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Home , Analog telephone adapter, Digital Signal 1, IP PBX, Voice over IP

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Printed in the USA. Contents

Introduction—What is VoIP? ...... 6 DDNS ...... 9 VoIP Standards ...... 6 DHCP ...... 9 VoIP Glossary ...... 6 Digital subscriber level 1 channel ...... 9 A-law ...... 6 Digital subscriber level zero ...... 9 ADSL ...... 6 Digital subscriber ...... 9 ADSL2+ ...... 6 access multiplexer ...... 9 Aggregate ...... 6 Discrete MultiTone ...... 9 American gauge ...... 6 DMT ...... 9 Analog ...... 6 DNS ...... 9 Asymmetric digital subscriber line ...... 6 Domain name service ...... 9 Asynchronous communication ...... 7 Domain name system ...... 9 Asynchronous transfer mode ...... 7 Downstream ...... 9 ATA ...... 7 DS0 ...... 9 ATM ...... 7 DS1 ...... 9 Audio menu ...... 7 DSL ...... 10 ARU ...... 7 DSLAM ...... 10 Audio response unit ...... 7 DTMF ...... 10 Audio teleconferencing or audio conferencing ...... 7 Dual tone multi-frequency ...... 10 AWG ...... 7 Dynamic DNS ...... 10 BLEC ...... 7 Dynamic domain naming system ...... 10 Broadband ...... 7 Dynamic host configuration protocol ...... 10 Building local exchange carrier ...... 7 E1 ...... 10 Cat 5 ...... 7 EFM ...... 10 CCITT ...... 8 management system ...... 10 Central office ...... 8 EMS ...... 10 CLE ...... 8 Enterprise session border controller ...... 10 CLEC ...... 8 eSBC ...... 10 CLI ...... 8 ...... 10 CO ...... 8 Ethernet demarcation ...... 10 ...... 8 Ethernet extenders ...... 10 Coder/decoder ...... 8 Ethernet in the first mile ...... 11 Comfort noise ...... 8 Ethernet telephone ...... 11 Comité Consultatif Internationale de Téléphonie et de Fax server ...... 11 Télégraphie ...... 8 First mile ...... 11 Command line interface ...... 8 Foreign exchange office ...... 11 Competitive local exchange carrier ...... 8 Foreign exchange station ...... 11 Conference bridge ...... 8 ...... 11 CPE ...... 9 Full duplex ...... 11 Customer located equipment ...... 9 FXO ...... 11 Customer premise equipment ...... 9 FXS ...... 11

3 VoIP Glossary—Terms and Definitions

G.711 ...... 11 LEC ...... 14 G.723.1 ...... 11 Local exchange carrier ...... 15 G.729 ...... 11 Loopback ...... 15 G.dmt ...... 12 Main distribution frame ...... 15 G.lite ...... 12 Management information base ...... 15 G.SHDSL ...... 12 MCU ...... 15 Gatekeeper ...... 12 MDF ...... 15 Gateway—VoIP ...... 12 MDU ...... 15 GRE ...... 12 Mean opinion score ...... 15 GSM 6.10 ...... 12 Messaging ...... 15 GUI ...... 12 MHU ...... 15 H.245 tunneling ...... 12 MIB ...... 15 H.323 ...... 12 Microfilter ...... 15 HTTP ...... 12 ...... 15 HTTPS ...... 12 Modulator/demodulator ...... 15 I-Phone ...... 12 MOS ...... 15 IAC ...... 12 MSAR ...... 16 ILEC ...... 13 MSBR ...... 16 Incumbent local exchange carrier ...... 13 MTU ...... 16 Integrated access concentrator ...... 13 Multiplex ...... 16 IVR ...... 13 Multi-commercial unit ...... 16 Interactive voice response ...... 13 Multi-dwelling unit ...... 16 International Union ...... 13 Multi-hospitality unit or multi-hotel unit ...... 16 ...... 13 Multi-service access ...... 16 ...... 13 Multi-service business router ...... 16 Internet service provider ...... 13 Multi-tenant unit ...... 16 Internet telephone ...... 13 Network functions virtualization ...... 16 Internetwork ...... 13 Network management system ...... 16 Interoperability ...... 13 Network operations center ...... 16 ...... 13 provider ...... 16 Internet ...... 13 NFV ...... 16 IP ...... 14 NMS ...... 16 iPhone ...... 14 NOC ...... 16 IP-PBX ...... 14 ...... 16 IP address ...... 14 NSP ...... 16 IP phone ...... 14 OAM ...... 16 IP streaming ...... 14 OC-3 ...... 16 IP telephony ...... 14 OC-12 ...... 16 IPsec ...... 14 Optical Carrier-3 ...... 16 ISP ...... 14 Optical Carrier-12 ...... 16 ITU ...... 14 PABX ...... 17 ...... 14 Packet ...... 17 ...... 14 ...... 17 Layer ...... 14 Packet transfer mode ...... 17

4 VoIP Glossary—Terms and Definitions

PBX ...... 17 SIP ...... 20 Plain old telephone system ...... 17 SLA ...... 20 Point of presence ...... 17 ...... 20 Point-to-point protocol ...... 17 ...... 20 Point-to-point protocol over Ethernet ...... 17 SOHO or SoHo ...... 20 PoP ...... 17 SONET ...... 20 POP ...... 17 Speech recognition ...... 20 Post office protocol ...... 17 ...... 20 Post, telegraph, and telephone ...... 17 T1 ...... 20 POTS ...... 17 T3 ...... 20 POTS splitter ...... 17 TC-PAM ...... 21 PPP ...... 17 TCP ...... 21 PPPoE ...... 18 Telco ...... 21 Private automatic branch exchange ...... 18 Telephony ...... 21 Private branch exchange ...... 18 Touch tone ...... 21 Protocol ...... 18 TR-069 ...... 21 PSTN ...... 18 Traffic shaping ...... 21 PSTN backup ...... 18 Transmission control protocol ...... 21 PSTN failover ...... 18 µ-law or u-law ...... 21 PSTN (POTS) gateway ...... 18 UNIX ...... 21 PTM ...... 18 Upstream ...... 21 PTT ...... 18 UTP ...... 21 Public switched telephone network ...... 18 VAD ...... 21 Punchdown block ...... 18 VDSL2 ...... 21 QoS ...... 18 Vectoring ...... 21 ...... 18 Very high digital subscriber line 2 ...... 22 RBOC ...... 18 VioIP ...... 22 Real-time ...... 18 Video-over-IP ...... 22 Regional Bell operating company ...... 18 ...... 22 Registered jack-11 ...... 19 Voice quality ...... 22 Registered jack-45 ...... 19 VoIP gateway ...... 22 RJ-11 ...... 19 VoIP router ...... 22 RJ-45 ...... 19 VoIP telephone ...... 22 Router ...... 19 VoIP switch ...... 22 RSVP ...... 19 VPN ...... 22 RTP ...... 19 WAN ...... 22 SDH ...... 19 Web browser ...... 22 Service level agreement ...... 19 Web server ...... 22 Service provider ...... 19 ...... 22 Session initiation protocol ...... 19 World wide web ...... 22 SFP ...... 19 WWW ...... 23 Sidetone ...... 19 X.21 ...... 23 Signaling system #7 (SSY7) ...... 19 Silence suppression ...... 19

