over the Internet to detect the absence of audio and conserve bandwidth by preventing the transmission of "silent packets" over the network. Most conversations include about 50% silence; VAD (also called "silence suppression") can be enabled to monitor signals for voice activity so that when silence is detected for a specified amount of time, the application informs the Packet Voice Protocol and prevents the encoder output
150 from being transported across the network. Voice activation detection can also be used to forward idle noise characteristics (sometimes called ambient or comfort noise) to a remote IP telephone or gateway. The universal standard for digitized voice, 64 Kbps, is a constant bit rate whether the speaker is actively speaking, is pausing between thoughts, or is totally silent. Without idle noise giving the illusion of a constant transmission stream during silence suppression, the listener would be likely to think the line had gone dead.
Silicon Cockroach Silicon cockroach is a term invented by networking expert John Sidgmore to describe the tiny portable electronic devices that are expected to become popular in the next few years, creating new behavior patterns while putting new demands on network bandwidth capacity. Sidgmore's pervasive cockroaches are expected to multiply and become a significant driver of Internet growth with the average person carrying as many as five separate devices at a time within a few years. Examples include not only smart phones, personal digital assistants (PDAs), and other handheld devices but also "smart" home appliances, computerized clothing, and other less visible sensors. Such devices may communicate locally with Bluetooth RF or with infrared wireless, or at some point be plugged into longer-range wireless or wired networks.
Simplex In telecommunication, duplex communication means that both ends of the communication can send and receive signals at the same time. full-duplex communication is the same thing. half-duplex is also bidirectional communication but signals can only flow in one direction at a time. Simplex communication means that communication can only flow in one direction and never flow back the other way. An ordinary telephone conversation is a duplex communication. Most inexpensive speakerphones in conference rooms are half-duplex communication. (If you're speaking, you can't hear anyone else interrupt. You have to pause to let others speak.)
SLA A Service Level Agreement (SLA) is a contract between a network service provider and a customer that specifies, usually in measurable terms, what services the network service provider will furnish. Many Internet service providers (ISP)s provide their customers with an SLA. More recently, IS departments in major enterprises have adopted the idea of writing a Service Level Agreement so that services for their customers (users in other departments within the enterprise) can be measured, justified, and perhaps compared with those of outsourcing network providers. Some metrics that SLAs may specify include: What percentage of the time services will be available The number of users that can be served simultaneously Specific performance benchmarks to which actual performance will be periodically compared
151 The schedule for notification in advance of network changes that may affect users Help desk response time for various classes of problems Dial-in access availability Usage statistics that will be provided
Slamming Slamming is the practice by some U.S. long-distance phone carriers of switching users to their service without the user's knowledge or authorization. Slamming methods include "free trials" and offers for credit cards that offer prize or give-away points for each dollar of charges from a given carrier. Phone solicitors often get "approval" from children, baby-sitters, and domestic employees. Occasionally, your carrier may be switched if your current carrier has been sold to another company.
Slot Time In Ethernet and its Carrier Sense Multiple Access/Collision Detect (CSMA/CD) approach to managing which device can use the communication link next, slot time is the amount of time a device waits after a collision before retransmitting. The transmitting device determines the appropriate amount of slot time by adding the amount of time it took for another device to detect a collision, the amount of time it took for the device to notify the original transmitting device of the collision, and the amount of time it took to transmit a jam sequence. Should a second collision occur, the device doubles the slot time in an effort to avoid future collisions; this process is called "backoff". In half-duplex Ethernet, where data can only travel in one direction at once, slot time becomes an important parameter in determining the how many devices can share a network.
SNR Pl. see S/N
Small Form Factor Pl. see SFF
Small Form-Factor Pluggable Pl. see SFF
Smart Building A smart home or building is a home or building, usually a new one, that is equipped with special structured wiring to enable occupants to remotely control or program an array of automated home electronic devices by entering a single command. For example, a homeowner on vacation can use a Touchtone phone to arm a home security system, control temperature gauges, switch appliances on or off, control lighting, program a home theater or entertainment system, and perform many other tasks. The field of home automation is expanding rapidly as electronic technologies converge. The home network encompasses communications, entertainment, security, convenience, and information systems. A technology known as Powerline Carrier Systems (PCS) is used to send coded signals along a home's existing electric wiring to programmable switches, or outlets. These signals convey
152 commands that correspond to "addresses" or locations of specific devices, and that control how and when those devices operate. A PCS transmitter, for instance, can send a signal along a home's wiring, and a receiver plugged into any electric outlet in the home could receive that signal and operate the appliance to which it is attached. One common protocol for PCS is known as X10, a signaling technique for remotely controlling any device plugged into an electrical power line. X10 signals, which involve short radio frequency (RF) bursts that represent digital information, enable communication between transmitters and receivers. In Europe, technology to equip homes with smart devices centers on development of the European Installation Bus, or Instabus. This embedded control protocol for digital communication between smart devices consists of a two-wire bus line that is installed along with normal electrical wiring. The Instabus line links all appliances to a decentralized communication system and functions like a telephone line over which appliances can be controlled. The European Installation Bus Association is part of Konnex, an association that aims to standardize home and building networks in Europe. Echelon Corp., the creator of the LonWorks system, is helping drive adoption of an open interoperability standard among vendors in the control networks industry. LonWorks is an open standard for network automation and control for the building, transportation, industrial and home markets. The American National Standards Institute (ANSI) has adopted the protocol underlying LonWorks control networks as an industry standard. The LonMark Interoperability Association is made up of more than 200 controls companies mission working on standard to integrate multi-vendor systems based on LonWorks networks.
Smartphone The term smartphone is sometimes used to characterize a wireless telephone set with special computer-enabled features not previously associated with telephones. In addition to functioning as an ordinary telephone, a smartphone's features may include: Wireless e-mail, Internet, Web browsing, and fax Intercom function Personal information management Online banking LAN connectivity Graffiti style data entry Local data transfer between phone set and computers Remote data transfer between phone set and computers Remote control of computers Remote control of home or business electronic systems Interactivity with unified messaging
Smart Home Pl. hee smart building
SMR Specialized Mobile Radio (SMR) is any two-way radio system in which two or more mobile/portable wireless transceivers are linked by a single repeater. The repeater
153 is elevated above average terrain; this maximizes the area of coverage. Operating frequencies are in the VHF (very-high-frequency) or UHF (ultra-high-frequency) range, that is, between approximately 30 MHz and 3 GHz. In some ways, an SMR system is like a cellular telephone network. But there are important differences. An SMR system is simpler than a cellular telephone network. There is only one repeater in a SMR system, and it links only the mobile/portable units for that system, not to other repeaters. In SMR, the range of each individual mobile/portable transceiver is greater than the range of a cell phone set. But total system coverage is usually far more limited than that of a cellular network, because there is no linking among repeaters. SMR systems use channel pairs. Each transceiver has a transmit frequency and a receive frequency. These frequencies differ by a fixed amount, called the offset. The transmit and receive frequencies are in the same band, that is, relatively close to each other in the radio spectrum. The transmit and receive frequencies of each mobile or portable transceiver in a system are all identical. An SMR system uses half-duplex communication and a PTT (push-to-talk) mode. Neither party can hear the other while transmitting. An example of half-duplex operation is a radio conversation between two people using simple walkie-talkies. SMR is used by taxi dispatchers, parcel delivery companies, fire departments, paramedic squads, police departments, and amateur radio operators.
SMS SMS (Short Message Service) is a service for sending messages of up to 160 characters (224 characters if using a 5-bit mode) to mobile phones that use Global System for Mobile (GSM) communication. GSM and SMS service is primarily available in Europe. SMS is similar to paging. However, SMS messages do not require the mobile phone to be active and within range and will be held for a number of days until the phone is active and within range. SMS messages are transmitted within the same cell or to anyone with roaming service capability. They can also be sent to digital phones from a Web site equipped with PC Link or from one digital phone to another. Typical uses of SMS include: Notifying a mobile phone owner of a voicemail message Notifying a salesperson of an inquiry and contact to call Notifying a doctor of a patient with an emergency problem Notifying a service person of the time and place of their next call Notifying a driver of the address of the next pickup
An SMS gateway is a Web site that lets you enter an SMS message to someone within the cell served by that gateway or that acts as an international gateway for users with roaming capability.
Soft Handoff In cellular telephone communication, soft handoff refers to the overlapping of repeater coverage zones, so that every cell phone set is always well within range of at least one repeater (also called a base station). In some cases, mobile sets transmit signals to, and receive signals from, more than one repeater at a time. Soft handoff technology is used by code-division multiple access (CDMA) systems. Older
154 networks use frequency division multiplex (FDM) or time division multiplex (TDM). In CDMA, all repeaters use the same frequency channel for each mobile phone set, no matter where the set is located. Each set has an identity based on a code, rather than on a frequency (as in FDM) or sequence of time slots (as in TDM). Because no change in frequency or timing occurs as a mobile set passes from one base station to another, there are practically no dead zones. As a result, connections are almost never interrupted or dropped.
Software-Defined Radio Pl. see SDR
Solar Fade Solar fade, also called sun interference, is a phenomenon that occurs in satellite communications on certain occasions when the downlink signal is aligned with the sun's position and it is overcome by signal noise from the sun. The term is used mainly in reference to geostationary (GEO) satellite systems. The sun is a powerful emitter of electromagnetic energy at all wavelengths, including those in the microwave portion of the radio-frequency (RF) spectrum, where most satellite communication is carried out. Normally, the sun does not affect the reception of microwave signals, because microwave-receiving antennas are rarely pointed right at the sun. But once in awhile, a signal source and the sun line up, and then they compete. At the equinoxes, around March 21 and September 21 of every year, the sun is directly over the earth's equator. GEO satellites orbit over the equator. Thus, for about a week before and after the equinoxes, the sun lines up almost exactly with any given GEO satellite once a day for users living at the equator. For subscribers in the northern hemisphere, the same thing happens for a couple of weeks before March 21 and after September 21. In the southern hemisphere, the effect is observed just after March 21 and before September 21. Unless the satellite downlink satellite signal is exceptionally strong, RF noise from the sun overpowers it, and reception is degraded or interrupted. After a few minutes, the sun's course across the sky takes it past the satellite, and normal reception resumes. Solar fade never occurs more than once a day for any GEO satellite, and presents a problem for only a few days out of the year. Nevertheless, it can be frustrating to satellite system users. It is important to realize that solar fade is not caused by a malfunction in system hardware or programming. Compare with rain fade.
SONET SONET is the American National Standards Institute standard for synchronous data transmission on optical media. The international equivalent of SONET is synchronous digital hierarchy (SDH). Together, they ensure standards so that digital networks can interconnect internationally and that existing conventional transmission systems can take advantage of optical media through tributary attachments. SONET provides standards for a number of line rates up to the maximum line rate of 9.953 gigabits per second (Gbps). Actual line rates approaching 20 gigabits per second are possible. SONET is considered to be the foundation for the physical layer of the broadband ISDN (BISDN). asynchronous transfer mode runs as a layer on top of SONET as well as on top of other
155 technologies. SONET defines a base rate of 51.84 Mbps and a set of multiples of the base rate known as "Optical Carrier levels (OCx)."
Spatial Division Multiple Access Pl. see SDMA
Specialized Mobile Radio Pl. see SMR
Splitter In telephony, a splitter, sometimes called a "plain old telephone service splitter," is a device that divides a telephone signal into two or more signals, each carrying a selected frequency range, and can also reassemble signals from multiple signal sources into a single signal. Users getting connected to the Internet with Asymmetric Digital Subscriber Line (ADSL) service may, in some cases, have a splitter installed at their home or business. Users elsewhere may be able to get splitterless service (which means that a splitter doesn't need to be installed). For ADSL, the splitter divides the incoming signal into low frequencies to send to voice devices and high frequencies for data to the computer. The telephone company's central office also uses a POTS splitter to send low-frequency voice signals on to the voice telephone network and to send high-frequency data to a Digital Subscriber Line Access Multiplexor (DSLAM) for transmission to the Internet.
