Broadband and Remote Access Technologies
Broadband cable and DSL technologies are very popular and widely deployed and used in, Europe, Australia, New Zealand, North America, and parts of Central America. Broadband technologies enable faster Internet access, Voice over IP (VoIP) capabilities, and the ability to easily stream video and music, amongst many numerous things. The ISCW exam objectives covered in this chapter are:
Describe Cable (HFC) technologies Describe xDSL technologies Configure ADSL (i.e., PPPoE or PPPoA) Verify basic teleworker configurations
This chapter is divided in the following sections:
. Data Transmission Basics . Cable Technology . The Public Switched Telephone Network . Digital Subscriber Line . Sending Data over DSL Networks . Configuring PPPoE and PPPoA for DSL
Data Transmission Basics
Data transmission refers to the process of sending data or the progress of the sent data signals after they have been transmitted. It is imperative to have a solid understanding of some of the different technologies and principles pertaining to data transmission in order to completely understand Cable and DSL transmission broadband technologies. Although going into detail on all specifics pertaining to data transmission is beyond the scope of the ISCW, this section covers and briefly describes the following relevant terms and technologies:
. Analog and Digital Signaling . Data Modulation . Multiplexing . Baseband and Broadband . Noise (Interference) . Attenuation . Coaxial Cable . Twisted Pair Cable . Fiber Optic Cable
Analog and Digital Signaling
On data networks, information can be transmitted using either analog signaling or digital signaling. Computers generate and interpret digital signals as electric current, which is measured in volts. The stronger the electrical signal, the higher the voltage. After the signal has been generated, it travels over copper cabling as electrical current; over fiber optic cabling as light pulses (waves); or through the atmosphere as electromagnetic (radio) waves. The following diagram shows a digital signal and illustrates the 1s and 0s in digital communication:
1 1 1
0 0 Amplitude Time
Analog data signals are also generated as voltage. However, unlike digital signals, the voltage varies in analog signals and is represented as a wavy line when plotted on a graph. All analog signals are characterized by four main characteristics. These four core characteristics are amplitude, frequency, wavelength, and pulse.
The amplitude is a measure of the signals (waves) strength at any given time. The frequency is the number of time the wave’s amplitude cycles from its starting point, through its highest amplitude and its lowest amplitude, back to its starting point over a fixed period of time.
Frequency is expressed in cycles per seconds, or hertz (Hz). Wavelength is the difference between the corresponding points on a wave’s cycle, for example, between one peak and the next peak. Wavelengths are expressed in meters or feet. The wavelength is inversely proportional to the frequency, meaning that the higher the frequency, the shorter the wavelength, and vice versa.
Finally, the term phase refers to the progress of a wave over time in relationship to a fixed point. If, for example, two waves start at the same time, with both being at their highest amplitude, the two waves would be in phase. However, if both waves started at the same time, with the first wave starting at its lowest amplitude and the second wave starting at its highest amplitude, the waves would be said to be 180 degrees out of phase. These concepts are illustrated in the following diagram showing two different analog waves in red and blue:
Degrees
0 90 180 180 360
90 Degree Phase
+5V Wavelength
Amplitude
Voltage (V)
-5V Frequency
Time (Seconds)
Data Modulation
Given the advances in technology, data is primarily sent using digital transmission. However, there are still some network technologies, such as telephone lines, that only use analog transmission. The issue arises in the fact that the digital signals must be able to communicate over analog transmission networks, and vice versa. For example, when using dial-up Internet access, the computer uses digital transmission, even though it is connected to an analog transmission network, i.e. the telephone line. In such situations, a modem is required to modulate digital signals into analog signals at the transmitting end, and demodulate analog signals into digital signals at the receiving end. The word modem actually stands for modulator and demodulator, which is a reflection of the functions performed by these devices, which include dial-up modems, cable modems and DSL modems, for example.