5 VoIP Glossary—Terms and Definitions

Introduction—What is VoIP? VoIP Standards Voice over IP or VoIP is a term used in IP telephony • ITU-T H.320 Standards for Video Conferencing for a set of facilities that use the Internet Protocol (IP) • H.323 ITU Standards to deliver voice information. In general, this means sending voice information in digital form in discrete • H.324 ITU Standards packets rather than in the traditional circuit-commit- • VPIM Technical Specification ted protocols of the public switched telephone net- work (PSTN). A major advantage of VoIP and VoIP Glossary Internet telephony is that they avoid the tolls charged by ordinary telephone service. A-law The term VoIP derives from the VoIP Forum, an A-law is an companding format in which effort by major equipment providers, to promote the the is compressed on input and expanded back use of ITU-T H.323, the standard for sending voice to its original form on output. A logarithmic mapping (audio) and video using IP on the public Internet and between 8-bit data space and 13-bit sample space as within an intranet. The Forum also promotes the user described in the CCITT G.711 recommendation.þA- of directory service standards so that users can locate law has 13 bits of dynamic range (78 dB). See also μ- other users and the use of touch-tone signals for auto- law or u-law on page 21 and G.711 on page 11. matic call distribution and voice mail. In addition to IP, VoIP uses the real-time protocol ADSL (RTP) to ensure that packets get delivered in a timely for Asymmetric Digital Subscriber Line. A way. Because the nature of public networks such as the type of DSL utilizing DMT providing a higher down- Internet makes it difficult to guarantee Quality of Ser- stream (to the customer site) than upstream (to the vice (QoS), better service is usually possible with pri- service provider). A single twisted-pair offers up to 12 vate networks managed by an enterprise or by an Mbps downstream and 1.8 Mbps upstream. Internet telephony service provider (ITSP). ADSL2+ A technique used by at least one equipment manufac- turer, Adir Technologies (formerly Netspeak), to help The latest ADSL technology. Like the previous ver- ensure faster packet delivery is to use the ping utility sions of ADSL and ADSL2, ADSL2+ uses DMT pro- to contact all possible network gateway computers viding a higher downstream (to the customer site) that have access to the public network and choose the than upstream (to the service provider). A single fastest path before establishing a Transmission Con- twisted pair offers up to 24 Mbps downstream and trol Protocol (TCP) sockets connection with the 3.3 Mbps upstream. other end. Aggregate Enterprises use VoIP gateways to enter into the VoIP Combining two or more bit streams into a single bit environment. A gateway receives packetized voice stream. transmissions from users within the company and then routes them to other parts of the company’s American wire gauge intranet (local area or wide area network) or—using a See AWG on page 7. T-carrier system or E-carrier interface—sends them over the public switched telephone network. See ATA on page 7.

Asymmetric digital subscriber line See ADSL.

Introduction—What is VoIP? 6 VoIP Glossary—Terms and Definitions

Asynchronous communication Audio teleconferencing or audio conferencing A data communications method in which bits are sent The original technology used for audio teleconferenc- without using a clock signal for synchronization. ing was based on PBX (Private Branch Exchange) Instead, each character is transmitted surrounded by a conferencing circuits. Setting up conference calls start and stop bit that designates the beginning and through the PBX is cumbersome, the voice quality ending points of the information. This as opposed to degrades as the number of people on a call increases, synchronous communication where blocks of data are and there are capacity limitations, so specialized con- transmitted using a synchronizing clock. ference bridges were developed to improve capacity and voice quality. Conference bridges, however, Asynchronous transfer mode require trained operator intervention to schedule and See ATM. invoke most features. As a result, individual corpora- tions found the cost of ownership prohibitive, and the ATA market for such products has been concentrated on Acronym for analog telephone adapter, an ATA enables service bureau providers. Today’s PC-based systems you to use analog phones for VoIP without having to provide the freedom of conference bridges. By install- purchase an IP phone. But an ATA cannot provide ing a conference server on your voice networks, you many of the added features (such as transfer and hold can set up, attend, and manage your own conferences buttons) that an IP phone provides. over any touch-tone telephone. Additionally, users can schedule meetings using desktop software from their ATM e-mail systems, or from a web browser. Acronym for Asynchronous Transfer Mode. A connec- tion-oriented switching and technique AWG using 53-byte cells to transmit different types of data Acronym for American Wire Gauge. An indication of concurrently across a single physical link. ATM fram- wire diameter. The heavier the gauge, the lower the ing carries between 15–40% overhead depending on AWG number, and the lower the impedance. packet size. BLEC Audio menu Acronym for Building Local Exchange Carrier. A tele- A verbal choice provided by a recording over the phone service provider whose equipment and network phone. Audio choice menus are common in auto- are contained within a building or complex. mated attendant, IVR and fax-on-demand systems. They are prompts for caller input. Audio menus can Broadband instruct you to speak commands or press keys on a Broadband is short for broad . When used touch-tone keypad as commands. in a VoIP context, broadband refers to the informa- tion capacity (2 Mbps or higher) of a communication ARU channel. This high-capacity, two-way medium sup- Acronym for Audio Response Unit. A computer tele- ports a wide range of frequencies, typically from audio phony system incorporating voice store-and-forward up to video frequencies. technology. There are passive and interactive ARUs. Passive ARUs simply play out while interac- Building local exchange carrier tive ARUs play messages based on input from callers. See BLEC.

Audio response unit Cat 5 See ARU. Short for Category 5. A level of unshielded twisted- pair wiring performance as defined by EIA/TIA-568.