Splitterless Splitterless refers to a type of Digital Subscriber Line (DSL) telephone service that does not require the installation of a plain old telephone service splitter at the customer location. Splitterless installation means that installation does not require "a truck roll" (that is, the cost of a visit from an installer ).
SS7 On the public switched telephone network (PSTN), Signaling System 7 (SS7) is a system that puts the information required to set up and manage telephone calls in a separate network rather than within the same network that the telephone call is made on. Signaling information is in the form of digital packets. SS7 uses what is called out-of-band signaling, meaning that signaling (control) information travels on a separate, dedicated 56 or 64 Kbps channel rather than within the same channel as the telephone call. Historically, the signaling for a telephone call has used the same voice circuit that the telephone call traveled on (this is known as in-band signaling). Using SS7, telephone calls can be set up more efficiently and with greater security. Special services such as call forwarding and wireless roaming service are easier to add and manage. SS7 is now an international telecommunications standard. SS7 is used for these and other services: Setting up and managing the connection for a call Tearing down the connection when the call is complete Billing Managing call forwarding, calling party name and number display, three-way calling, and other Intelligent Network (IN) services
156 Toll-free (800 and 888) and toll (900) calls Wireless as well as wireline call service including mobile telephone subscriber authentication, personal communication service (PCS), and roaming
SS7 messages contain such information as: How should I route a call to 914 331-4985? The route to network point 587 is crowded. Use this route only for calls of priority 2 or higher. Subscriber so-and-so is a valid wireless subscriber. Continue with setting up the call.
Because control signals travel in a separate network from the call itself, it is more difficult for anyone to violate the security of the system. (See 2600 and phreak for cracking techniques that are defeated by SS7.) The Integrated Services Digital Network (ISDN) also uses out-of-band signaling, extending it all the way to the end user on the ISDN D-channel while voice and data flow on B channels. How It Works? SS7 consists of a set of reserved or dedicated channel known as signaling links and the network points that they interconnect. There are three kinds of network points (which are called signaling points): Service Switching Points (SSPs), Signal Transfer Points (STPs), and Service Control Points (SCPs). SSPs originate or terminate a call and communicate on the SS7 network with SCPs to determine how to route a call or set up and manage some special feature. Traffic on the SS7 network is routed by packet switches called STPs. SCPs and STPs are usually mated so that service can continue if one network point fails.
StarBand StarBand is a broadband Internet service provider (ISP) that uses geostationary satellites to provide always-on connection independent of other media. Established in late 2000, StarBand was the first widely-available service for the general public that made use of satellite links for both upstream and downstream data. StarBand connection requires a Universal Serial Bus (USB) port or an Ethernet card, a special modem, a dish antenna measuring approximately 2 feet high by 3 feet wide by 3 feet deep, and two coaxial cables that run between the dish and the modem. Professional installation is recommended. upstream data speeds, at the time of writing, were reported by users as 30 to 100 kilobits per second (Kbps), with 50 Kbps being typical. downstream data speeds depend on the Web site visited, the complexity of downloaded pages, and the time of day. Normal downloads range from 150 Kbps to 500 Kbps. Because StarBand uses a satellite for both the upstream and the downstream links, the latency is considerable. A geostationary satellite orbits 22,300 miles above the Earth's equator. For a user at a temperate latitude, signals must make four trips through space, each of approximately 23,000 miles, between a mouse click and the appearance of Web site data on the display. This introduces a delay of 0.5 second, because the speed of electromagnetic (EM) radiation is finite. But that is the mathematical minimum. In reality, the latency is usually longer. The upstream data must travel from the user's dish to the satellite, and then back down to the StarBand hub, where it is transmitted over land-based
157 media. The downstream data must travel from the Web site under observation to the StarBand hub over land-based media, then up to the satellite, and back down to the user. The result is latency that sometimes exceeds 1 second. This is not a system defect, but technical reality. StarBand is not recommended for highly interactive applications such as gaming. But for people in remote regions with no other option, StarBand can provide good broadband Internet service. The author of this definition has had the service since March of 2001, and it has proven invaluable. The only alternative is dial-up through antiquated telephone lines, resulting in real-world browsing speeds averaging 10 Kbps or somewhat higher. StarBand cannot compete with a well-installed DSL or cable modem connection, but it has provided the author with a browsing speed increase of over 1,000 percent, even taking latency into account.
STP Shielded twisted pair is a special kind of copper telephone wiring used in some business installations. An outer covering or shield is added to the ordinary twisted pair telephone wires; the shield functions as a ground. Twisted pair is the ordinary copper wire that connects home and many business computers to the telephone company. To reduce crosstalk or electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Each signal on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. Shielded twisted pair is often used in business installations. The more common kind of wire that is installed to your home is unshielded twisted pair. Twisted pair is now frequently installed with two pairs to the home, with the extra pair making it possible for you to add another line (perhaps for modem use) when you need it. Twisted pair comes with each pair uniquely color-coded when it is packaged in multiple pairs. Different uses such as analog, digital, and Ethernet require different pair multiples. Although twisted pair is often associated with home use, a higher grade of twisted pair is often used for horizontal wiring in LAN installations because it is less expensive than coaxial cable. The wire you buy at a local hardware store for extensions from your phone or computer modem to a wall jack is not twisted pair. It is a side-by-side wire known as silver satin. The wall jack can have as many five kinds of hole arrangements or pinouts, depending on the kinds of wire the installation expects will be plugged in (for example, digital, analog, or LAN) . (That's why you may sometimes find when you carry your notebook computer to another location that the wall jack connections won't match your plug.) (This definition closely duplicates the definition for twisted pair.)
Storage Tunneling Pl. see FCIP
Subcarrier A subcarrier is one telecommunication signal carrier that is carried on top of another carrier so that effectively two signals are carried at the same time. At the receiving end, the main carrier and subcarrier signals are demodulated separately. A subcarrier can be used for some purpose entirely different from the purpose of the main carrier. For example, broadcast audio signals in the AM and FM ranges
158 can carry a subcarrier along with the main audio signal. According to the U.S. Federal Communications Commission, these subcarrier channels can be used for many different purposes, "including (but not limited to) paging, inventory distribution, bus dispatching, stock market reports, traffic control signal switching,...and muzak."
Sun Interference Pl. see solar fade
Superhet The term superheterodyne refers to a method of designing and building wireless communications or broadcast equipment, particularly radio receivers. Sometimes a receiver employing this technology is called a "superheterodyne" or "superhet." In many wireless applications, the equipment must function over a range, or band, of frequencies. But it is easier to process a modulated signal at a single frequency than over a band, especially if the highest frequency in the band is much different than the lowest frequency. To overcome this inherent difficulty when engineering variable-frequency wireless equipment, the desired data-carrying signal can be combined with the output of an unmodulated, variable-frequency oscillator (VFO) in a circuit called a mixer. When this is done, output is produced at a fixed frequency representing the difference between the input frequencies. When the correct range of frequencies is chosen for the VFO, a receiver can be designed that will intercept incoming signals over a specific band. For example, if the desired input frequency range is 20 MHz to 25 MHz, a VFO can be built that generates an unmodulated carrier at 29 to 34 MHz. When the incoming signal is mixed with the VFO output, the resultant has a constant frequency of 9 MHz, representing the difference between the frequencies of the inputs. The 9-MHz output retains the modulation characteristics of the incoming signal. In this case, the receiver is said to have an intermediate frequency (IF) of 9 MHz. This IF signal can be amplified and filtered more easily than signals having frequencies that vary from 20 to 25 MHz. The theory and design of superheterodyne equipment is a rather sophisticated business. Numerous books are devoted to this topic; some engineers make their entire living designing oscillators, mixers, and amplifiers that use this technology. For more information about superheterodyne engineering practice, a professional-level textbook or a formal course is recommended.
Superheterodyne Pl. see superhet
SVC In a network, a switched virtual circuit (SVC) is a temporary virtual circuit that is established and maintained only for the duration of a data transfer session. A permanent virtual circuit (PVC) is a continuously dedicated virtual circuit. A virtual circuit is one that appears to be a discrete, physical circuit available only to the user but that is actually a shared pool of circuit resources used to support multiple users as they require the connections. Switched virtual circuits are part of an X.25 network. Conceptually, they can also be implemented as part of a frame relay network.
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Switch In telecommunications, a switch is a network device that selects a path or circuit for sending a unit of data to its next destination. A switch may also include the function of the router, a device or program that can determine the route and specifically what adjacent network point the data should be sent to. In general, a switch is a simpler and faster mechanism than a router, which requires knowledge about the network and how to determine the route. Relative to the layered Open Systems Interconnection (OSI) communication model, a switch is usually associated with layer 2, the Data-Link layer. However, some newer switches also perform the routing functions of layer 3, the Network layer. Layer 3 switches are also sometimes called IP switches. On larger networks, the trip from one switch point to another in the network is called a hop. The time a switch takes to figure out where to forward a data unit is called its latency. The price paid for having the flexibility that switches provide in a network is this latency. Switches are found at the backbone and gateway levels of a network where one network connects with another and at the subnetwork level where data is being forwarded close to its destination or origin. The former are often known as core switches and the latter as desktop switches. In the simplest networks, a switch is not required for messages that are sent and received within the network. For example, a local area network may be organized in a token ring or bus arrangement in which each possible destination inspects each message and reads any message with its address. Circuit-Switching verses Packet-Switching A network's paths can be used exclusively for a certain duration by two or more parties and then switched for use to another set of parties. This type of "switching" is known as circuit-switching and is really a dedicated and continuously connected path for its duration. Today, an ordinary voice phone call generally uses circuit- switching. Most data today is sent, using digital signals, over networks that use packet-switching. Using packet-switching, all network users can share the same paths at the same time and the particular route a data unit travels can be varied as conditions change. In packet-switching, a message is divided into packets, which are units of a certain number of bytes. The network addresses of the sender and of the destination are added to the packet. Each network point looks at the packet to see where to send it next. Packets in the same message may travel different routes and may not arrive in the same order that they were sent. At the destination, the packets in a message are collected and reassembled into the original message.
Switched Virtual Circuit Pl See SVC
160 T T T-carrier system The T-carrier system, introduced by the Bell System in the U.S. in the 1960s, was the first successful system that supported digitized voice transmission. The original transmission rate (1.544 Mbps) in the T-1 line is in common use today in Internet service provider (ISP) connections to the Internet. Another level, the T-3 line, providing 44.736 Mbps, is also commonly used by Internet service providers. Another commonly installed service is a fractional T-1, which is the rental of some portion of the 24 channels in a T-1 line, with the other channels going unused. The T-carrier system is entirely digital, using pulse code modulation and time-division multiplexing. The system uses four wires and provides duplex capability (two wires for receiving and two for sending at the same time). The T-1 digital stream consists of 24 64-Kbps channels that are multiplexed. (The standardized 64 Kbps channel is based on the bandwidth required for a voice conversation.) The four wires were originally a pair of twisted pair copper wires, but can now also include coaxial cable, optical fiber, digital microwave, and other media. A number of variations on the number and use of channels are possible. In the T-1 system, voice signals are sampled 8,000 times a second and each sample is digitized into an 8-bit word. With 24 channels being digitized at the same time, a 192-bit frame (24 channels each with an 8-bit word) is thus being transmitted 8,000 times a second. Each frame is separated from the next by a single bit, making a 193-bit block. The 192 bit frame multiplied by 8,000 and the additional 8,000 framing bits make up the T-1's 1.544 Mbps data rate. The signaling bits are the least significant bits in each frame. You can compare these rates with “t”. To see the relationship between T-carrier, E- carrier, and DS0 multiples, see digital signal X.