Data modulation is a technology used to modify analog signals to make them more suitable for carrying data over a communication path. In modulation, a simple wave called a carrier wave is combined with the information or data wave to produce a unique signal that gets transmitted from one node to another. The carrier wave contains preset properties, such as the frequency, amplitude, and phase and when combined with the information wave, any one of the carrier wave properties is then modified resulting in a new blended signal that contains the properties of both the carrier wave and the data wave. When the signal reaches its destination, the receiver separates the data (information) from the carrier wave via demodulation.
Multiplexing
Multiplexing is a form of transmission that allows multiple signals to travel simultaneously over a single medium. In order to carry multiple signals, the physical media is logically separated or segmented into smaller channels, also commonly referred to as sub-channels.
In order to combine and transmit multiple signals over a single medium a multiplexer (mux) is required at the transmitting end of the channel. At the receiving end, a demultiplexer (demux) is required to separate the combined signals and regenerate them in their original form. Multiplexing allows networks to increase the amount of data that can be transmitted in a given amount of time over a given bandwidth.
There are many different types of multiplexing available and the type that is used depends on the media, transmission and reception that the equipment can handle. While going into all the different types of multiplexing is beyond the requirements of the ISCW, we are going to describe Frequency Division Multiplexing because of its relevance in Cable and DSL networks.
Frequency Division Multiplexing (FDM) assigns a unique frequency band to each individual communications sub-channel. Signals are modulated with different carrier frequencies and are then multiplexed to simultaneously travel over a single channel. Each signal is then demultiplexed at the receiving end. FDM was first used by telephone companies when they discovered that it allowed them to send multiple voice signals over a single cable. That meant that rather than running separate lines for each residence they could send as many as 24 multiplexed signals over a single neighborhood line. Each signal was then demultiplexed before being brought into the home.
With recent technological advances, telephone companies can use FDM to multiplex signals on the phone line that enters a home. Voice communications use the frequency band of 300 Hz – 3300 Hz, although the most common representation of this range is 300 Hz – 3KHz. Because everything above the 3 KHz range was simply unused space, telephone companies used FDM to allow them to send data signals in this space without interrupting voice communications, allowing for DSL service over existing telephone lines. In a similar manner to telephone companies, cable operators also use FDM to multiplex signals over a single channel and provide television, voice and data services over cable networks.
Baseband and Broadband
Baseband is a transmission form in which signals are sent through direct current (DC) applied to the wire. Because DC requires the exclusive use of the wire, baseband systems can only transmit one signal, or channel, at a time and every device on the baseband system shares the same channel. When one node on a baseband system is transmitting data, all other nodes must wait for that transmission to end before they can send any data that they may need to send. A common example of baseband systems is half-duplex Ethernet.
Broadband is a form of transmission in which signals are modulated as radio frequency (RF) analog waves that use different frequency ranges. Unlike baseband, broadband technology does not encode information as digital pulses. Broadband systems handle a relatively wide range (band) of frequencies, which may be divided into channels or frequency bins. While broadband is generally more expensive than baseband, it can carry more data and span greater distances than baseband systems. An example of a broadband system is cable television.
Noise (Interference)
Noise is an undesirable influence that may distort or degrade a signal. While there are many different types of noise, one of the most common is electromagnetic interference (EMI), which is caused by waves that emanate from electrical devices such as televisions, motors, power lines, and fluorescent lights, for example. While EMI greatly affects analog transmissions, it does not affect digital transmissions as much. Additionally, fiber optic cabling (which will be described last in this section) is completely unaffected by electromagnetic interference.
Attenuation
Attenuation is the loss of the strength of a signal as it travels away from its source. This is one of the most common transmission flaws. Fortunately, however, there are solutions that can be used to rectify this problem.