VoIP Glossary 7 VoIP Glossary—Terms and Definitions

Cat 5 cable is used for transmission speeds up to 1000 software algorithm that converts an audio signal into a Mbps (1 Gbps). digital format suitable for transportation using a spe- cific set of protocols. Equipment on the receiving end, CCITT which must also use these same protocols, restores the Acronym for the Comité Consultatif Internationale de digital signals back to audio (or analog) format. Téléphonie et de Télégraphie, an organization based in differ based on the quality of the sound they Geneva, Switzerland that develops world-wide data produce, how much bandwidth they require, the pro- communications standards. CCITT is part of the cessing power needed, and so on. The main voice ITU. Three main sets of standards have been estab- compression schemes used in VoIP networks are: lished: CCITT Groups 1-4 standards apply to facsim- G.711, which is the same compression used on tradi- ile transmissions; the CCITT V series of standards tional wired telephone networks and used 64-kbps apply to and error detection and correction coding; G.729, which compresses voice to an 8-kbps methods; and the CCITT X series standards apply to rate but still delivers near toll quality voice over most local area networks. networks; and G.723, which compresses voice at 6.3 or 5.3-kbps, but also has the poorest quality compared Central office to the other two codecs. See CO. Coder/decoder CLE See Codec. Acronym for Customer Located Equipment. Comfort noise CLEC See Silence suppression on page 19. Acronym for competitive local exchange carrier, tele- communications carriers that provide local exchange Comité Consultatif Internationale de Téléphonie et service in competition with an ILEC, using just the de Télégraphie CLEC’s own switching and network or a combination See CCITT. of the CLEC’s switching facilities and the CLEC’s network facilities or an ILEC’s unbundled network Command line interface facilities. Examples of CLECs are Teleport, GTE, See CLI. and AT&T. Competitive local exchange carrier CLI See CLEC. Acronym for command line interface. Configuration interface for devices such as CPE’, IAD’s, etc. Conference bridge A device used to connect multiple parties over the CO phone. A proctor or operator can man conference Acronym for central office, a telco facility that handles bridges or they can be supervised. There are stand- the switching of telephone calls on the PSTN for a alone conference bridges and conference bridge func- small regional area. The CO houses servers, storage tions built in to some PBXs (Private Branch systems, switching equipment, emergency power sys- Exchange). These systems have circuitry for summing tems, and related devices that are used to run tele- and balancing the energy (noise) on each channel so phone systems. everyone can hear each other. More sophisticated con- ference bridges have the ability to “idle” the transmit Codec side of channels of non-speaking parties. An acronym for coder/decoder (also defined as compres- sor/decompressor). In a VoIP context, a codec is a DSP

VoIP Glossary 8 VoIP Glossary—Terms and Definitions

CPE Digital subscriber level 1 channel Acronym for customer premise equipment, a CPE is a See DS1 on page 9. networking device, such as a modem, POTS splitter, or other device, that is installed at a customer site. Digital subscriber level zero CPEs, which terminate the telco or broadband net- See DS0. work, pass communications streams to VoIP gateways, switches, routers, PBXs, DSUs or CSUs, telephone Digital subscriber line sets, personal computers, set-top boxes, and other See DSL on page 10. devices at the customer’s premises. Digital subscriber line access multiplexer Customer located equipment See DSLAM on page 10. See CLE on page 8. Discrete MultiTone Customer premise equipment See DMT. See CPE. DMT DDNS Acronym for Discrete MultiTone. DSL technology An acronym for dynamic domain naming system or using digital signal processors to divide the signal into dynamic DNS. When you connect via broadband or 256 sub-channels. (Ex: ADSL, VDSL) ISDN to your ISP, you are normally assigned a dynamic IP address. Depending on how your ISP has DNS set it up, this address can change each time you con- Acronym for domain name system or domain name ser- nect to the ISP or when a time limit expires. If you are vice, DNS is a process that maps hostnames (which running a local server, when the IP address changes, are more easily remembered than numbered remote users will not be able to locate your new IP addresses) to IP addresses. address. Some people pay their ISPs for a static IP address that never changes. Others use various Domain name service services on the Internet that provide dynamic DNS See DNS. service. Once registered, you choose a name (myhost- name.dyndns.org for example) that is mapped to your Domain name system current IP address so remote users can get to your net- See DNS. work via that address. Thereafter, when a new IP address is assigned, software on your server automati- Downstream cally notifies the DDNS service of the change, at In the direction toward the customer premises. which the service remaps to the new address, so remote users never notice the change. DS0 Short for Digital Subscriber Level Zero. A 64-kbps unit DHCP of transmission bandwidth. A worldwide standard Acronym for dynamic host configuration protocol, a speed for digitizing one voice conversation, and more method for automatically assigning IP addresses to recently, for data transmission. Twenty-four DS0s (24 devices on a TCP/IP network. When a new device x 64 kbps) equal one DS1. connects to the network, the DHCP server allocates an IP address from a list of available addresses. The DS1 device keeps this IP address until the session ends. Short for Digital Signal 1. DS1 is the primary digital After the device disconnects, the IP address is released telephone standard used in the United States and and available for use.

VoIP Glossary 9 VoIP Glossary—Terms and Definitions

Japan and is able to transmit up to 24 multiplexed configurations for E1 lines are E1 PRI, and unchan- voice and data calls over telephone lines. nelized E1

DSL EFM Acronym for digital subscriber line, a high speed digital Acronym for Ethernet in the First Mile. Uses one of the switched service that uses existing copper pairs to con- Ethernet family network protocols between a telecom- nect subscriber CPE (customer premises equipment) munications company and a customer’s premises. to the CO (central office). DSL handles more data From the customer’s point of view it is their “first” downstream (data flowing towards the subscriber) mile, although from the ’s point of view than upstream (flowing towards the network). it is known as the “last mile”.

DSLAM Element management system Acronym for digital subscriber line access multiplexer. See EMS. Also known as an IAC (Integrated Access Concentrator). A piece of equipment located in the Central Office EMS (CO) that combines (or multiplexes) multiple DSL Acronym for Element Management System. An EMS subscriber lines into a single high-speed connection consists of systems and applications for managing net- (usually an OC-3 or OC-12 optical trunk). When the work elements (NE) on the network element-manage- telco receives a DSL signal, an xDSL modem with a ment layer (NEL) of the POTS splitter detects and routes voice calls and data. management network (TMN). It sends voice calls to the PSTN and data to the DSLAM, where it passes through the ATM to the Enterprise session border controller Internet, then back through the DSLAM and xDSL See eSBC. modem before returning to the customer’s computer eSBC DTMF Acronym for Enterprise Session Border Controller. Con- Acronym for dual tone multi-frequency, the signal a trols the communication between different networks, telephone company receives when a telephone’s typically between customers LAN and WAN. touchpad keys are pressed. Also known as Touch Tone. Ethernet Dual tone multi-frequency A type of network that supports high-speed commu- See DTMF. nication among systems. It is a widely implemented standard for LANs. Dynamic DNS See DDNS on page 9. Ethernet demarcation Provides a clear separation between the user and the Dynamic domain naming system network, allowing carriers to extend network visibility See DDNS on page 9. up to the user premises.