T1 The T1 (or T-1) carrier is the most commonly used digital line in the United States, Canada, and Japan. In these countries, it carries 24 pulse code modulation (PCM) signals using time-division multiplexing (TDM) at an overall rate of 1.544 million bits per second (Mbps). T1 lines use copper wire and span distances within and between major metropolitan areas. A T1 Outstate System has been developed for longer distances between cities. It's probable that your Internet access provider is connected to the Internet as a point-of-presence (POP) on a T1 line owned by a major telephone network.
T3 Pl. see Tcarrier system
TAPI
161 TAPI (Telephony Application Program Interface) is a standard program interface that lets you and your computer "talk" over telephones or video phones to people or phone-connected resources elsewhere in the world. Assuming your computer is equipped with TAPI and your setup includes the right application and hardware, you may be able: Call someone by clicking on their picture or other image Use a similar graphical user interface (GUI) to set up a conference call and then attend the call at the scheduled time See who you're talking to individually or at a conference call Add a voice note to an e-mail note you send or listen to a voice note attached to an e-mail note you receive Program your computer to automatically receive phone calls from certain numbers (but not from others) Send and receive faxes Do these things from a portable wireless cellular telephone telephone/computer as well as from a desktop computer
Developed jointly by Intel and Microsoft, TAPI is included with the Windows 95/98 and Windows NT operating system. Using TAPI, programmers can take advantage of different telephone systems, including ordinary public switched telephone network, digital ISDN, and private branch exchange without having to understand all their details. Each phone system hardware provider (for example, the modem maker or ISDN card maker) provides a specific software driver that interfaces directly with the hardware. TAPI provides a high-level interface for dialing and disconnecting. Instead of having to encode an ATDT dial string and the ATH disconnect string, the programmer codes a much simpler "function call." In addition to the interface for applications, TAPI includes a Service Provider Interface (SPI) for hardware vendors who are writing the driver software. The TAPI Dynamic Link Library (DLL) maps the API to the SPI and coordinates input/output traffic.
TDM Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing. The circuit that combines signals at the source (transmitting) end of a communications link is known as a multiplexer. It accepts the input from each individual end user, breaks each signal into segments, and assigns the segments to the composite signal in a rotating, repeating sequence. The composite signal thus contains data from multiple senders. At the other end of the long- distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high- bandwidth cable. If many signals must be sent along a single long-distance line, careful engineering is required to ensure that the system will perform properly. An asset of TDM is its flexibility. The scheme allows for variation in the number of signals being sent along the line, and constantly adjusts the time intervals to make
162 optimum use of the available bandwidth. The Internet is a classic example of a communications network in which the volume of traffic can change drastically from hour to hour. In some systems, a different scheme, known as frequency-division multiplexing (FDM), is preferred.
TDMA TDMA (time division multiple access) is a technology used in digital cellular telephone communication that divides each cellular channel into three time slots in order to increase the amount of data that can be carried. TDMA is used by Digital- American Mobile Phone Service (D-AMPS), Global System for Mobile communications (GSM), and Personal Digital Cellular (PDC). However, each of these systems implements TDMA in a somewhat different and incompatible way. An alternative multiplexing scheme to FDMA with TDMA is CDMA (code division multiple access), which takes the entire allocated frequency range for a given service and multiplexes information for all users across the spectrum range at the same time. TDMA was first specified as a standard in EIA/TIA Interim Standard 54 (IS-54). IS-136, an evolved version of IS-54, is the United States standard for TDMA for both the cellular (850 MHz) and personal communications services (1.9 GHz) spectrums. TDMA is also used for Digital Enhanced Cordless Telecommunications (DECT).
Telco In the United States and possibly other countries, "telco" is a short form for telephone company. Sometimes it means a local telephone company, such as a Bell operating company or an independent local telephone company. Sometimes it means any telephone company, including one offering long-distance services.
Telecenter A telecenter (US spelling) or telecentre (UK spelling) is a work location usually in a different place than the organization's main office that provides convenient occasional access for telecommuting to work equipment that they don't have at home or on the road. For example, a home telecommuter might need to print and reproduce printed copies of a document occasionally using a high-speed printer not available at home. Or a mobile worker might occasionally check the telecenter for fax mail or to send a fax. A telecenter could also have teleconference facilities.
Teleconference A teleconference is a telephone meeting among two or more participants involving technology more sophisticated than a simple two-way phone connection. At its simplest, a teleconference can be an audio conference with one or both ends of the conference sharing a speaker phone. With considerably more equipment and special arrangements, a teleconference can be a conference, called a videoconference, in which the participants can see still or motion video images of each other. Because of the high bandwidth of video and the opportunity for larger and multiple display screens, a videoconference requires special telecommunication arrangements and a special room at each end. As equipment and high-bandwidth cabling become more commonplace, it's possible that videoconferences can be held from your own computer or even in a mobile setting.
163 One of the special projects of Internet2 is to explore the possibility of having teleconferences in which all participants actually appear to be in the same room together. Today's audio teleconferences are sometimes arranged over dial-up phone lines using bridging services that provide the necessary equipment for the call.
Telephone jacks Pl. See RJ
Telemetry Telemetry is the wireless transmission and reception of measured quantities for the purpose of remotely monitoring environmental conditions or equipment parameters. The term is also used in reference to the signals containing such data. Telemetry is used with satellites, space probes, and mobile robots. Telemetry is employed in manned spacecraft, such as the Space Shuttle and the International Space Station (ISS), to monitor the physical condition of astronauts and to ensure the maintenance of a proper working environment for them. Telemetry is also used with the Hubble Space Telescope (HST). A telemetry transmitter consists of a set of measuring instruments, an encoder that translates instrument readings into analog or digital signals, a modulator, and a wireless transmitter with an antenna. A telemetry receiver consists of an antenna, a set of radio-frequency (RF) amplifiers, a demodulator, and recording devices. A computer can be used to process and store received information.
Telephony Telephony is the technology associated with the electronic transmission of voice, fax, or other information between distant parties using systems historically associated with the telephone, a handheld device containing both a speaker or transmitter and a receiver. With the arrival of computers and the transmittal of digital information over telephone systems and the use of radio to transmit telephone signals, the distinction between telephony and telecommunication has become difficult to make. Internet telephony is the use of the Internet rather than the traditional telephone company infrastructure and rate structure to exchange spoken or other telephone information. Since access to the Internet is available at local phone connection rates, an international or other long-distance call will be much less expensive than through the traditional call arrangement. On the Internet, three new services are now or will soon be available: The ability to make a normal voice phone call (whether or not the person called is immediately available; that is, the phone will ring at the location of the person called) through the Internet at the price of a local call The ability to send fax transmissions at very low cost (at local call prices) through a gateway point on the Internet in major cities The ability to send voice messages along with text e-mail
Terabit In measuring data transmission speed, a terabit is one trillion binary digits, or 1,000,000,000,000 (that is, 1012) bits. A terabit is used for measuring the amount of
164 data that is transferred in a second between two telecommunication points or within network devices. For example, several companies are building a network switch that passes incoming packets through the device and out again at a terabits-per- second speed. Terabits per second is usually shortened to Tbps. Although the bit is a unit of the binary number system, bits in data communications have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two.
Terbo The suffix terbo appears in the V.32terbo modem protocol and indicates the third version of the V.32 protocol. Terbo is an invented word based on the Old Latin ter meaning "three times" and the word turbo (Latin for "whirling top" or "whirlwind") meaning "speed." Also see bis, a suffix used in several modern protocols that means "second version."
Time-Division Multiplexing Pl. see TDM
Time Division Multiple Access Pl. see TDMA
Token Ring A token ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a binary digit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The token ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 token ring technology provides for data transfer rates of either 4 or 16 megabits per second. Very briefly, here is how it works: Empty information frames are continuously circulated on the ring. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.
165 The token scheme can also be used with bus topology LANs. The standard for the token ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802.5. The Fiber Distributed-Data Interface (FDDI) also uses a token ring protocol.
Transit Transit is the connection to and use of a telecommunication path provided by a vendor. Transit may be billed separately or, where peering is also provided, may be billed as part of the peering charge.
Triangulation Triangulation is a process by which the location of a radio transmitter can be determined by measuring either the radial distance, or the direction, of the received signal from two or three different points. Triangulation is sometimes used in cellular communications to pinpoint the geographic position of a user. The drawings below illustrate the basic principle of triangulation. In the scenario shown by the top drawing, the distance to the cell phone is determined by measuring the relative time delays in the signal from the phone set to three different base stations. In the scenario shown by the bottom drawing, directional antennas at two base stations can be used to pinpoint the location of the cell phone.
Triangulation is difficult to carry out unless the person using the cell phone wants to be located. This might be the case, for example, in an emergency situation. Triangulation is the method by which the so-called 911 cell phones work.
166 Triangulation apparatus can be confused by the reflection of signals from objects such as large steel-frame buildings, water towers, communications towers, and other obstructions. For this reason, at least two independent triangulation determinations should be made to confirm the position of a cell phone or other radio transmitter. A more sophisticated form of triangulation is used by the Global Positioning System (GPS).
Trunk In telephone systems, a trunk is a line that carries multiple voice or data channel between two telephone exchange switching systems. In digital communications, a trunk is often a T-carrier system.
Transceiver A transceiver is a combination transmitter/receiver in a single package. The term applies to wireless communications devices such as cellular telephones, cordless telephone sets, handheld two-way radios, and mobile two-way radios. Occasionally the term is used in reference to transmitter/receiver devices in cable or optical fiber systems. In a radio transceiver, the receiver is silenced while transmitting. An electronic switch allows the transmitter and receiver to be connected to the same antenna, and prevents the transmitter output from damaging the receiver. With a transceiver of this kind, it is impossible to receive signals while transmitting. This mode is called half duplex. Transmission and reception often, but not always, are done on the same frequency. Some transceivers are designed to allow reception of signals during transmission periods. This mode is known as full duplex, and requires that the transmitter and receiver operate on substantially different frequencies so the transmitted signal does not interfere with reception. Cellular and cordless telephone sets use this mode. satellite communications networks often employ full-duplex transceivers at the surface-based subscriber points. The transmitted signal (transceiver-to-satellite) is called the uplink, and the received signal (satellite-to-transceiver) is called the downlink.
Transponder A transponder is a wireless communications, monitoring, or control device that picks up and automatically responds to an incoming signal. The term is a contraction of the words transmitter and responder. Transponders can be either passive or active. A passive transponder allows a computer or robot to identify an object. Magnetic labels, such as those on credit cards and store items, are common examples. A passive transponder must be used with an active sensor that decodes and transcribes the data the transponder contains. The transponder unit can be physically tiny, and its information can be sensed up to several feet away. Simple active transponders are employed in location, identification, and navigation systems for commercial and private aircraft. An example is an RFID (radio- frequency identification) device that transmits a coded signal when it receives a request from a monitoring or control point. The transponder output signal is tracked, so the position of the transponder can be constantly monitored. The input (receiver) and output (transmitter) frequencies are preassigned. Transponders of this type can operate over distances of thousands of miles.
167 Sophisticated active transponders are used in communications satellites and on board space vehicles. They receive incoming signals over a range, or band, of frequencies, and retransmit the signals on a different band at the same time. The device is similar to a repeater of the sort used in land-based cellular telephone networks. The incoming signal, usually originating from a point on the earth's surface, is called the uplink. The outgoing signal, usually sent to a point or region on the surface, is the downlink. These transponders sometimes operate on an interplanetary scale.