In order to boost the strength of analog signals, an amplifier is used. An amplifier is simply an electronic device that increases the voltage or strength of the signals. Cable operators use amplifiers to boost the strength of signals. It is important to know that amplifiers also increase the strength of any noise that is associated with the signal. In other words, when an analog signal is amplified, the noise that it has accumulated is also amplified, which may actually cause the analog signal to worsen significantly. This is especially observed after several amplifications. The following diagram illustrates the use of amplifiers to boost analog signals:
Amplitude
0 0 0 0 0 0
0 0 0 0
Amplifier Amplifier
Distance
Digital transmission does not use amplifiers to boost the signal strength. Instead, devices called repeaters are used. In addition to this, when digital signals are repeated they are transmitted in their original form – without any accumulated noise – in a process called regeneration. Both amplifiers and repeaters operate at Layer 1 of the OSI Model. The following diagram illustrates the use of repeaters to boost digital signals:
Digital signal starts to lose strength and begins to look like an analog signal
Amplitude
0 0 0
0 0 Regenerates the digital signal Repeater
Distance
Coaxial Cable
Coaxial cable, commonly referred to as coax, consists of a central metal core, often made of copper, surrounded by an insulator, a braided metal shielding called braiding or shield, and an outer cover, referred to as the sheath or jacket. The coax core may be constructed of one solid metal wire or several thin strands of metal wire, as illustrated in the following diagram:
The coax core carries the electromagnetic signal and the braided metal shielding acts as a shield against noise, as well as a ground for the signal. The insulator layer consists of a plastic material such as Polyvinyl Chloride (PVC) or Teflon. The insulation protects the core from the metal shielding because if the two made contact, the wire would short-circuit. The sheath, which protects the cable from physical damage, may also be made of PVC or other materials. Because of its shielding, most coax cable has a high resistance to noise. Additionally, coax also has the ability to carry signals further than twisted pair cabling, for example, before the signal needs to be amplified. Twisted pair cabling is described in the following section.
While there are hundreds of coax cable specifications, the only ones relevant to the ISCW course requirements are the RG specifications. RG stands for Radio Guide, which is appropriate because coax cable is used to guide radio frequencies in broadband transmission. The differences between the specifications lie in the material used for their shielding and conducting cores, which influence their transmission characteristics, such as attenuation, throughput and impedance. Impedance measures how easily a circuit conducts current when a voltage runs through it. Impedance is a way of telling you how much of the voltage introduced at one end will really make it to the other end. Impedance is measured in ohms. The size of the coax conducting core is referred to as American Wire Gauge (AWG) size. In essence, the larger the AWG size, the smaller the diameter of a piece of wire. The following section provides a brief description of coax cable specifications used with data networks:
. RG-6 – This type of coax cable has an impedance of 75 ohms and contains an 18 AWG conducting core, which is usually made of solid copper. RG-6 is typically used to deliver broadband cable and Internet services over long distances.
. RG-8 – This type of coax cable has an impedance of 50 ohms and a 10 AWG conducting core. This coax cable type is considered obsolete and will never be found on newer networks.
. RG-58 – This type of coax cable has an impedance of 50 ohms and a 24 AWG conducting core. As is the case with RG-8, this coax cable type is considered obsolete and will never be found on newer networks.
. RG-59 – This type of coax cable has an impedance of 75 ohms and a 20 or 22 AWG core, typically made of braided copper. While less expensive than RG-6, RG-59 suffers from greater attenuation. RG-59 is still used for somewhat short connections, such as the distribution of video signals from a certain receiver to multiple monitors in a building.
The two most common connectors used with coaxial cable are either BNC or F-type connectors. BNC stands for British Naval Connector; however, these connectors are seldom used with either RG-6 or RG-59 coaxial cable. The F connector is inexpensive, and yet has good 75 ohm impedance match up to 1 GHz and has usable bandwidth up to several GHz. The F-type, or simply F connector, is the most common means of connecting television signals. This connector is illustrated in the following picture:
Twisted Pair Cable
Twisted pair cable consists of color-coded pairs of insulated copper wires, with each having a diameter of between 0.4 mm and 0.8 mm. Two wires are twisted around each other to form a pair and all the pairs are encased in a plastic sheath. The number of pairs in a cable varies depending on the type of cable. Twisted pair cable is relatively inexpensive, flexible and easy to install. Additionally, it can travel significant differences before a repeater is needed; however, it cannot span greater distances that coaxial cable or fiber optic cable. All twisted pair cable falls into one of two categories: Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP).