Dynamic host configuration protocol Ethernet extenders See DHCP on page 9. Ethernet signals begin to degrade beyond 100 meters (328 feet). To achieve longer distances, an Ethernet E1 extender is required. Ethernet Extenders can be used A 2.048 Mbps signal that supports thirty-two 64 kbps to drive Ethernet up to 10 kilometers (6.4 miles) over timeslots, at least 30 of which can transmit and copper. Extenders are also commonly used to provide receive data or digitized voice. The most common Ethernet over a single voice-grade twisted-pair as

VoIP Glossary 10 VoIP Glossary—Terms and Definitions opposed to Cat 5 or higher rated cables. These devises FXO are typically transparent to higher layer protocols. Acronym for foreign exchange office. A telephone sig- naling interface that generates off-hook and on-hook Ethernet in the first mile indications at the foreign exchange station (FXS) at See EFM. the end of a telephone circuit.

Ethernet telephone FXS See IP phone on page 14. Acronym for foreign exchange station. Telecom equip- ment instance that delivers line power, generates dial- Fax server tone and voltage to a subscriber device (tele- A computer based fax machine. Fax servers are “shared phone). use” devices, typically installed on a LAN. Clients on the LAN can use the fax server from their PCs in G.711 much the same way they share a network-based G.711 is an ITU-T standard for audio compression (shared) printer. can be generated by users at that is mostly used in telephony. The standard defines their workstations and “printed” to the fax server for two algorithms: μ-law algorithm (used in America) transmission. Likewise, fax servers can route incoming and a-law algorithm (used in Europe and the rest of faxes to printers, file server directories, or to individ- the world). Both are logarithmic, but a-law was specif- ual users. Fax servers save users from having to print ically designed for improved computer processing. documents, carry them to the fax machine, and subse- quently wait for them to be transmitted after creating G.723.1 a cover page. The G.723.1 codec is commonly used in VoIP appli- cations because of its low-bandwidth requirements. First mile G.729 compresses voice audio in 30-millisecond seg- Sometimes referred to as Local Loop, the final leg of ments. Music or tones (DTMF Touch Tone or fax delivering communications connectivity to a resident tones, for example) cannot be transmitted reliably or customer. with this codec, so G.711 or out-of- methods are used instead to carry these signals. G.723.1 supports Foreign exchange office 6.3-kbps data rates (in 24-bytes segments) and 5.3- See FXO. kbps in 20-byte segments.

Foreign exchange station G.729 See FXO. The G.729 codec is commonly used in VoIP applica- tions because of its low-bandwidth requirements. Frame relay G.729 compresses voice audio in 10-millisecond seg- In data communications, Frame Relay is a packet ments. Music or tones (DTMF Touch Tone or fax switching method that uses available bandwidth only tones, for example) cannot be transmitted reliably when it is needed. This fast packet switching method with this codec, so G.711 or out-of-band methods are is efficient enough to transmit voice communications used instead to carry these signals. Standard G.729 with the proper network management. operates at 8 kbps, but G.729 has two variants: Annex A (G.729A) which is less processor intensive and Full duplex allows double the number of calls as plain G.729 and In telephony and data communications, full duplex Annex B (G.729B) which adds voice activity detection means the ability for both ends of a communication (VAD) and comfort noise generation (CNG) which to simultaneously send and receive information with- work together to reduce bandwidth used. You can out degrading the quality of the content. combine Annex B with G.729A to give G.729AB.

VoIP Glossary 11 VoIP Glossary—Terms and Definitions

The G.729 variants can generally interoperate with GSM 6.10 each othe but there are modified versions that provide Short for Groupe Speciale Mobile 6.10 or Global System and 11.8-kbps rates for better speech quality. Also for Mobile Communications 6.10, GSM 6.10 is a lossy very common is G.729A which is compatible with CBR (constant bit rate) codec in which data is com- G.729, but requires less computation (at the cost of pressed into a stream at a fixed rate. GSM 6.10 has a degraded speech quality). sample rate of 8k samples per second and a data rate of 13 kbps. GSM is also a popular global mobile G.dmt phone standard. A name for the line modulation specified by ITU rec- ommendation G.992.1. GUI Acronym for Graphical User Interface. Mostly used for G.lite CPE type device configuration A name for the line modulation specified by ITU rec- ommendation G.992.2. H.245 tunneling H.245 tunneling is the encapsulation of H.245 mes- G.SHDSL sages within H.225/Q.931 messages. If you have a G.SHDSL, or SHDSL, is a standardized method and enable H.245 tunneling, there is one less (ITU-T G.991.2) to transport symmetrical data rates TCP port that you need to allow for on copper pair access lines. G.SHDSL offers bit rates incoming connections. from 192 kbps to 15.3 Mbps over a 2-wire single pair and up to 60 Mbps over four bonded pairs H.323 H.323 is the ITU standard for the transmission of Gatekeeper real-time audio, video and data information over This component of H.323 manages the bandwidth packet switching-based networks. Such networks inbound and outbound from the LAN. Gatekeepers include IP-based Internet packet exchange-based local register clients and coordinate communications with area networks, enterprise networks, and metropolitan other gatekeepers. A gatekeeper performs the follow- and wide area networks. ing functions: admission control for authorizing cli- ents’ access to the LAN; bandwidth control for each HTTP network segment managed; client network address Acronym for Hyper Text Transfer Protocol. An applica- translation so users can dial network locations with tion-level protocol for the transmission of informa- easily remembered aliases (such as e-mail addresses) tion in distributed systems. instead of IP addresses; and call management that monitors H.323 calls and tracks rejected calls. HTTPS Acronym for Hyper Text Transfer Protocol Secure. The Gateway—VoIP secure version of HTTP, the protocol over which data Device that is converting traditional telephony in to is sent between your browser and the website to which VoIP. For instance, T1 ISDN lines to be converted you are connected. over to VoIP using SIP or H.323 I-Phone GRE See IP phone on page 14. Acronym for Generic Routing Encapsulation A proto- col that encapsulates other protocols in order to route IAC them over IP networks Acronym for Integrated Access Concentrator. An access gateway providing aggregation and switching for mul- tiple DSL connections.