Tropo Radio waves can propagate over the horizon when the lower atmosphere of the earth bends, scatters, and/or reflects the electromagnetic fields. These effects are collectively known as tropospheric propagation, or tropo for short. Tropospheric propagation can affect wireless communications, sometimes enhancing the usable range, but also compounding interference problems. The most well-known form of tropo is called bending. Air reduces radio-wave propagation speed compared with the speed in a vacuum. The greater the air density, the more the air slows the waves, and thus the greater is the index of refraction. The density and index of refraction are highest near the surface, and steadily decrease with altitude. This produces a tendency for radio waves at very-high frequencies (VHF, 30 to 300 MHz) and ultra-high frequencies (UHF, 300 MHz to 3 GHz) to be refracted toward the surface. A wave beamed horizontally can follow the curvature of the earth for hundreds of miles. The lower atmosphere scatters electromagnetic radiation over a vast range, including radio wavelengths. This effect is known as tropospheric scatter, or troposcatter. In general, troposcatter is most pronounced at UHF and microwave radio frequencies (300 MHz and above). A radio wave beamed slightly above the horizon can be scattered at altitudes up to several miles, making over-the-horizon communication possible. The greatest communications range can be realized over flat land or over water. Scattered waves are weak, so high-power transmitters and sensitive receivers are necessary. A less common, but often dramatic, form of tropo is called ducting or duct effect. This occurs when there is a defined, horizontal boundary between air masses having different densities. When a cool air mass is overlain by a warm air mass, as is the case along and near warm fronts and cold fronts, radio waves at VHF and UHF are reflected at the boundary if they strike it at a near-grazing angle from beneath (within the cooler air mass). Because radio waves are also reflected from the earth's surface, the result can be efficient propagation for hundreds or, in some cases, upwards of 1,000 miles, as the waves alternately bounce off the frontal boundary and the surface. Ducting can allow long-distance radio reception in the frequency-modulation (FM) broadcast band between 88 and 108 MHz. It can also affect the lower VHF television channels if receiving antennas (rather than cable networks) are used.
Tropospheric Propogation Pl. see tropo
Two Way Pager Pl. see pager
168
169 U U Ubiquitous Computing Pervasive computing is the trend towards increasingly ubiquitous (another name for the movement is ubiquitous computing), connected computing devices in the environment, a trend being brought about by a convergence of advanced electronic - and particularly, wireless - technologies and the Internet. Pervasive computing devices are not personal computers as we tend to think of them, but very tiny - even invisible - devices, either mobile or embedded in almost any type of object imaginable, including cars, tools, appliances, clothing and various consumer goods - all communicating through increasingly interconnected networks. According to Dan Russell, director of the User Sciences and Experience Group at IBM's Almaden Research Center, by 2010 computing will have become so naturalized within the environment that people will not even realize that they are using computers. Russell and other researchers expect that in the future smart devices all around us will maintain current information about their locations, the contexts in which they are being used, and relevant data about the users. The goal of researchers is to create a system that is pervasively and unobtrusively embedded in the environment, completely connected, intuitive, effortlessly portable, and constantly available. Among the emerging technologies expected to prevail in the pervasive computing environment of the future are wearable computers, smart homes and smart buildings. Among the myriad of tools expected to support these are: application-specific integrated circuitry (ASIC); speech recognition; gesture recognition; system on a chip (SoC); perceptive interfaces; smart matter; flexible transistors; reconfigurable processors; field programmable logic gates (FPLG); and microelectromechanical systems (MEMS). A number of leading technological organizations are exploring pervasive computing. Xerox's Palo Alto Research Center (PARC), for example, has been working on pervasive computing applications since the 1980s. Although new technologies are emerging, the most crucial objective is not, necessarily, to develop new technologies. IBM's project Planet Blue, for example, is largely focused on finding ways to integrate existing technologies with a wireless infrastructure. Carnegie Mellon University's Human Computer Interaction Institute (HCII) is working on similar research in their Project Aura, whose stated goal is "to provide each user with an invisible halo of computing and information services that persists regardless of location." The Massachusetts Institute of Technology (MIT) has a project called Oxygen. MIT named their project after that substance because they envision a future of ubiquitous computing devices as freely available and easily accessible as oxygen is today.
UHF The UHF (ultrahigh frequency) range of the radio spectrum is the band extending from 300 MHz to 3 GHz. The wavelengths corresponding to these limit frequencies
170 are 1 meter and 10 centimeters. In the UHF band, signals from earth-based transmitters are not returned by the ionosphere to the surface; they always pass into space. Conversely, signals from space always penetrate the ionosphere and reach the surface. The global "shortwave" propagation familiar to users of lower frequencies is unknown at UHF. The troposphere can cause bending, ducting, and scattering at UHF, extending the range of communication significantly beyond the visual horizon. Auroral, meteor-scatter, and EME (earth-moon-earth, also called moonbounce) propagation are sometimes observed, but these modes do not offer reliable communication and are of interest primarily to amateur radio operators. In the upper portion of the band, waves can be focused or collimated by dish antennas of modest size. The UHF band is extensively used for satellite communication and broadcasting, in cellular telephone and paging systems, and by third-generation (3G) wireless services. Because the frequency is high and the band is vast (a span of 2.7 gigahertz from the low end to the high end), wideband modulation and spread spectrum modes are practical. Channels and subbands within the UHF portion of the radio spectrum are allocated by the International Telecommunication Union (ITU).
U interface In Integrated Services Digital Network (ISDN) Basic Rate Interface service, a U interface is the electrical interface for the single twisted pair wire connection from a local phone company (the central office) to a home or business. Unlike a regular 64 Kbps analog phone connection, however, the twisted-pair using ISDN carries two 64 Kbps channels (known as B, or bearer, channels) and an additional 16 Kbps channel (the D, for data or delta, channel) for control signals. The U interface twisted-pair is usually connected at the home or business to a network terminator 1 (NT1) box, sometimes called a network terminating unit. (In the UK and some other countries, the NT1 is located at the central office.) The other side of the NT1 has plugs for four wires, which can be connected on a loop configuration known as an S-bus or S interface to up to eight devices (for example, two computers and six phones) or to a T interface. An NT1 can also be integrated into a modem or other device, in which case the ISDN connection can only serve that device. The U-loop or U-V loop, as it is sometimes called, uses the 2B1Q line code protocol, meaning that two binary digits are used to represent one quadratude - that is, four possible variations of signal level (amplitude and polarity). Communication is full-duplex, meaning that data can be arriving at the same time you are sending data.. The U-V loop replaces the traditional local loop. The maximum distance for the ISDN loop is 6,500 meters (about 18,000 feet). Pl. refer to table in “S Interface”.
Ultra Wideband Pl. see UWB
Universal ADSL Pl. see G.lite
Universal Network The "universal network" is the idea of a single network that integrates the existing voice and public telecommunications network (including the Internet), cable TV,
171 data networks, and video broadcast networks so that they work together well. Currently, each has a different kind of traffic and the older networks bear the burden of an out-of-date infrastructure. For example, the public voice network supports connections of phone-call duration and circuit-switching (although the same network also supports connectionless traffic and packet-switching for data). The video broadcast and cable TV networks deliver mainly the higher-bandwidth continous-flow traffic of streaming video and sound. Juniper Networks planned something close to a universal network in a fiber-optic network that includes very fast (multigigabit) switches using microchips customized for Internet traffic. The microchips included application-specific (ASIC) circuits made by IBM. The switches used wave-division multiplexing (WDM).
UMTS UMTS (Universal Mobile Telecommunications Service) is a so-called "third- generation (3G)," broadband, packet-based transmission of text, digitized voice, video, and multimedia at data rates up to 2 megabits per second (Mbps) that will offer a consistent set of services to mobile computer and phone users no matter where they are located in the world. Based on the Global System for Mobile (Global System for Mobile communication) communication standard, UMTS, endorsed by major standards bodies and manufacturers, is the planned standard for mobile users around the world by 2002. Once UTMS is fully implemented, computer and phone users can be constantly attached to the Internet as they travel and, as they roaming service, have the same set of capabilities no matter where they travel to. Users will have access through a combination of terrestrial wireless and satellite transmissions. Until UMTS is fully implemented, users can have multi-mode devices that switch to the currently available technology (such as GSM 900 and 1800) where UMTS is not yet available. Today's cellular telephone systems are mainly circuit-switched, with connections always dependent on circuit availability. packet-switched connection, using the Internet Protocol (Internet Protocol), means that a virtual connection is always available to any other end point in the network. It will also make it possible to provide new services, such as alternative billing methods (pay-per-bit, pay-per-session, flat rate, asymmetric bandwidth, and others). The higher bandwidth of UMTS also promises new services, such as video conferencing. UMTS promises to realize the Virtual Home Environment (Virtual Home Environment) in which a roaming user can have the same services to which the user is accustomed when at home or in the office, through a combination of transparent terrestrial and satellite connections. Trials of UMTS technology, using advanced mobile phone/computing device prototypes, are being conducted in 1999 by Nortel Networks and BT (British Telecommunications). The electromagnetic radiation spectrum for UMTS has been identified as frequency bands 1885-2025 MHz) for future IMT-2000 systems, and 1980-2010 MHz and 2170-2200 MHz for the satellite portion of UMTS systems.
Universal Mobile Telecommunications Service Pl. see UMTS
Unstructured Supplementary Service Data Pl. see USSD
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Upstream Pl. see downstream
USSD USSD (Unstructured Supplementary Service Data) is a Global System for Mobile (GSM) communication technology that is used to send text between a mobile phone and an application program in the network. Applications may include prepaid Roaming or mobile chatting. USSD is similar to Short Messaging Service (SMS), but, unlike SMS, USSD transactions occur during the session only. With SMS, messages can be sent to a mobile phone and stored for several days if the phone is not activated or within range. The Wireless Application Protocol (WAP) supports USSD. USSD is defined in the GSM standard documents GSM 02.90 and GSM 03.90.
UWB Ultra wideband (also known as UWB or as digital pulse wireless) is a wireless technology for transmitting large amounts of digital data over a wide spectrum of frequency bands with very low power for a short distance. Ultra wideband radio not only can carry a huge amount of data over a distance up to 230 feet at very low power (less than 0.5 milliwatts), but has the ability to carry signals through doors and other obstacles that tend to reflect signals at more limited bandwidths and a higher power. Ultra wideband can be compared with another short-distance wireless technology, Bluetooth, which is a standard for connecting handheld wireless devices with other similar devices and with desktop computers. Ultra wideband broadcasts digital pulses that are timed very precisely on a carrier signal across a very wide spectrum (number of frequency channels) at the same time. Transmitter and receiver must be coordinated to send and receive pulses with an accuracy of trillionths of a second. On any given frequency band that may already be in use, the ultra wideband signal has less power than the normal and anticipated background noise so theoretically no interference is possible. Time Domain, a company applying to use the technology, uses a microchip manufactured by IBM to transmit 1.25 million bits per second, but says there is the potential for a data rate in the billions of bits per second. Ultra wideband has two main types of application: Applications involving radar, in which the signal penetrates nearby surfaces but reflects surfaces that are farther away, allowing objects to be detected behind walls or other coverings. Voice and data transmission using digital pulses, allowing a very low powered and relatively low cost signal to carry information at very high rates within a restricted range.
In the U.S., the Federal Communications Commission approved the commercial use of ultra wideband on February 14, 2002.