The following diagram illustrates twisted pair cabling:
STP consists of twisted wire pairs that are not only individually insulated, but are also surrounded by a shielding made of a metallic substance such as foil, although some STP cabling uses a braided copper shielding. This shielding serves two purposes. The first is that it acts as a barrier to electromagnetic forces, and the second is that it is used to contain the electrical energy of the signals inside the cable.
UTP cabling consists of one or more insulated wire pairs encased in a plastic sheath. Unlike STP, UTP does not contain additional shielding for the twisted pairs which makes it both less expensive as well as less resistant to interference than STP.
Fiber Optic Cable
Fiber optic cable, fiber, or optical fiber cable contains one or several strands of glass or plastic fibers at its center (core). Data is transmitted over fiber optic cable via pulsating light sent from a laser or light emitting diode (LED) through the central fiber(s). The fibers are surrounded by a layer of glass or plastic called cladding, which typically has a different density from the glass or plastic in the strands. The cladding reflects light back to the core in patterns that vary depending on the transmission mode. This reflection allows the fiber to bend around corners without diminishing the integrity of the light-based signal. Outside the cladding, a plastic buffer is used to protect both the cladding and the core. Fiber variations fall into two categories, which are single-mode and multi-mode.
Single-mode fiber (SMF) uses a narrow core that is less than 10 microns in diameter through which light generated by a laser travels, while reflecting very little, which prevents the light from dispersing as the signal travels along the fiber. This allows SMF to accommodate the highest bandwidths and longest distances (without using repeaters) of all network transmission media. SMF can span distances up to 40,000 meters (40Km).
Multi-mode fiber (MMF) contains a core that is between 50 and 115 microns in diameter, with the most common size being 62.5 microns. Pulses of light generated by a laser or LED travel across this core at different angles. Because of this MMF cannot match the great distances that SMF can span, even though its distance limitation exceeds other transmission methods, such as copper cabling. Typically, MMF is used to connect switches within a network. The following diagram illustrates transmission over SMF and MMF fiber optic cables:
Cladding Cladding
LASER
Single-mode fiber (SMF)
Cladding Cladding
LASER
Multi-mode fiber (MMF)
Cable Technology
Cable modems are network bridge devices that operate at Layer 1 and Layer 2 of the OSI Model and connect home networks to the Internet through the cable television system. On the network side, cable modems support Ethernet and on the cable side, DOCSIS. However, it should be noted that although cable modems generally follow the DOCSIS standard, implementations commonly can and do vary by provider. DOCSIS is a core component cable technology and will be described in detail later in this section.
Cable companies use hybrid fiber coaxial cable (HFC) networks to provide both fiber and coaxial connections to the customer. The coaxial portion is used to carry the television service and the fiber optical cable is used for the data connection. HFC networks will be described in
detail later in this section. Typical cable throughput varies anywhere between 1Mbps and 6Mbps for downloads and between 128Kbps and 768Kbps for uploads. In theory, however, cable modems have the ability to support up to 30Mbps download speeds. Cable networks are a shared multipoint circuit which means that the actual download and upload speeds depend on the level of activity on the network at any given point in time.
Video Cable Networks
CATV, which stands for Community Antenna Television, is now commonly referred to simply as Cable TV. Cable television is a system of providing television to consumers via radio frequency signals transmitted to televisions through fixed optical fiber and coaxial cables as opposed to the over-the-air method used in traditional television broadcasting in which a television antenna is required. Additionally, high-speed Internet, IP Telephony, and similar non-television services may also be provided. CATV is delivered to subscribers via HFC networks owned by the MSO using the DOCSIS specification. MSO, which stands for Multiple System Operator, is a term used to describe an operator of multiple cable television systems. Some common examples of MSOs include Time Warner Cable and Comcast Cable in the US, Rogers Communications in Canada, and Virgin Media in the UK.