VoIP Glossary 12 VoIP Glossary—Terms and Definitions

ILEC upon millions of computers and subnetworks— Acronym for incumbent local exchange carrier, the U.S. almost entirely supported by commercial funds except local telephone companies that comprised the seven in countries where deregulation has not occurred. The Bell Operating Companies (such as Pacific Bell or Internet is the substrate and chief communications GTE) until the 1996 Telecommunications Act estab- backbone for the WWW. lished CLECs. Also called Baby Bells or dominant car- riers. See also RBOC on page 18. Internet protocol See IP. Incumbent local exchange carrier See ILEC. Internet service provider See ISP on page 14. Integrated access concentrator See IAC. Internet telephone See IP phone on page 14. IVR Acronym for Interactive voice response. In computer Internetwork telephony, IVR is a horizontal application wherein An interconnected group of networks (also called an computer-based information is accessed over the internet). phone by using a telephone instead of a computer. An IVR platform uses computer telephony components Interoperability to translate callers’ touch-tones or voice commands The ability of equipment from different vendors to into computer queries after the callers listen to an communicate using common protocols. audio menu. For example: “Please enter your account number using the touch-tones on your telephone.” Intranet These queries are then “fetched” by the IVR platform A private network or internet using Internet standards from the host computer. In some cases, the informa- and software, but protected from public access. tion resides in the same platform (self-hosted). The information is converted into voice commands that Internet telephony are spoken over the phone to the caller. Any means of transmitting the human voice (real- time or near real-time) over the Internet. There are Interactive voice response several components: 1) On the client side, a multime- See IVR. dia-equipped PC with special client software will digi- tize your voice. This can be done with a voice modem International Telecommunication Union or other voice encoding method; 2) A direct or dial- See ITU on page 14. up connection to the Internet allows your voice to be transmitted in packet form to its destination; 3) Con- Internet nection with the far side is achieved by IP address The Internet consists of the world’s combined public search, common servers or to identify the IP-based packet-switched networks. The Internet is called party (and to “ring” that person’s phone); 4) A an outgrowth and combination of a variety of univer- similar arrangement on the far end completes the call sity and government sponsored computer networks. and allows both parties to speak. There are also Federal and private sector subsidies supported the PSTN/Internet gateways that allow regular telephone DARPA-NET, NSFnet (National Sciences Founda- callers to make phone-to-Internet-to-phone connec- tion,) and thousands of other subnetworks, which tions. There are PC-to-phone connections and were used to do inter-agency research and communi- phone-to-PC connections. cation. Today, the Internet is made up of millions

VoIP Glossary 13 VoIP Glossary—Terms and Definitions

IP ISP Acronym for Internet Protocol. An open networking Acronym for Internet service provider, a business that protocol used for internet packet delivery, IP specifies provides subscriber-based access to the Internet. Sub- the format of packets, also called datagrams, and the scribers can be individuals or businesses. According to addressing scheme. IPv4 is for IP version 4 (RFC 791) Jack Rickard, publisher of Boardwatch Magazine, and IPv6 is for IP version 6 (RFC 2460) ISPs operate at the fourth or lowest level of the Inter- net. At the third level, regional providers aggregate iPhone traffic from lower-order ISPs to the second, backbone See IP phone. level. The highest level in North America is the NAP (network access point), which acts as peer-to-peer IP-PBX points for the largest backbones. An IP-PBX is a business phone system that manages There are three ”official” NAPs located in San Fran- throughout the enterprise and acts as a cisco, California; Chicago, Illinois; and Pennsauken, gateway to data and voice networks. An IP-PBX is a New Jersey. ISPs use Internet routers, servers and combination switch/router and PBX that handles Rrack-mounted modems to provide a variety of ser- VoIP so you can place calls using over a packet- vices, including web site hosting, FTP service, e-mail switched network instead of the circuit- accounts, unified messaging, audio and video broad- switched PSTN. casting, and—in some cases—Internet telephony and fax gateway services. IP address The Internet protocol (IP) address (also called the ITU Internet address) is the numeric address—formatted as Acronym for the International Telecommunication four sets of numbers separated by periods Union, an agency of the United Nations based in (204.171.64.2, for example)—of a computer attached Geneva, Switzerland. The ITU sets standards related to a TCP/IP network. Every client and server station to facsimile communications. Previously the ITU was must have a unique IP address. Client workstations known as the CCITT. have a permanent (or static) address or one that is dynamically assigned to them each dial-up session. Jitter Jitter is a measure of variations in the delays that occur IP phone while delivering voice packets from sending to receiv- IP phones work and look like regular telephones, but ing ends. they have the ability to connect to the Internet and make cost-saving long-distance calls. Latency Latency or delay, is a measure of how much time it IP streaming video takes for a packet of data to get from one point to See VioIP on page 22. another. Together, latency and bandwidth define the speed and capacity of a network. IP telephony See Internet telephony on page 13. Layer A logical level in the Open Systems Interconnection IPsec model. Short for Internet Protocol Security. IPsec is a network protocol suite that authenticates and encrypts the LEC packets of data sent over a network. Acronym for local exchange carrier, A company that provides intra-LATA (local access transport area) tele- communications services.

VoIP Glossary 14 VoIP Glossary—Terms and Definitions

Local exchange carrier MHU See LEC. Acronym for Multi-Hospitality Unit or Multi-Hotel Unit. A hotel or motel. Loopback A diagnostic procedure that sends a test message back MIB to its point. Used to test various portions Acronym for Management Information Base. A data- of a data link in order to isolate an equipment or data base of managed objects used by SNMP to provide line problem. network management information and device control.