173 V V VBI The vertical blanking interval (VBI) is a portion of a television signal that can carry information other than video or audio, such as closed-caption text and stock market data. The interval in sending a video signal is required for the time it takes the electron gun in a television monitor's cathode ray tube (CRT) to move back up to the top of the tube. VBI data can be inserted by a cable TV provider and transmitted to a special receiver that connects to a computer's RS-232C port
VCSEL A vertical cavity surface emitting laser (VCSEL) is a specialized laser diode that promises to revolutionize fiber optic communications by improving efficiency and increasing data speed. The acronym VCSEL is pronounced 'vixel.' Older laser diodes, called edge-emitting diodes, emit coherent light or infrared (IR) energy parallel to the boundaries between the semiconductor layers. The VCSEL emits its coherent energy perpendicular to the boundaries between the layers. The vertical in VCSEL arises from the fact that laser diodes are typically diagrammed showing the boundaries as horizontal planes, so the output of the VCSEL appears to emerge vertically in these drawings. VCSELs have been constructed that emit energy at 850 nanometers (nm) and 1300 nm. These wavelengths correspond to energy in the near infrared (IR) portion of the electromagnetic spectrum. (The longest visible red is at approximately 770 nm.) Optical fibers transmit energy most efficiently at wavelengths around 1550 nm. Materials used to manufacture VCSELs include gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), and indium gallium arsenide nitride (InGaAsN). The VCSEL has several advantages over edge-emitting diodes. The VCSEL is cheaper to manufacture in quantity, is easier to test, and is more efficient. In addition, the VCSEL requires less electrical current to produce a given coherent energy output. The VCSEL emits a narrow, more nearly circular beam than traditional edge emitters; this makes it easier to get the energy from the device into an optical fiber. The main challenge facing engineers today is the development of a high-power VCSEL device with an emission wavelength of 1550 nm.
Vertical Cavity Surface Emitting Laser Pl. see VCSEL
Vertical Blanking Interval Pl. see VBI
VAD
174 In Voice over IP (VoIP), voice activation detection (VAD) is a software application that allows a data network carrying voice traffic over the Internet to detect the absence of audio and conserve bandwidth by preventing the transmission of "silent packets" over the network. Most conversations include about 50% silence; VAD (also called "silence suppression") can be enabled to monitor signals for voice activity so that when silence is detected for a specified amount of time, the application informs the Packet Voice Protocol and prevents the encoder output from being transported across the network. Voice activation detection can also be used to forward idle noise characteristics (sometimes called ambient or comfort noise) to a remote IP telephone or gateway. The universal standard for digitized voice, 64 Kbps, is a constant bit rate whether the speaker is actively speaking, is pausing between thoughts, or is totally silent. Without idle noise giving the illusion of a constant transmission stream during silence suppression, the listener would be likely to think the line had gone dead.
Very High Frequency Pl. see VHF
Very Small Aperture Terminal Pl. see VSAT
VHF The VHF (very high frequency) range of the radio spectrum is the band extending from 30 MHz to 300 MHz. The wavelengths corresponding to these limit frequencies are 10 meters and 1 meter. In the VHF band, electromagnetic fields are affected by the earth's ionosphere and troposphere. Ionospheric propagation occurs regularly in the lower part of the VHF spectrum, mostly at frequencies below 70 MHz. In this mode, the communication range can sometimes extend over the entire surface of the earth. The troposphere can cause bending, ducting, and scattering, extending the range of communication significantly beyond the visual horizon. Auroral, meteor-scatter, and EME (earth-moon-earth, also called moonbounce) propagation take place on occasion, but these modes do not offer reliable communication and are of interest primarily to amateur radio operators. The VHF band is popular for mobile two-way radio communication. A great deal of satellite communication and broadcasting is done at VHF. wideband modulation is used by some services; the most common example is fast-scan television broadcasting. Channels and subbands within the VHF portion of the radio spectrum are allocated by the International Telecommunication Union (ITU).
Video Telephony Video telephony is full-duplex, real-time audio-visual communication between or among end users. The idea of the video telephone, also called the videophone, is about as old as the telephone itself. The primary challenge facing developers of the video telephone is the fact that full-motion, high-resolution video data requires far more bandwidth than audio data. This is true whether the signals are analog or digital. The bandwidth of a video signal can be minimized by using the lowest image resolution that will give acceptable results, by settling for grayscale rather than color imagery, and by transmitting non-moving images at intervals of several
175 seconds rather than a continuous, full-motion image. A video signal of this type, which is a form of slow-scan television (SSTV), can be transmitted and received over ordinary copper telephone lines. Broadband Internet solutions such as DSL (Digital Subscriber Line), cable, and land-based wireless make it possible to transmit and receive video data at higher resolutions and more rapid refresh rates than is the case with the ordinary telephone system. Broadband satellite solutions can also work, although the latency inherent in geostationary-satellite systems produces an image and voice delay that is objectionable to some users. Even with the best broadband technology available to consumers today, the image quality is modest, resembling the picture on a small fast-scan television (FSTV) receiver. The technology for SSTV video telephony has been widely available for years, but videophones are not in widespread use. Possibly the main reason is that most people do not want to be seen when they are on the telephone, unless it is for business reasons (videoconferencing). Another apparent reason is the cost of the hardware, which requires a display screen and a video camera. A third reason is the generally low image quality and slow refresh rate. As high-speed Internet access becomes more widely available, fast-scan television (FSTV) or high- definition television (HDTV) video telephony is likely to become more attractive for potential home and business users.
Videophone Pl. see video telephony
Virtual circuit A virtual circuit is a circuit or path between points in a network that appears to be a discrete, physical path but is actually a managed pool of circuit resources from which specific circuits are allocated as needed to meet traffic requirements. A permanent virtual circuit (PVC) is a virtual circuit that is permanently available to the user just as though it were a dedicated or leased line continuously reserved for that user. A switched virtual circuit (SVC) is a virtual circuit in which a connection session is set up for a user only for the duration of a connection. PVCs are an important feature of frame relay networks and SVCs are proposed for later inclusion.
Visor Visor is the trade name of a handheld computer manufactured by Handspring. The Visor is similar to the Palm computer and uses the Palm operating system (Palm OS). Data can be easily transferred between the Visor and other computers, including Palm handhelds, using HotSync technology. Interfacing is possible with Macintosh as well as with IBM-compatible desktop and notebook computers. The Visor can be tailored to meet the needs of the individual user. It contains an expansion slot called Springboard that allows the addition of modules, each of which is designed for a specific function. The modules are plug-and-play and are hot-swappable (they are automatically recognized by the Visor, and they can be exchanged without powering-down or rebooting). Data can be stored and backed up with an optional flash memory module.
Voice Activation Detection Pl. see VAD
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Voice Portal A voice portal (sometimes referred to as a vortal) is a Web site or other service that a user can reach by telephone for information such as weather, sport scores, or stock quotes. For example, a mobile user with a cellular telephone might dial in to a voice portal Web site and request information using voice or Touchtone keys and receive the requested information from a special voice-producing program at the Web site. Whereas a user with a smartphone can connect to the Internet and get information on a small visual display (perhaps with a WAP interface), the user of a voice portal needs only a regular cellular phone. After requesting information by speaking or pressing keys, the voice portal responds with voice information or, in some cases, with an e-mail message.
Voice-Activated e-mail Pl. see Voice-enabled e-mail
Voice-enabled e-mail Voice-enabled e-mail (sometimes referred to as voice-activated e-mail) uses voice recognition and speech synthesis technologies to enable users to access their e- mail from any telephone. In general, the various products available work similarly: The subscriber dials a phone number to access a voice portal, then, to collect their e-mail messages, they press a couple of keys and, perhaps, say a phrase like "Get my e-mail." Speech synthesis software converts e-mail text to a voice message, which is played back over the phone. The user may navigate through options (such as skipping messages, or hearing a list of senders, for example) through voice commands or key selections. Users dictate their replies, which are delivered to the recipients as voice messages. Voice-enabled e-mail is especially useful for mobile workers, because it makes it possible for them to access their messages easily from virtually anywhere (as long as they can get to a phone), without having to invest in expensive equipment such as laptop computers or personal digital assistants (PDAs). Proponents hope that new services like voice-enabled e-mail and unified messaging will turn out to be the killer apps that will dissolve the perceived barriers between data networks and traditional voice networks.
Vortal On the Web, a vortal (vertical industry portal) is a Web site that provides a gateway or portal to information related to a particular industry such as health care, insurance, automobiles, or food manufacturing. (A vertical industry is one that is focused on a relatively narrow range of goods and services, whereas a horizontal industry is one that aims to produce a wide range of goods and services. Because most industry tends to specialize, most industry tends to be vertical.) A term that might also be used is interest community Web site since any vertical industry brings together people sharing an interest in buying, selling, or exchanging information about that particular industry. Vortals are also seen as likely business-to-business communities - for example, small business people with home offices might be attracted to a comprehensive vortal that provided ideas and product information
177 related to setting up and maintaining the home office. By whatever name, Web sites that promise to give the user a single place to communicate with and about a single industry are predicted to becomes big businesses themselves. Gartner Inc. estimates that 300 such sites already exist and predicts as many as 10,000 within the next several years. An early leader is publicly traded VerticalNet, a company that uses the same content format and design for a number of vortal sites. Vortal is also short for voice portal.
VSAT VSAT (Very Small Aperture Terminal) is a satellite communications system that serves home and business users. A VSAT end user needs a box that interfaces between the user's computer and an outside antenna with a transceiver. The tranceiver receives or sends a signal to a satellite transponder in the sky. The satellite sends and receives signals from an earth station computer that acts as a hub for the system. Each end user is interconnected with the hub station via the satellite in a star topology. For one end user to communicate with another, each transmission has to first go to the hub station which retransmits it via the satellite to the other end user's VSAT. VSAT handles data, voice, and video signals. VSAT is used both by home users who sign up with a large service such as DirecPC and by private companies that operate or lease their own VSAT systems. VSAT offers a number of advantages over terrestrial alternatives. For private applications, companies can have total control of their own communication system without dependence on other companies. Business and home users also get higher speed reception than if using ordinary telephone service or ISDN.
V.xx The V Series Recommendations from the ITU-TS are summarized in the table below. They include the most commonly used modem standards and other telephone network standards. Prior to the ITU-T standards, the American Telephone and Telegraph Company and the Bell System offered its own standards (Bell 103 and Bell 212A) at very low transfer rates. Another set of standards, the Microcom Networking Protocol, or MNP Class 1 through Class 10 (there is no Class 8), has gained some currency, but the development of an international set of standards means these will most likely prevail and continue to be extended. (Some modems offer both MNP and ITU-T standards.) In general, when modems handshake, they agree on the highest standard transfer rate that both can achieve. Beginning with V.22bis, ITU-T transfer rates increase in 2400 bps multiples. (bis refers to a "second version." terbo refers to a "third version.")
Standard Meaning Provides 1200 bits per second at 600 baud (state changes per V.22 second) The first true world standard, it allows 2400 bits per second at 600 V.22bis baud V.32 Provides 4800 and 9600 bits per second at 2400 baud Provides 14,400 bits per second or fallback to 12,000, 9600, 7200, V.32bis and 4800 bits per second
178 Provides 19,200 bits per second or fallback to 12,000, 9600, 7200, V.32terbo and 4800 bits per second; can operate at higher data rates with compression; was not a CCITT/ITU standard Provides 28,800 bits per second or fallback to 24,000 and 19,200 bits V.34 per second and backwards compatility with V.32 and V.32bis Provides up to 33,600 bits per second or fallback to 31,200 or V.34 V.34bis transfer rates The trunk interface between a network access device and a packet network at data rates greater than 19.2 Kbps. V.35 may use the V.35 bandwidths of several telephone circuits as a group. There are V.35 Gender Changers and Adapters. Same transfer rate as V.32, V.32bis, and other standards but with V.42 better error correction and therefore more reliable Provides up to 56,000 bits per second downstream (but in practice V.90 somewhat less). Derived from the x2 technology of 3Com (US Robotics) and Rockwell's K56flex technology.