Cable video services are delivered via broadband. Frequency-division multiplexing (FDM) is used in cable networks to combine multiple signals onto a carrier wave in a wide range of radio frequencies (RF) over the HCF network. FDM allows cable operators to allocate bandwidth to multiple channels or frequencies onto a single physical cable, or wire. The following diagram illustrates the components typically found in video cable networks:
MSO HCF Network RHE LHE Antenna Site
Transportation RHE Network
RHE
Transportation Network
Fiber Optic Node
Distribution Hub Distribution Hub
Fiber Optic Node Distribution Network Amplifier
Distribution Diagram Legend Network Fiber Optic Cable Coaxial Cable
Referencing the diagram illustrated above, the remote headend (RHE) sends local programming information to the antenna site owned by an MSO. The RHE is simply the television network operator, for example, CNN or BBC. The antenna site is a location owned by the MSO that receives transmissions via satellite dishes, antennas, analog video servers, digital video servers, and also via local programming. The programming information is received by the antenna site from the RHE in the downstream path.
In cable terminology, the term downstream may refer to the forward-path signals that carry programming information from the antenna site to the LHE or; however, it is most commonly used to refer to signals from the headend office to the home (subscriber). Downstream signals are carried on a 50 MHz to 860 MHz band (although some texts state 88 MHz to 860 MHz). Upstream signals, on the other hand, refer to the return-path signals that carry information from the home (subscriber) to the headend office. These may include control signals to order a movie, for example. Upstream signals travel from the subscriber (customer) to the provider and use frequencies in the range of 5 MHz to 42 MHz in the U.S, and in other parts of the world, the range is 5 MHz to 65 MHz.
NOTE: The forward-path and the return-path signals are actually carried over the same coaxial cable between the optical node and the home. Unlike television signals, typically, several
hundreds of users share a 6 MHz downstream data channel and one or more upstream data channels. The downstream channel occupies the space of a single television transmission channel (NTSC) in the cable operator's channel lineup and provides up to 40 Mbps of throughput, although typical rates are up to 6 Mbps. The Media Access Control (MAC) sub- layer of the DOCSIS Data Link Layer coordinates shared access to the upstream bandwidth. NTSC and the DOCSIS Data Link Layer will be described in detail later in this chapter.
Reverting back to the explanation of the diagram above, the antenna site sends this information to the local headend (LHE) owned by the MSO via the transportation network, which connects the antenna site(s) to the LHE. The transmissions from the antenna site are typically analog although digital information can also be received. This network may be comprised of coaxial supertrunk or fiber optic cabling. The LHE is the MSO facility where received television signals are processed, formatted, and distributed over to the cable network.
The LHE performs signal conversion from RF (analog) to light (fiber) and the television signal is distributed to subscribers via the transportation network to a fiber optic node located relatively close to the subscriber. The transportation network between the LHE and the fiber optic nodes may or may not have one or more distribution hubs, which simply facilitate the flow of the signal from the LHE to the fiber optic nodes.
When the fiber optic node receives the signal from the LHE, it converts the signal from light (fiber) back to RF (coaxial). The RF is then sent via coaxial cable to the subscriber’s home, where it may pass amplifiers, taps and splitters before reaching the destination cable modem. In cable networking, an amplifier is any device that changes, usually increases, the amplitude of a signal. In other words, amplifiers magnify an input signal so that the output signal is larger.
Tapping is used to split the input RF power over the coax cable to support multiple outlets. A tap terminates into a small coaxial drop using an F connector. The drop is then connected to the house where a ground block protects the system from stray voltages. Taps pass the RF signal and block the AC power unless there are telephony devices that need the back-up power reliability provided by the coaxial power system. Within the house, splitters can be used if more than one television needs to be connected, or to separate television, voice and data signals.
The overall network owned by the MSO is comprised of fiber optic cable and coaxial cable. This network is referred to as the hybrid fiber coaxial, or HFC, network. HFC is a telecommunications industry term for a broadband network which combines optical fiber and coaxial cable. HCF will be described in detail later in this chapter.