Main distribution frame Microfilter See MDF. A microfilter (also called a splitter) filters out the broadband signal from the normal phone signal, Management information base enabling you to use both on the same line. It comes See MIB. with two sockets, one for the broadband router or modem and one for the phone, fax, set top box, or MCU answering machine. Acronym for Multi-Commercial Unit. A building or complex with commercial tenants, such as an office Modem building or shopping mall. A modem (modulator/demodulator) is equipment that converts digital signals to analog signals and vice- MDF versa. Modems are used to send data signals (digital) Acronym for Main Distribution Frame. The point over the telephone network, which is usually analog. where all local loops are terminated at a central office. A modem modulates binary signals into tones that can be carried over the telephone network. At the MDU other end, the demodulator part of the modem con- Acronym for Multi-Dwelling Unit. A building hous- verts the tones to binary code. ing multiple residences, such as an apartment build- ing. Modulator/demodulator See Modem. Mean opinion score See MOS. MOS Acronym for mean opinion score. Voice quality is Messaging reported as an MOS on a scale from 1 to 5 where 1 is In computer telephony, any means of storing and for- the worst and 5 the best quality. Originally, telephone warding messages. This includes fax mail, voice mail, companies would hire people to listen to test phone and broadcast messaging. This horizontal application calls and rank the quality of those calls. They would is the most popular of all voice solutions. Messaging then take the average or the mean opinion of the lis- systems provide for the storing and forwarding of teners and assign that as the call’s quality. Today, “non-real time” communication. For example, a MOS scores can be arrived at based on network per- recorded voice message can be stored for later play- formance measures such as jitter and latency using an back either locally or remotely, or a fax can be received international telephony standard called the E-Model and stored before it is re-transmitted to the ultimate (the ITU-T G.107 standard). recipient. Messages can vary in content and media type—the distinction being that they are recorded or stored for pick up in the future.

VoIP Glossary 15 VoIP Glossary—Terms and Definitions

MSAR Network operations center Acronym for Multi-Service Access Router. Device that See NOC. integrates at least one WAN technology such as VDSL for multiple services at the customer premise. Network service provider See NSP. MSBR Acronym for Multi-Service Business Router. Customer NFV premise device for multi service purposes, but without Acronym for Network Functions Virtualization. An WAN interface. emerging networking technology in which functional- ity conventionally carried out in dedicated network MTU elements is performed in software hosted on com- Acronym for Multi-Tenant Unit. A building housing puter hardware or virtual machines. multiple tenants, such as an office building. Some- times used as a generic term to encompass Multi- NMS Commercial Units (MCUs), Multi-Dwelling Units Acronym for Network Management System. A com- (MDUs), and Multi-Hospitality Units (MHUs). puter system used for monitoring and controlling net- work devices. Multiplex Multiplexing combines input signals from many NOC sources onto a single communications path, thereby Acronym for Network Operations Center. The point at enabling a single communications channel to carry which a network is monitored and controlled. several messages simultaneously. Node Multi-commercial unit A connection or switching point on the network. See MCU. NSP Multi-dwelling unit Acronym for Network Service Provider. A local tele- See MDU. phone company or ISP that provides network services to subscribers. Multi-hospitality unit or multi-hotel unit See MHU on page 15. OAM Acronym for Operations, Administration and Mainte- Multi-service access router nance. See MSAR. OC-3 Multi-service business router A fiber optic line capable of 155 Mbps of bandwidth. See MSBR. OC-12 Multi-tenant unit A fiber optic line carrying 622.08 Mbps of band- See MTU. width.

Network functions virtualization Optical Carrier-3 See NFV. See OC-3.

Network management system Optical Carrier-12 See NMS. See OC-12.

VoIP Glossary 16 VoIP Glossary—Terms and Definitions

PABX phony platforms such as automated attendants are See PBX. able to route incoming calls automatically, too.

Packet Plain old telephone system A logically grouped unit of data. Packets contain a See POTS. payload (the information to be transmitted), origina- tor, destination, and synchronization information. Point of presence The idea with packets is to transmit them over a net- See PoP. work so each individual packet can be sent along the most optimal route to its destination. Packets are con- Point-to-point protocol structed on one end of the communication and de- See PPP. constructed on the receiving end based on the header addressing information at the front of each packet. Point-to-point protocol over Ethernet Routers in the network will store and forward packets See PPPoE. based on network delays, errors, and re-transmittal requests from the receiving end. PoP An acronym for point of presence. The Network Ser- Packet switching vice Provider’s access point on a Network Access Pro- A means of economically sending and receiving data vider’s network. over multiple network channels. Packet switching takes data and breaks it down into packets—small POP bundles of information containing the payload and Short for post office protocol, an Internet standard for routing information. The packets are then transmit- storage and retrieval of messages. ted to the receiving end, where they are converted back to the original data format. One feature of Post office protocol packet switching is that packets can be received out of See POP. order and then be quickly arranged into the correct order. There are slow packet switching networks—like Post, telegraph, and telephone the old SNA networks—and fast packet networks See PTT. based on Frame Relay and ATM. Although tradition- ally used for data, packet networks—especially well- POTS managed ones—are suitable for real-time transmission An acronym for Plain Old Telephone Service. Standard of voice and video. telephone service over the PSTN, with an analog bandwidth of less than 4 kHz. See also PSTN. Packet transfer mode See PTM on page 18. POTS splitter A device that filters out the DSL signal and allows PBX POTS frequencies to pass through. In telephony, a private branch exchange (PBX) (also called a private automatic branch exchange or PABX) PPP system behaves as a customer’s premises over trunk An acronym for Point-to-Point Protocol. A protocol for lines (thus the term branch). At first, PBXs mimicked packet transmission over serial links, specified by RFC a small telephone company switchboard. Users would 1661. use an operator to make telephone calls to the PSTN (public switched telephone network). Now, users dial directly, without using an operator; computer tele-

VoIP Glossary 17 VoIP Glossary—Terms and Definitions

PPPoE reach their final destination. PTM framing caries An acronym for Point-to-Point Protocol over Ethernet. between 2-9% overhead depending on packet size. A method for establishing sessions and encapsulating PPP packets over an Ethernet, specified by RFC PTT 2516. An acronym for Post, Telegraph, and Telephone. A national communications authority, sometimes gov- Private automatic branch exchange ernment-controlled and monopolistic, which acts as a See PBX on page 17. common carrier.

Private branch exchange Public switched telephone network See PBX on page 17. See PSTN.

Protocol Punchdown block A set of rules that determines the behavior of devices An array of connectors used for connecting cable cir- in achieving and maintaining communication. cuits of a network interface.