179 W W Walkie Talkie Pl. see Handie Talkie
WAP WAP (Wireless Application Protocol) is a specification for a set of communication protocols to standardize the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access, including e-mail, the World Wide Web, newsgroups, and Internet Relay Chat (IRC). While Internet access has been possible in the past, different manufacturers have used different technologies. In the future, devices and service systems that use WAP will be able to interoperate. The WAP layers are: Wireless Application Environment (WAE) Wireless Session Layer (WSL) Wireless Transport Layer Security (WTLS) Wireless Transport Layer (WTP)
The WAP was conceived by four companies: Ericsson, Motorola, Nokia, and Unwired Planet (now Phone.com). The Wireless Markup Language (WML) is used to create pages that can be delivered using WAP. There are other approaches to an industry standard besides WAP, including i-Mode.
War Driving War driving is the act of locating and possibly exploiting connections to wireless local area networks while driving around a city or elsewhere. To do war driving, you need a vehicle, a computer (which can be a laptop), a wireless Ethernet card set to work in promiscuous mode, and some kind of an antenna which can be mounted on top of or positioned inside the car. Because a wireless LAN may have a range that extends beyond an office building, an outside user may be able to intrude into the network, obtain a free Internet connection, and possibly gain access to company records and other resources. Some people have made a sport out of war driving, in part to demonstrate the ease with which wireless LANs can be compromised. With an omnidirectional antenna and a geophysical positioning system (GPS), the war driver can systematically map the locations of 802.11b wireless access points. Companies that have a wireless LAN are urged to add security safeguards that will ensure only intended users have access. Safeguards include the use of the Wired Equivalent Privacy (WEP) encryption standard, the setup and use of a virtual private network (VPN) or IPsec, and a firewall or DMZ. The term derives from a somewhat similar approach to breaching the telephone system called war dialing.
WASP
180 A wireless application service provider (WASP) is part of a growing industry sector resulting from the convergence of two trends: wireless communications and the outsourcing of services. A WASP performs the same service for wireless clients as a regular application service provider (ASP) does for wired clients: it provides Web- based access to applications and services that would otherwise have to be stored locally. The main difference with WASP is that it enables customers to access the service from a variety of wireless devices, such as a smartphone or personal digital assistant (PDA). Although the business world is increasingly mobile, many corporations are resisting the idea of wireless communication, because of concerns about set-up and maintenance costs and the need for in-house expertise. WASPs offer businesses the advantages of wireless service with less expense and fewer risks. Because mobile applications are subscribed to, rather than purchased, up- front costs are lower; because the WASP provides support, staffing and training costs are lower. WASP services may include: Constant system monitoring Diagnostics and resolution User support Text formatting for various devices Problem detection and reporting
There are still issues to be resolved. Coverage areas remain limited, for example, and data synchronization among devices can be problematic. Nevertheless, WASPs provide an easier, safer, and cheaper way for organizations to add mobile components, and a number of major companies are opting for them. UPS, Sprint, and eBay are among the early subscribers to WASP services. Interestingly, some ASPs have begun to offer WASP services, while others are purchasing them. WASP is also an acronym for the Web Standards Project.
WATS WATS (wide-area telephone service) is a specialized form of fixed-rate long- distance telecommunication service. WATS lines are commonly used by businesses and government agencies. Some individuals and small corporations also have WATS subscriptions. There are three types of WATS lines: IN-WATS (for incoming calls), OUT-WATS (for outgoing calls), or a combination of both services. IN-WATS lines have telephone numbers with certain area codes reserved expressly for that purpose, such as 800, 888, or 877. People calling these numbers are not charged a long-distance toll. Instead, the recipient (subscriber) is charged a fixed monthly rate up to a certain number of hours of usage. Beyond the limit, an additional toll is imposed. OUT-WATS lines are, in effect, fixed-rate long-distance subscriptions. With most WATS lines, calling-zone restrictions apply. For example, it might not be possible to make or accept WATS calls to or from locations within the state where the subscriber is located, or to or from locations outside the country where the subscriber is located.
Wave Division Multiplexing Pl. see DWDM
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WAX Wireless Abstract XML (WAX) is an abstract markup language and associated tools that facilitate wireless application development. WAX comes as an integral part of Morphis, an open source transcoding platform from Kargo, Inc. Because WAX tags perform at a higher level of abstraction than those of earlier wireless markup languages, WAX translates to common languages, such as Hypertext Markup Language (HTML), Wireless Markup Language (WML), and Handheld Device Markup Language (HDML) through Extensible Stylesheet Language (XSL) style sheets and XSL Transformations (XSLT). The major features of WAX include: the WAX language itself; translation stylesheets, which are used to translate the WAX language into the most suitable language for the requesting device; the device registry, which includes an XML database of device particulars; dynamic image and text selection, which allows content to be written a single time for multiple transformations; and the application foundation, a WAX servlet that creates a foundation for WAX applications. Kargo claims that WAX transforms content for various wireless graphical user interfaces (GUIs) more easily and seamlessly than other wireless languages. WAX tags have complex and variable functionality: for example, a single tag can create different display options on separate devices. Because WAX is an extensible language, elements of other languages can easily be incorporated into applications.
Webtone Webtone is immediate and continuous access to the Internet in the same way that we think of dialtone when we pick up a phone receiver. To have webtone in the same way that we have dialtone, most users of the term believe that not only access is required, but also sufficient bandwidth to meet user demands as well as the same quality of service we expect today from the telephone system. Since the telephone system and the Internet are tending to converge, some believe that eventually webtone will include dialtone. Webtone also implies Internet access from mobile devices, supercomputers, kitchen appliances, and perhaps eventually the very walls that surround us.
WEP Wired Equivalent Privacy (WEP) is a security protocol, specified in the IEEE Wireless Fidelity (Wi-Fi) standard, 802.11b, that is designed to provide a wireless local area network (WLAN) with a level of security and privacy comparable to what is usually expected of a wired LAN. A wired local area network (LAN) is generally protected by physical security mechanisms (controlled access to a building, for example) that are effective for a controlled physical environment, but may be ineffective for WLANs because radio waves are not necessarily bound by the walls containing the network. WEP seeks to establish similar protection to that offered by the wired network's physical security measures by encrypting data transmitted over the WLAN. Data encryption protects the vulnerable wireless link between clients and access points; once this measure has been taken, other typical LAN security mechanisms such as password protection, end-to-end encryption, virtual private networks (VPNs), and authentication can be put in place to ensure privacy. A
182 research group from the University of California at Berkeley recently published a report citing "major security flaws" in WEP that left WLANs using the protocol vulnerable to attacks (called wireless equivalent privacy attacks). In the course of the group's examination of the technology, they were able to intercept and modify transmissions and gain access to restricted networks. The Wireless Ethernet Compatibility Alliance (WECA) claims that WEP - which is included in many networking products - was never intended to be the sole security mechanism for a WLAN, and that, in conjunction with traditional security practices, it is very effective.
WCDMA WCDMA (wideband code-division multiple access), an ITU standard derived from code-division multiple access (CDMA), is officially known as IMT-2000 direct spread. WCDMA is a third-generation (3G) mobile wireless technology offering much higher data speeds to mobile and portable wireless devices than commonly offered in today's market. WCDMA can support mobile/portable voice, images, data, and video communications at up to 2 Mbps (local area access) or 384 Kbps (wide area access). The input signals are digitized and transmitted in coded, spread-spectrum mode over a broad range of frequencies. A 5 MHz-wide carrier is used, compared with 200 kHz-wide carrier for narrowband CDMA.
WDM Pl. see DWDM
Web Slate A Web slate is a wireless Internet appliance that consists of a liquid crystal display (LCD) with a touch screen that allows the user to view and interact with Web pages
Wide-area telephone service Pl. see WATS
Wideband Wideband is a transmission medium or channel that has a wider bandwidth than one voice channel (with a carrier wave of a certain modulated frequency). This term is usually contrasted with narrowband.
Wideband CDMA Pl. see WCDMA
Wideband Code-Division Multiple Access Pl. see wideband CDMA
Wink In telecommunications, a wink is a signal in the form of a brief interruption in an otherwise continuous signal. Winks can be used to indicate certain conditions, or to cause specific actions to be performed by a telephone switching system. Wink pulsing is a means of alerting an operator or user that a certain condition exists. It consists of a series of brief interruptions in an otherwise continuous signal. For example, a multi-line telephone has several buttons, one for each line, and each
183 with a backlight. When one of the buttons is lit to indicate that there is a call on that line, the illumination is interrupted at regular intervals (usually about 1/2 second) to get the user's attention. A wink release is the tone sent to a phone from the central office indicating that the other end of a connection has hung up. Usually, the phone receiving the wink release will then hang up, too.
Wink Release Pl. see wink
Wired Equivalent Privacy Pl. see Wep
Wireless Wireless is a term used to describe telecommunications in which electromagnetic waves (rather than some form of wire) carry the signal over part or all of the communication path. Some monitoring devices, such as intrusion alarms, employ acoustic waves at frequencies above the range of human hearing; these are also sometimes classified as wireless. The first wireless transmitters went on the air in the early 20th century using radiotelegraphy (Morse code). Later, as modulation made it possible to transmit voices and music via wireless, the medium came to be called "radio." With the advent of television, fax, data communication, and the effective use of a larger portion of the spectrum, the term "wireless" has been resurrected.Common examples of wireless equipment in use today include: Cellular phones and pagers -- provide connectivity for portable and mobile applications, both personal and business Global Positioning System (GPS) -- allows drivers of cars and trucks, captains of boats and ships, and pilots of aircraft to ascertain their location anywhere on earth Cordless computer peripherals -- the cordless mouse is a common example; keyboards and printers can also be linked to a computer via wireless Cordless telephone sets -- these are limited-range devices, not to be confused with cell phones Home-entertainment-system control boxes -- the VCR control and the TV channel control are the most common examples; some hi-fi sound systems and FM broadcast receivers also use this technology Remote garage-door openers -- one of the oldest wireless devices in common use by consumers; usually operates at radio frequencies Two-way radios -- this includes Amateur and Citizens Radio Service, as well as business, marine, and military communications Baby monitors -- these devices are simplified radio transmitter/receiver units with limited range satellite television -- allows viewers in almost any location to select from hundreds of channels wireless LANs or local area networks -- provide flexibility and reliability for business computer users
184 Wireless technology is rapidly evolving, and is playing an increasing role in the lives of people throughout the world. In addition, ever-larger numbers of people are relying on the technology directly or indirectly. (It has been suggested that wireless is overused in some situations, creating a social nuisance.) More specialized and exotic examples of wireless communications and control include: Global System for Mobile Communication (GSM) -- a digital mobile telephone system used in Europe and other parts of the world; the de facto wireless telephone standard in Europe General Packet Radio Service (GPRS) -- a packet-based wireless communication service that provides continuous connection to the Internet for mobile phone and computer users Enhanced Data GSM Environment (EDGE) -- a faster version of the Global System for Mobile (GSM) wireless service Universal Mobile Telecommunications System (UMTS) -- a broadband, packet- based system offering a consistent set of services to mobile computer and phone users no matter where they are located in the world Wireless Application Protocol (WAP) -- a set of communication protocols to standardize the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access i-Mode -- the world's first "smart phone" for Web browsing, first introduced in Japan; provides color and video over telephone sets
Wireless can be divided into: fixed wireless -- the operation of wireless devices or systems in homes and offices, and in particular, equipment connected to the Internet via specialized modems Mobile wireless -- the use of wireless devices or systems aboard motorized, moving vehicles; examples include the automotive cell phone and PCS (personal communications services) Portable wireless -- the operation of autonomous, battery-powered wireless devices or systems outside the office, home, or vehicle; examples include handheld cell phones and PCS units IR wireless -- the use of devices that convey data via IR (infrared) radiation; employed in certain limited-range communications and control systems
Wireless Abstract XML Pl. see WAX
Wireless Application Protocol Pl. see WAP
Wireless Application Service Provider Pl. see WASP WASP is also an acronym for the Web Standards Project.