Voice and Data Cable Networks
Now that we have an understanding of the components in video cable networks, the following diagram illustrates a data cable network:
LHE Back Office IP Services Network CMTS
Diagram Legend
Fiber Optic Cable
HCF Ethernet Cable
Network Coaxial Cable
Analog cable
0 0 0
0 0 Amplifier
Splitter EMTA
Tap
Computer Telephone
Bob’s House
Cable Modem
Computer
Alice’s House
Referencing the diagram illustrated above, when subscribers have data and voice services via the cable network, a combined customer premises equipment device known as an Embedded Multimedia Terminal Adapter (EMTA) will often be used. An EMTA is simply a cable modem and a VoIP adapter (MTA, Multimedia Terminal Adapter) bundled into a single device.
Cable modems (CMs) communicate with routers, called Cable Modem Termination Systems (CMTS), over the HFC plant using the DOCSIS standard. Cable modems are considered the CPE, or Customer Premise Equipment, and they perform the modulation and demodulation of signals received from the computer, etc, to and from the CMTS.
An example of a CMTS is a Cisco Universal Broadband Router (uBR), such as the Cisco uBR7200, or uBR10k, with features that enable it to communicate with an HFC cable network
via a Cisco cable modem card. The Cisco cable modem cards allow providers to connect cable modems on the HFC network to a Cisco uBR at the MSO headend. The modem card provides the interface between the Cisco uBR protocol control information (PCI) bus and the radio frequency (RF) signal on the DOCSIS HFC network.
An HCF fiber node sits between the CMTS and CM or EMTA with each CM or EMTA assigned to a specific fiber node. One or more fiber nodes are assigned to a given CMTS.
For data transmissions, in the upstream path, the CM or EMTA modulates the digital data from the computer received via the Ethernet connection to the data RF signal over the coaxial connection. The RF signal is forwarded onto the CMTS via the HCF network. When the CMTS receives the data, it CMTS demodulates the RF data back into digital data and sends it to the IP network or Internet. In the downstream path, the CMTS receives data from the Internet or IP network, and performs modulation of that digital data into an RF signal. The CM or EMTA receives the analog signal and demodulates the data RF signal back into digital data format and forwards it to the end user via the Ethernet connection. This process is illustrated in the following diagram:
Upstream
Downstream
Digital Analog Digital
Internet HFC CMTS Cable Computer Modem
Modulates analog signal from Modulates digital signal from HFC to digital signal; PC to analog signal; modulates digital signal from demodulates analog signal Internet to analog signal from HFC to digital signal
The IP network that the CMTS is connected to typically allows the MSO to provide back office services, which include DHCP to dynamically allocate network addressing information, ToD, which stands for Time of Day and is used to provide timestamping services, and TFTP servers, which provide configuration and software files. Although thoroughly covered at the CCNA level, a timestamp is a sequence of characters, denoting the date and/or time at which a certain event occurred. This data is usually presented in a consistent format, allowing for easy comparison of two different records and tracking progress over time; the practice of recording timestamps in a consistent manner along with the actual data is called timestamping. Timestamps are typically used for logging events, in which case each event in a log is marked with a timestamp.
NOTE: In situations where the subscriber has both voice and CATV services, an RF splitter separates the voice, video and data traffic as necessary such that data traffic is sent to the modem, voice to the EMTA, and video is sent to the television set(s), for example.
Hybrid Fiber Coaxial Networks
DOCSIS, which stands for Data over Cable Service Interface Specification, is an international standard that defines the communications and operation support interface requirements for a data over cable system. It permits the addition of high-speed data transfer to an existing Cable TV (CATV) system. It is employed by many cable television operators to provide Internet access (see cable internet) over their existing hybrid fiber coaxial (HFC) infrastructure. DOCSIS will be described in detail later in this chapter.
One advantage of coax over other types of transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors. This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other transmission lines, and provides protection of the signal from external electromagnetic interference.