PSTN QoS An acronym for Public Switched Telephone Network. A Measure of how well a transmission system performs network shared among many users who can use tele- in terms of transmission quality and service availabil- phones to establish connections between two points. ity. As a concept, QoS means that transmission rates, Also known as dial network. error rates, and other characteristics can be measured, improved, and, within limits, guaranteed in advance. PSTN backup QoS incorporates bandwidth, latency, and jitter to See PSTN failover. describe a network’s ability to customize the treatment of specific classes of data. For example, QoS can be PSTN failover used to prioritize audio transmissions over Web PSTN failover lines are used as backup connections in browsing traffic. case your Internet connection is not available. These are optional ports on ATA devices or IP phones that Quality of service connect directly to the analog PSTN lines coming See QoS. from the telco. This setup requires having both a reg- ular analog telephone lines and an account with a RBOC VoIP Service Provider. Short for Regional Bell Operating Company, the seven companies formed to manage the local exchanges PSTN (POTS) gateway originally owned by AT&T. Software that enables H.323 clients to make outgoing calls, and for incoming calls to be routed to Real-time H.323 clients. Communications wherein perceptible delays between the sender and receiver are minimal and easily toler- PTM ated are considered to take place in real-time. Regular An acronym for Packet Transfer Mode. PTM is a type telephone calls are real time. Point-to-point fax trans- of that is not synchronized by a missions are near to real-time. Voice messaging is not clock. Data is broken into variable-size units of data, real-time. and those packets are relayed from one node to another, often in multiple parallel paths, until they Regional Bell operating company See RBOC.

VoIP Glossary 18 VoIP Glossary—Terms and Definitions

Registered jack-11 (voice, video, and so on). The minimum rate for See Registered jack-11. SDH is 155 Mbps, the maximum is 2.488 Gbps.

Registered jack-45 Service level agreement See RJ-45. See SLA on page 20.

RJ-11 Service provider The designation for connecting a tip and ring circuit A company that provides services to Internet, tele- to a standard, modular, 6-position jack. phone, and users.

RJ-45 Session initiation protocol Eight-position used for data trans- See SIP on page 20. mission over standard twisted or flat pairs. SFP Router An acronym for Small Form-factor Pluggable. A speci- A device that connects LANs by dynamically routing fication for modular optical transceivers. data according to destination and available routes. Sidetone RSVP Sidetone is a technique in which the speaker’s voice is Short for resource reservation protocol, RSVP is a proto- played through the telephone earpiece locally so that col used in VoIP to manage QoS. Defined in RFC the speaker, subconsciously hearing his or her voice 2205: Resource ReSerVation Protocol (RSVP), RSVP instead of silence, will use the telephone works by requesting that required bandwidth and more comfortably. latency be “reserved” for the VoIP by every network device between the two endpoints. Signaling system #7 (SSY7) The basis for routing traffic with out-of-band signal- RTP ing. Its forerunner, CCIS (Common Channel Interof- Acronym for real-time transport protocol, RTP encap- fice Signaling), used 4.8 kbps data links to transmit sulates VoIP data packets inside UDP packets. call set up and tear down messages to switching office Defined in RFC 3550—RTP: A Transport Protocol for adjunct computers and packet switches. SS7 in itself is Real-Time Applications, RTP provides end-to-end net- not a network service offering, but rather the underly- work transport functions suitable for applications ing infrastructure upon which many existing and pro- transmitting real-time data, such as audio, video or posed offerings are based. For example, local Basic simulation data, over multicast or unicast network Rate ISDN (BRI) services can tap into SS7, so 64 services. RTP does not address resource reservation kbps packetized data can be routed with the help of and does not guarantee quality-of-service for real-time the network’s out-of-band signaling capability. In services. The data transport is augmented by a control addition, nationwide Primary Rate ISDN (PRI) ser- protocol (RTCP) to allow monitoring of the data vices can use the same backbone. delivery in a manner scalable to large multicast net- works, and to provide minimal control and identifica- Silence suppression tion functionality. 70–80% of a phone conversation consists of silence, the rest being used for talking. To save bandwidth, SDH silence activity detection (SAD) and voice activity An acronym for synchronous digital hierarchy. This is detection (VAD) techniques are used to avoid trans- the European standard for metropolitan fiber rings, mitting the silent portions of the conversation, used by ILECs. SDH handles multiple data types thereby saving up to 35% of the bandwidth. A packet

VoIP Glossary 19 VoIP Glossary—Terms and Definitions is sent called a silence indicator (SID) to notify the IP networks to the PSTN and manages traffic con- other end that the voice activity power level has taining a mix of voice, fax, data, and video. dropped below a certain threshold (-50 dbm, for example) thereby marking a silent period. VAD SOHO or SoHo requires a 5-ms look-ahead buffer so this adds delay to Acronym for small office, home office, a term that refers the voice path, so VAD is generally used only on to people who work in very small businesses or WAN circuits. One problem with SAD/VAD is that home offices. the natural silences that occur between talking actu- ally contains sounds such as breathing and other back- SONET ground noises that users are not consciously aware of. An acronym for synchronous optical network. This is Therefore, when VAD replaces these subliminal noises the ANSI standard for metropolitan fiber rings, used with pure silence, it is off-putting to the users; so tech- by ILECs. Traffic is carried in an electronic packet niques are employed to add white (or pink) random transported over fiber, using dual counter-rotating noise (called comfort noise) locally to simulate this rings. The SONET standard defines a hierarchy of background noise. interface rates that allow data streams at different rates to be multiplexed. It establishes Optical Carrier (OC) SIP levels from 51.8 Mbps to 2.48 Gbps (10 Gbps Short for session initiation protocol, SIP is an Internet for OC-192). Engineering Task Force (IETF) standard protocol for initiating an interactive user session that involves mul- Speech recognition timedia elements such as video, voice, chat, gaming, Speech recognition describes a technology that enable and virtual reality. Like HTTP or SMTP, SIP works in callers to speak words that are used to the Application layer of the Open Systems Intercon- control applications. nection (OSI) communications model. The Applica- tion layer is the level responsible for ensuring that Store and forward communication is possible. SIP can establish multi- The method for storing a message or transmission for media sessions or Internet telephony calls and modify later playback or transmission. As opposed to real- or terminate them. time communication, store and forward is the basis for all messaging systems, including email, fax-on- SLA demand, unified messaging, etc. In data communica- An acronym for Service Level Agreement. The contract tions, store and forward applies to momentary buffer- between the subscriber or operator and service pro- ing of packets or other data strings. vider specifying the agreed to service level commit- ments and related business agreement. T1 A digital transmission with a capacity of 1.544 Mbps Softphone used in North America. A T1 may be channelized A softphone is a software application that loads the into 24 DS0s, each capable of carrying a single voice VoIP service onto your desktop computer or laptop. conversation or data stream. T1 may be carried on Softphone applications enable you to place VoIP calls or two twisted pairs. from your computer to any location if you have access to a broadband connection. T3 North American standard for DS-3. Operates at a sig- Softswitch naling rate of 44.736 Mbps, or the equivalent of Short for software switch (also referred to as a media 28 T1s. gateway controller, gatekeeper, and call agent), a softs- witch is a software-based switching platform that links