185 Wireless ASP Pl. see WASP
Wireless Cable Pl. see MMDS
Wireless LAN A wireless LAN is one in which a mobile user can connect to a local area network (LAN) through a wireless (radio) connection. A standard, IEEE 802.11, specifies the technologies for wireless LANs. The standard includes an encryption method, the Wired Equivalent Privacy algorithm. High-bandwidth allocation for wireless will make possible a relatively low-cost wiring of classrooms in the United States. A similar frequency allocation has been made in Europe. Hospitals and businesses are also expected to install wireless LAN systems where existing LANs are not already in place. Using technology from the Symbionics Networks, Ltd., a wireless LAN adapter can be made to fit on a Personal Computer Memory Card Industry Association (PCMCIA) card for a laptop or notebook computer.
Wireless Local Area Network Pl. see Wireless LAN
Wireless Service Provider Pl. see WSP
Wireless Transport Layer Security Pl. see WTLS
WLAN Pl. see wireless LAN
Wrap Plug A wrap plug, also known as a loopback plug, is a special plug that can be inserted into a port on a communications device to perform a diagnostic test called a loopback test. There are numerous possible configurations, depending on the hardware and the nature of the test to be performed. Wrap plugs are manufactured commercially for specific systems and tests. A wrap plug can also be "home- brewed" by taking a cable with the correct type of plug attached, cutting the cable, stripping the wires, and then twisting certain wires together to short out specific pins in the port. In some cases, attenuators, consisting of networks of resistors, are used in place of direct short-circuits. This simulates the path loss in a real-life communications circuit. The effect of a wrap plug is to cause transmitted (output) data to be returned as received (input) data, simulating a complete communications circuit using a single computer. In any case, the manufacturer's instructions must be closely followed to be sure valid test results are obtained, and to avoid damage to the equipment under test.
186 WSP A wireless service provider (WSP) is a company that offers transmission services to users of wireless devices (handheld computers and telephones) through radio frequency (RF) signals rather than through end-to-end wire communication. Generally, a WSP offers either cellular telephone telephone service, personal communication service (PCS) service, or both. The term also seems applicable to satellite television and Internet access providers.
WTLS Wireless Transport Layer Security (WTLS) is the security level for Wireless Application Protocol (WAP) applications. Based on Transport Layer Security (TLS) v1.0 (a security layer used in the Internet, equivalent to Secure Socket Layer 3.1), WTLS was developed to address the problematic issues surrounding mobile network devices - such as limited processing power and memory capacity, and low bandwidth - and to provide adequate authentication, data integrity, and privacy protection mechanisms. Wireless transactions, such as those between a user and their bank, require stringent authentication and encryption to ensure security to protect the communication from attack during data transmission. Because mobile networks do not provide end-to-end security, TLS had to be modified to address the special needs of wireless users. Designed to support datagrams in a high latency, low bandwidth environment, WTLS provides an optimized handshake through dynamic key refreshing, which allows encryption keys to be regularly updated during a secure session.
187 X X X2 x2 is a technology from US Robotics (now 3Com) for the downstream transmission of data over ordinary phone lines at 56 Kbps (thousands of bits per second). The 56 Kbps speed is achieved in the downstream direction only (to your home or business). Upstream speed is at the regular maximum speed of 33.6 Kbps. (The actual achieved downstream speed is reported by users to be about 53 Kbps.) x2 provided input to and has been replaced by the V.90 ITU-TS standard. 56 Kbps technologies exploit the fact that most telephone company offices are interconnected with digital lines. Assuming your Internet connection provider has a digital connection to its telephone company office, the downstream traffic from your local Internet access provider can use a new transmission technique on your regular twisted pair phone line that bypasses the usual digital-to-analog conversion. A V.90-equipped modem doesn't need to demodulate the downstream data. Instead, it decodes a stream of multi-bit voltage pulses generated as though the line was equipped for digital information. (Upstream data still requires digital-to- analog modulation.) Unlike Integrated Services Digital Network, the V.90 technology does not require any additional installation or extra charges from your local phone company. On the other hand, the maximum transmission speed of ISDN is twice that of V.90 at 128 Kbps. You also have the flexibility of combining digital and voice transmission on the same line.
X.25 Packet Switching X.25 Packet Switched networks allow remote devices to communicate with each other across high speed digital links without the expense of individual leased lines. Packet Switching is a technique whereby the network routes individual packets of HDLC data between different destinations based on addressing within each packet.
188
The protocol known as X.25 encompasses the first three layers of the OSI 7- layered architecture as defined by the International Organization for Standardization (ISO) as follows: Layer 1: The Physical Layer is concerned with electrical or signaling. It includes several standards such as V.35, RS232 and X.21. Layer 2: The Data Link Layer, which is an implementation of the ISO HDLC standard called Link Access Procedure Balanced (LAPB) and provides an error free link between two connected devices. Layer 3: The Network Layer which provides communications between devices connected to a common network. In the case of X.25, this layer is referred to as the X.25 Packet Layer Protocol (PLP) and is primarily concerned with network routing functions and the muliplexing of simultaneous logical connections over a single physical connection.
The user end of the network is known as Data Terminal Equipment (DTE) and the carrier's equipment is Data Circuit-terminating Equipment (DCE). The X.25 PLP permits a DTE user on an X.25 network to communicate with a number of remote DTEs simultaneously. Connections occur on logical channels of two types: Switched virtual circuits (SVCs) - SVCs are very much like telephone calls; a connection is established, data are transferred and then the connection is released. Each DTE on the network is given a unique DTE address which can be used much like a telephone number. Permanent virtual circuits (PVCs) - a PVC is similar to a leased line in that the connection is always present. The logical connection is established permanently by the Packet Switched Network administration. Therefore, data may always be sent, without any call setup.
To establish a connection on an SVC, the calling DTE sends a Call Request Packet, which includes the address of the remote DTE to be contacted. The destination DTE decides whether or not to accept the call (the Call Request packet includes the sender's DTE address, as well as other information that the called DTE can use to decide whether or not to accept the call). A call is accepted by issuing a Call Accepted packet, or cleared by issuing a Clear Request packet. Once the originating DTE receives the Call Accepted packet, the virtual circuit is established and data transfer may take place. When either DTE wishes to terminate the call, a Clear Request packet is sent to the remote DTE, which responds with a Clear Confirmation packet. The destination for each packet is identified by means of the Logical Channel Identifier (LCI) or Logical Channel Number (LCN). This allows the PSN to route the each packet to its intended DTE.
X.25 relies on the underlying robustness of HDLC LAPB to get data from node to node through the X.25 network. An X.25 packet makes up the data field of an HDLC frame. Additional flow control and windowing are provided for each Logical Channel at the X.25 level. Maximum packet sizes vary from 64 bytes to 4096 bytes, with 128 bytes being a default on most networks. Both maximum packet size and packet level windowing may be negotiated between DTEs on call set up. X.25 gives
189 a virtual high quality digital network at low cost. It is economical for the same reason that it is usually cheaper to use the mail than to run your own postal service: there are tremendous savings to be made if multiple parties share the same infrastructure. In most parts of the world, X.25 is paid for by a monthly connect fee plus packet charges. There is usually no holding charge, making X.25 ideal for organizations that need to be on line all the time. Another useful feature is speed matching: because of the store-and-forward nature of Packet Switching, plus excellent flow control, DTEs do not have to use the same line speed. So you can have, for instance, a host connected at 56kbps communicating with numerous remote sites connected with cheaper 19.2kbps lines. X.25 has been around since the mid 1970's and so is pretty well debugged and stable. There are literally no data errors on modern X.25 networks.
X.25 does have some drawbacks. There is an inherent delay caused by the store- and-forward mechanism. On most single networks the turn-around delay is about 0.6 seconds. This has no effect on large block transfers, but in flip-flop types of transmissions the delay can be very noticeable. Frame Relay (also called Fast Packet Switching) does not store and forward, but simply switches to the destination part way through the frame, reducing the transmission delay considerably. Another problem for the networks is a large requirement for buffering to support the store-and-forward data transfer. One of the reasons that Frame Relay is so cost effective is that storage requirements are minimal. X.25 is a data pump: there has to be some higher level that is making sense of the bits. There are standards for allowing certain applications to make use of X.25. Among them is IBM's QLLC protocol that defines how SNA traffic can be carried over X.25 networks. Another is the asynchronous X.25 PAD. Sangoma supports asynchronous virtual PAD implementations under UNIX® type operating systems such as SCO® UNIX, Linux® and BSDI® Unix. We also support IP and IPX routing over X.25 as part of our WANPIPE™ packages. X.25 and TCP/IP are similar in that they are both packet switched protocols. However, they differ in a number of areas: TCP/IP has only end-to end error checking and flow control, while X.25 is error checked from node to node. TCP/IP has a much more complicated flow control and window mechanism than X.25, to compensate for the fact that a TCP/IP network is completely passive. The electrical and link levels are tightly specified in the X.25 specifications, while TCP/IP is designed to travel over many different kinds of media, with many different types of link service (e.g. Ethernet, Frame relay, X.25, A™ , FDDI etc.) . xDSL Pl. see DSL
190 Numbers & Signs 2.5G 2.5G describes the state of wireless technology and capability usually associated with General Packet Radio Services (GPRS) - that is, between the second and third generations of wireless technology. The second generation or 2G-level of wireless is usually identified as Global System for Mobile (GSM) service and the third generation or 3G-level is usually identified as Universal Mobile Telecommunication Service (UMTS). Each generation provides a higher data rate and additional capabilities. There is also a fourth generation (4G) of technology in the planning and research stages. GPRS offers data speeds at 28 Kbps (and possibly higher) and is expected to be introduced in the 2001 through 2003 timeframe.
3G 3G is a short term for third-generation wireless, and refers to near-future developments in personal and business wireless technology, especially mobile communications. This phase is expected to reach maturity between the years 2003 and 2005. The third generation, as its name suggests, follows the first generation (1G) and second generation (2G) in wireless communications. The 1G period began in the late 1970s and lasted through the 1980s. These systems featured the first true mobile phone systems, known at first as "cellular mobile radio telephone." These networks used analog voice signaling, and were little more sophisticated than repeater networks used by amateur radio operators. The 2G phase began in the 1990s, and much of this technology is still in use. The 2G cell phone features digital voice encoding. Examples include CDMA, TDMA, and GSM. Since its inception, 2G technology has steadily improved, with increased bandwidth, packet routing, and the introduction of multimedia. The present state of mobile wireless communications is often called 2.5G. Ultimately, 3G is expected to include capabilities and features such as: Enhanced multimedia (voice, data, video, and remote control) Usability on all popular modes (cellular telephone, e-mail, paging, fax, videoconferencing, and Web browsing) Broad bandwidth and high speed (upwards of 2 Mbps) Routing flexibility (repeater, satellite, LAN) Operation at approximately 2 GHz transmit and receive frequencies Roaming capability throughout Europe, Japan, and North America
While 3G is generally considered applicable mainly to mobile wireless, it is also relevant to fixed wireless and portable wireless. The ultimate 3G system might be operational from any location on, or over, the earth's surface, including use in homes, businesses, government offices, medical establishments, the military, personal and commercial land vehicles, private and commercial watercraft and marine craft, private and commercial aircraft (except where passenger use restrictions apply), portable (pedestrians, hikers, cyclists, campers), and space stations and spacecraft. Proponents of 3G technology promise that it will "keep people connected at all times and in all places." Researchers, engineers, and
191 marketeers are faced with the challenge of accurately predicting how much technology consumers will actually be willing to pay for. (Recent trends suggest that people sometimes prefer to be disconnected, especially when on vacation.) Another concern involves privacy and security issues. As technology becomes more sophisticated and bandwidth increases, systems become increasingly vulnerable to attack by malicious hackers (known as crackers) unless countermeasures are implemented to protect against such activity.