In HFC networks, the fiber optic network extends from the cable providers master headend, sometimes to regional headends, to a neighborhood’s hub site, and finally to an HCF fiber optic node. This fiber optic node can serve between 25 to 2000 homes, depending on its capabilities. Some master headends also house telephony equipment for providing telecommunications services such as Voice over IP (VoIP) to the community they are provisioned to serve. The coaxial cable is deployed from the optical fiber feeders to each subscriber. The hybrid networks provide the bandwidth and reliability of optical fiber at a lower cost than a pure fiber network.
There are certain standards that MSOs use when sending the various received RF signals on the fiber optic and coaxial copper cables. The original, and most widely used, method to transport video over the HFC network is by modulation of standard analogue TV channels which is similar to the method used for transmission of over-the-air broadcast Analog television channels. The MSO uses NTSC, PAL, SECAM and MPEG-2/MPEG-4 coding over Quadrature Amplitude Modulation (QAM) as the modulation signaling standards for broadband television. QAM will be described later in this chapter.
NTSC, which stands for National Television System(s) Committee, was first developed in 1941. An NTSC television channel, as transmitted, occupies a total bandwidth of 6 MHz. If you recall, earlier in this chapter we learned that several hundreds of users share a 6 MHz downstream channel and one or more upstream channels. The downstream channel occupies the space of a single television transmission channel in the cable operator's channel lineup and provides bandwidth speeds of up to 40 Mbps. Although NTSC broadcasters were required by the FCC to shut down their analog transmitters in 2009, this standard is still used in other countries.
PAL, short for Phase Alternating Line, is an analogue television encoding system. Although this has been phased out in the US, PAL is still used in broadcast television systems in large parts of the world. When deployed in PAL–based systems, one analogue TV channel occupies an 8 MHz-wide frequency band. PAL is used in EuroDOCSIS.
SECAM, French for Sequential Color with Memory, is an analog color television system first used in France. Like PAL, in SECAM-based systems, one analogue TV channel occupies an 8 MHz-wide frequency band. Although SECAM transmissions are more robust over longer distances than NTSC or PAL, this standard is not used in the U.S.
MPEG-2 is widely used as the format of digital television signals that are broadcast over-the-air, via cable, and by direct broadcast satellite TV systems. MPEG stands for Moving Picture Experts Group. MPEG-4 is a patented collection of methods defining compression of audio and visual (AV) digital data; however, MPEG-4 is still a developing standard. Quadrature amplitude modulation (QAM) is an analog and a digital modulation scheme. The technical specifics pertaining to MPEG-2, MPEG-4 and QAM are beyond the scope of the ISCW and will not be described in detail in this guide.
Radio Frequency
Radio frequency (RF) is a frequency of electromagnetic radiation within the range of about 3 Hz to 300 GHz. When an RF current is supplied to an antenna, it gives rise to an electromagnetic (EM) field that propagates through space. The EM field is sometimes called an RF field or simply referred to as a radio wave. The frequency of an RF signal is inversely proportional to the wavelength of the electromagnetic field to which it corresponds. The RF spectrum is divided into several ranges, which are also referred to as bands. The following table illustrates the eight bands in the RF spectrum:
Designation Abbr. Frequencies Free-space Wavelengths Very Low Frequency VLF 9 kHz - 30 kHz 33 km - 10 km Low Frequency LF 30 kHz - 300 kHz 10 km - 1 km Medium Frequency MF 300 kHz - 3 MHz 1 km - 100 m High Frequency HF 3 MHz - 30 MHz 100 m - 10 m Very High Frequency VHF 30 MHz - 300 MHz 10 m - 1 m Ultra High Frequency UHF 300 MHz - 3 GHz 1 m - 100 mm Super High Frequency SHF 3 GHz - 30 GHz 100 mm - 10 mm Extremely High Frequency EHF 30 GHz - 300 GHz 10 mm - 1 mm
Of the eight bands listed, only VHF and UHF are of concern, as these are used for television broadcasts. Very High Frequency (VHF) operates in the 30 MHz - 300 MHz rang. UHF, or Ultra High Frequency, operates in the 300 MHz - 3 GHz range. The SHF and EHF bands are often
referred to as the microwave spectrum. You are not expected to go into detail on the different bands within the RF spectrum.