VoIP Glossary 20 VoIP Glossary—Terms and Definitions

TC-PAM Transmission control protocol An acronym for Trellis-Coded Pulse-Amplitude Modu- See TCP. lation, Modulation format that is used in HDSL2 and G.SHDSL. It is a variant of trellis coded modulation μ-law or u-law (TCM) which uses a one-dimensional pulse-ampli- μ-law or u-law is an audio codec companding format tude modulation (PAM) symbol space, as opposed to in which the signal is compressed on input and a two-dimensional quadrature amplitude modulation expanded back to its original form on output. More (QAM) symbol space. TC-PAM is spectrally compati- correctly known as mu-law, it performs logarithmic ble with all other forms of DSL. mapping between an 8-bit data space and 14-bit sam- ple space as described in the CCITT G.711 recom- TCP mendation.þμ-law has 14 bits of dynamic range (84 An acronym for transmission control protocol, The dB).þSee also A-law on page 6 and G.711 on page 11. transport layer protocol developed for the ARPAnet which comprises layers 4 and 5 of the OSI model. UNIX TCP controls sequential data exchange in TCP/IP for A multi-user, multi-tasking operating system origi- remotely hosts in a peer-to-peer network. See also IP nally developed in 1969 by Ken Thompson of AT&T on page 14. Bell Laboratories. UNIX is used in telephone com- pany and mission-critical applications. Telco Telephone company. Upstream In the direction of the telephone network or server. Telephony Taken from Greek root words meaning far sound, UTP telephony means the process of converting or trans- An acronym for Unshielded Twisted Pair. mitting voice or other signals over a distance, and then re-converting them to an audible sound at the far VAD end. See Silence suppression on page 19.

Touch tone VDSL2 See DTMF on page 10. An acronym for Very High Bit Rate Digital Subscriber Line 2. Similar to ADSL, VDSL2 is a type of DSL uti- TR-069 lizing DMT. At longer distances the transmission pro- Technical Report 069, is a specification for remote vides a higher downstream (to the customer site) than management of end user devices (CPEs). upstream (to the service provider). On short loops VDSL2 can offer symmetrical performance. A single Traffic shaping twisted pair offers up to 200 Mbps downstream. A Traffic shaping (also known as metering, shaping, and new 35b profile allows for up to 400 Mbps down- smoothing) refers to the use of queues to reduce surges stream. that can clog a network. Data is buffered in a queue and then released to the network in set amounts to Vectoring ensure that the traffic will fit within the promised traf- Also known as G.vector or VDSL2 vectoring. Vectoring fic limits for the particular connection. Traffic shaping is a transmission method that employs the coordina- is used in ATM, Frame Relay, and other types tion of line signals for the reduction of self-FEXT of networks. (far-end crosstalk) levels in both the downstream and upstream directions.

VoIP Glossary 21 VoIP Glossary—Terms and Definitions

Very high bit rate digital subscriber line 2 mended because they support VoIP prioritization, See VDSL2 on page 21. which means they are alotted bandwidth to ensure VoIP quality, regardless of other traffic. VioIP Short for video over IP (also called IP streaming video), VoIP telephone VioIP enables video signals to be transported and See IP phone on page 14. managed over IP networks. With VioIP, live or pre- recorded video content is captured and compressed, VoIP switch then transmitted via the network to one or more A VoIP switch is very much like an ordinary PBX in receiving stations for viewing. that it enables users to make telephone calls to the PSTN. The difference is that devices that connect to a Video-over-IP PBX are hardwired (or connected permanently) to it, See VioIP. whereas with a VoIP switch, the IP protocol enables connections to be softwired and easily modified. Voice activity detection See Silence suppression on page 19. VPN Acronym for , a network that is Voice quality constructed by using public wires to connect nodes. A Human speech uses a bandwidth of 100 Hz to 10 VPN is a private data network that makes use of the kHz (if you include harmonics) with most of the public telecommunication infrastructure, using secu- speech occurring in the 100 Hz to 3 kHz range. The rity measures that involve encrypting data before more bandwidth that is allocated to reproduce speech, sending it across the Internet and decrypting the data the more faithfully the sound compares to the origi- at the other end. nal, which is known as fidelity. Voice quality is a mea- sure of how well a person can hear the voice on the WAN opposite end of the connection after it has been sub- An acronym for Wide Area Network. A network that jected to signal compression and the effects of echo, spans a large geographic area. latency (or delay), and jitter. Voice quality is most com- monly stated as a metric called the mean opinion Web browser score (MOS). See also MOS on page 15. Client software used to view information on WWW servers. Web browsers are also packaged with email VoIP gateway clients, newsreaders and IP telephony clients. A VoIP gateway translates protocols between tele- phony and packet networks to deliver voice and simi- Web server lar services. In compliance with the H.323 standard, On the world wide web, a server dedicated to storing gateways serve as network endpoints that support real- data (such as web pages in HTML format) and dis- time communication between other endpoints. VoIP tributing it to users. Web browsers are able to down- gateways support voice communication between ter- load video, text, still images and audio from web minals on a packet network (usually IP) and terminals pages. Some servers support unified messaging. on a circuit network (the PSTN, for example). Wide area network VoIP router See WAN. A VoIP router connects IP networks together. When an IP phone or ATA connects to the network, it does World wide web so through a router. Some IP phones and ATAs have See WWW on page 23. built-in routing capabilities and their use is recom-

VoIP Glossary 22 VoIP Glossary—Terms and Definitions

WWW Short for World Wide Web, WWW (also called W3) is an information space in which the items of interest, referred to as resources, are identified by global identi- fiers called uniform resource identifiers (URI). Although the WWW is often considered to be the same thing as the Internet, it is actually part of the Internet. The WWW comprises the resources that can be accessed using Gopher, FTP, HTTP, telnet, , WAIS and other tools, and the network of hypertext (HTTP) servers.

X.21 An ITU-T standard for a high-speed, 15-position, DCE/DTE interface.

VoIP Glossary 23 q