2600 2600 is the frequency in hertz (cycles per second) that AT&T formerly put as a steady signal on any long-distance telephone line that was not currently in use. Prior to widespread use of out-of-band signaling, AT&T used in-band signaling, meaning that signals about telephone connections were transmitted on the same line as the voice conversations. Since no signal at all on a line could indicate a pause in a voice conversation, some other way was needed for the phone company to know when a line was free for use. So AT&T put a steady 2600 hertz signal on all free lines. Knowing this, certain people developed a way to use a whistle or other device to generate a 2600 hertz tone on a line that was already in use, making it possible to call anywhere in the world on the line without anyone being charged. Cracking the phone system became a hobby for some in the mostly under-20 set who came to be known as phreaks. In the 1960s, a breakfast cereal named Captain Crunch included a free premium: a small whistle that generated a 2600 hertz signal. By dialing a number and then blowing the whistle, you could fool the phone company into thinking the line was not being used while, in fact, you were now free to make a call to any destination in the world. Today, long-distance companies use Signaling System 7, which puts all channel signals on a separate signaling channel, making it more difficult to break into the phone system.
4G 4G is the short term for fourth-generation wireless, the stage of broadband mobile communications that will follow the still-burgeoning third generation (3G) that is expected to reach maturity between 2003-2005. 4G services are expected to be introduced first in Japan, as early as 2006 - four years ahead of the previous target date. The major distinction of 4G over 3G communications is increased data transmission rates, just as it is for 3G over 2G and 2.5G (the present state of wireless services, hovering somewhere between 2G and 3G). According to NTT- DoCoMo, the leading Japanese wireless company, the current download speed for i-Mode (mobile internet service) data is - theoretically - 9.6 Kbps, although in practice the rates tend to be slower. 3G rates are expected to reach speeds 200 times that, and 4G to yield further increases, reaching 20-40 Mbps (about 10-20 times the current rates of ADSL service). 4G is expected to deliver more advanced versions of the same improvements promised by 3G, such as enhanced multimedia, smooth streaming video, universal access, and portability across all types of devices. Industry insiders are reluctant to predict the direction that less- than-immediate future technology might take, but 4G enhancements are expected to include worldwide Roaming capability. As was projected for the ultimate 3G
192 system, 4G might actually connect the entire globe and be operable from any location on - or above - the surface of the earth.
802.11 802.11 is a family of specifications for wireless local area networks (WLANs) developed by a working group of the Institute of Electrical and Electronics Engineers (IEEE). There are currently four specifications in the family: 802.11, 802.11a, 802.11b, and 802.11g. All four use the Ethernet protocol and CSMA/CA (carrier sense multiple access with collision avoidance) for path sharing. The most recently approved standard, 802.11g, offers wireless transmission over relatively short distances at up to 54 megabits per second (Mbps) compared with the 11 megabits per second of the 802.11b standard. Like 802.11b, 802.11g operates in the 2.4 GHz range and is thus compatible with it. The 802.11b standard - often called Wi-Fi - is backward compatible with 802.11. The modulation used in 802.11 has historically been phase-shift keying (PSK). The modulation method selected for 802.11b is known as complementary code keying (CCK), which allows higher data speeds and is less susceptible to multipath-propagation interference. The 802.11a specification applies to wireless ATM systems and is used in access hubs. 802.11a operates at radio frequencies between 5 GHz and 6 GHz. It uses a modulation scheme known as orthogonal frequency-division multiplexing (OFDM) that makes possible data speeds as high as 54 Mbps, but most commonly, communications takes place at 6 Mbps, 12 Mbps, or 24 Mbps.
802.11a Pl. see 802.11
802.15 802.15 is a communications specification that was approved in early 2002 by the Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA) for wireless personal area networks (WPANs). The initial version, 802.15.1, was adapted from the Bluetooth specification and is fully compatible with Bluetooth 1.1. Bluetooth is a well-known and widely used specification that defines parameters for wireless communications among portable digital devices including notebook computers, peripherals, cellular telephones, beepers, and consumer electronic devices. The specification also allows for connection to the Internet. The IEEE 802.15 Working Group proposes two general categories of 802.15, called TG4 (low rate) and TG3 (high rate). The TG4 version provides data speeds of 20 Kbps or 250 Kbps. The TG3 version supports data speeds ranging from 11 Mbps to 55 Mbps. Additional features include the use of up to 254 network devices, dynamic device addressing, support for devices in which latency is critical, full handshaking, security provisions, and power management. There will be 16 channels in the 2.4- GHz band, 10 channels in the 915-MHz band, and one channel in the 868-MHz band. The IEEE plans to refine the 802.15 specification to work with the Specification and Description Language (SDL), particularly SDL-88, SDL-92, and SDL-2000 updates of the International Telecommunication Union (ITU) recommendation Z.100.
802.1X
193 The 802.1X standard is designed to enhance the security of wireless local area networks (WLANs) that follow the IEEE 802.11 standard. 802.1X provides an authentication framework for wireless LANs, allowing a user to be authenticated by a central authority. The actual algorithm that is used to determine whether a user is authentic is left open and multiple algorithms are possible. 802.1X uses an existing protocol, the Extensible Authentication Protocol (EAP, RFC 2284), that works on Ethernet, token ring, or wireless LANs, for message exchange during the authentication process. In a wireless LAN with 802.1X, a user (known as the supplicant) requests access to an access point (known as the authenticator). The access point forces the user (actually, the user's client software) into an unauthorized state that allows the client to send only an EAP start message. The access point returns an EAP message requesting the user's identity. The client returns the identity, which is then forwarded by the access point to the authentication server, which uses an algorithm to authenticate the user and then returns an accept or reject message back to the access point. Assuming an accept was received, the access point changes the client's state to authorized and normal traffic can now take place. The authentication server may use the Remote Authentication Dial-In User Service (RADIUS), although 802.1X does not specify it.
802.3 802.3 is a standard specification for Ethernet, a method of physical communication in a local area network (LAN), which is maintained by the Institute of Electrical and Electronics Engineers (IEEE). In general, 802.3 specifies the physical media and the working characteristics of Ethernet. The original Ethernet supports a data rate of 10 megabits per second (Mbps) and specifies these possible physical media: 10BASE-2 (Thinwire coaxial cable with a maximum segment length of 185 meters) 10BASE-5 (Thickwire coaxial cable with a maximum segment length of 500 meters) 10BASE-F (optical fiber cable) 10BASE-T (ordinary telephone twisted pair wire) 10BASE-36 (broadband multi-channel coaxial cable with a maximum segment length of 3,600 meters)
This designation is an IEEE shorthand identifier. The "10" in the media type designation refers to the transmission speed of 10 Mbps. The "BASE" refers to baseband signalling, which means that only Ethernet signals are carried on the medium (or, with 10BASE-36, on a single channel). The "T" represents twisted-pair; the "F" represents fiber optic cable; and the "2", "5", and "36" refer to the coaxial cable segment length (the 185 meter length has been rounded up to "2" for 200). Also see 100BASE-T and Gigabit Ethernet.
10-Gigabit Ethernet 10-Gigabit Ethernet, being standardized in IEEE 802.3a, is a developing telecommunication technology that offers data speeds up to 10 billion bits per second. Built on the Ethernet technology used in most of today's local area networks (LANs), 10-Gigabit Ethernet is described as a "disruptive" technology that
194 offers a more efficient and less expensive approach to moving data on backbone connections between networks while also providing a consistent technology end-to- end. Using optical fiber, 10-Gigabit Ethernet can replace existing networks that use ATM switches and SONET multiplexers on an OC-48 SONET ring with a simpler network of 10-Gigabit Ethernet switches and at the same time improve the data rate from 2.5 Gbps to 10 Gbps. 10-Gigabit Ethernet is expected to be used to interconnect local area networks (LANs), wide area networks (WANs), and metropolitan area networks (MANs). 10-Gigabit Ethernet uses the familiar IEEE 802.3 Ethernet media access control (MAC) protocol and its frame format and size. Like Fast Ethernet and Gigabit Ethernet, 10-Gigabit Ethernet uses full-duplex transmission, which makes possible a considerable distance range. On multimode fiber, 10-Gigabit Ethernet will support distances up to 300 meters; on single mode fiber, it will support distances up to 40 kilometers. Smaller Gigabit Ethernet networks can feed into a 10-Gigabit Ethernet network.
10BASE5 10BASE-5, one of several physical media specified by IEEE 802.3 for use in an Ethernet local area network (LAN), consists of Thickwire coaxial cable with a maximum segment length of 500 meters. Like other specified media, 10BASE-2 supports Ethernet's 10 Mbps data rate. In addition to 10BASE-5, 10 megabit Ethernet can be implemented with these media types: 10BASE-2 (Thinwire coaxial cable with a maximum segment length of 185 meters) 10BASE-F (optical fiber cable) 10BASE-T (ordinary telephone twisted pair wire) 10BASE-36 (broadband multi-channel coaxial cable with a maximum segment length of 3,600 meters)
This designation is an Institute of Electrical and Electronics Engineers (IEEE) shorthand identifier. The "10" in the media type designation refers to the transmission speed of 10 Mbps. The "BASE" refers to base band signaling, which means that only Ethernet signals are carried on the medium (or, with 10BASE-36, on a single channel). The "T" represents twisted-pair; the "F" represents fiber optic cable; and the "2", "5", and "36" refer to the coaxial cable segment length (the 185 meter length has been rounded up to "2" for 200). Also see 100BASE-T and Gigabit Ethernet.
10BASEF 10BASE-F, one of several physical media specified by IEEE 802.3, is the use of optical fiber in an Ethernet local area network (LAN). Like other specified media, 10BASE-F supports Ethernet's 10 Mbps data rate. In addition to 10BASE-F, 10 megabit Ethernet can be implemented with these media types: 10BASE-2 (Thinwire coaxial cable with a maximum segment length of 185 meters)
195 10BASE-5 (Thicknet coaxial cable with a maximum segment length of 500 meters) 10BASE-T (ordinary telephone twisted pair wire) 10BASE-36 (broadband multi-channel coaxial cable with a maximum segment length of 3,600 meters)
This designation is an Institute of Electrical and Electronics Engineers (IEEE) shorthand identifier. The "10" in the media type designation refers to the transmission speed of 10 Mbps. The "BASE" refers to baseband signalling, which means that only Ethernet signals are carried on the medium (or, with 10BASE-36, on a single channel). The "T" represents twisted-pair; the "F" represents fiber optic cable; and the "2", "5", and "36" refer to the coaxial cable segment length (the 185 meter length has been rounded up to "2" for 200). Also see 100BASE-T and Gigabit Ethernet.
10BASET 10BASE-T, one of several physical media specified in the IEEE 802.3 standard for Ethernet local area networks (LANs), is ordinary telephone twisted pair wire. 10BASE-T supports Ethernet's 10 Mbps transmission speed. In addition to 10BASE-T, 10 megabit Ethernet can be implemented with these media types: 10BASE-2 (Thinwire coaxial cable with a maximum segment length of 185 meters) 10BASE-5 (Thickwire coaxial cable with a maximum segment length of 500 meters) 10BASE-F (optical fiber cable) 10BASE-36 (broadband coaxial cable carrying multiple baseband channels for a maximum length of 3,600 meters)
This designation is an Institute of Electrical and Electronics Engineers (IEEE) shorthand identifier. The "10" in the media type designation refers to the transmission speed of 10 Mbps. The "BASE" refers to baseband signalling, which means that only Ethernet signals are carried on the medium. The "T" represents twisted-pair; the "F" represents fiber optic cable; and the "2", "5", and "36" refer to the coaxial cable segment length (the 185 meter length has been rounded up to "2" for 200).
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