DOCSIS Specifications
Now known as CableLabs Certified Cable Modems, the Data-over-Cable Service Interface Specification, or DOCSIS, was created out of Motorola’s Cable Data Link Protocol (CDLP) Physical Layer technology and the MAC Layer was created by LANCity for use with NTSC broadcasts in the U.S. In Europe, and other parts of the world, a version of the technology compatible with the PAL broadcast standard, is used. This version is commonly referred to as EuroDOCSIS. Additionally, it is important to know that Japan has developed its own version of DOCSIS that is further distinguished from either of the existing services in either Europe or the US. This version is beyond the scope of the ISCW.
DOCSIS defines the interface standards for cable modems and supporting equipment involved in high speed data transfer and distribution over CATV networks. It allows additional high- speed data transfer over an existing CATV system and is widely used by television operators to offer Internet access through an already existing HFC infrastructure. Other devices that recognize and support DOCSIS include High-Definition TV's (HDTVs) and Web-enabled set- top boxes for televisions. The DOCSIS architecture consists of three primary components:
1. A Cable Modem (CM) 2. A Cable Modem Termination System (CMTS) 3. Back Office Services
These three components have already been described in this chapter. DOCSIS defines protocol for bi-directional signal exchange between the CM and CMTS through the use of cable. There are three versions of the DOCSIS standard: version 1.0 (which was revised as version 1.1), version 2.0 and version 3.0.
DOCSIS version 1.0 was issued in March 1997, with revision 1.1, which added Quality of Service (QoS) capabilities, following in April 1999. DOCSIS 1.1 features an increased upstream data transmission and improved security. This version facilitates multiple services such as voice and streaming. The end result is faster transmission and reception with a greater inventory of features. DOCSIS version 1.1 allows for downstream traffic transfer rates of 27-36 Mbps over a radio frequency (RF) path in the 50 MHz to 860 MHz range, and upstream traffic transfer rates between 320 Kbps-10 Mbps (average 5 Mbps) over a RF path between 5 and 42 MHz. The DOCSIS 1.1 standard can coexist with DOCSIS 1.0.
Because of increased demand for symmetric services such as IP telephony, DOCSIS was revised to enhance upstream transmission speeds and DOCSIS 2.0 was introduced. DOCSIS 2.0 has an added capacity for symmetric services by operating at 64 QAM, backed by a new 6 MHz wide downstream channel. Enhanced modulation and improved error correction ensures that this
standard offers increased bandwidth for IP traffic. The upstream traffic rate for DOCSIS 2.0 is above 30 Mbps which is 3 times better than DOCSIS 1.1 and 6 times faster than DOCSIS 1.0. The downstream traffic rate for DOCSIS 2.0 can support up to 40 Mbps. DOCSIS 2.0 is interoperable and backward compatible with the DOCSIS 1.0 and DOCSIS 1.1 specifications.
The DOCSIS 3.0 specification outlined the methodology for wideband downstream channel bonding, upstream channel bonding, along with other features such as IPv6, IP Multicasting and AES encryption. DOCSIS 3.0 can achieve downstream speeds of up to 160 Mbps by bonding 6 MHz channels together, or in the case of EuroDOCSIS, 8 MHz channels. The DOCSIS 3.0 specification upstream channel bonding has the ability to provide up to 120 Mbps of shared throughput for cable operators.
One of the major goals of the DOCSIS 3.0 specification is that due to the increased downstream speeds through wideband deployments, it will allow cable operators to better compete against fiber optic provider services, such as Verizon FiOS, which has already deployed its 50 Mbps downstream and 20 Mbps upstream service in six markets, while providing a better service to subscribers. However, the reality of the situation is that at the present moment, the speeds vary based on the current infrastructure of the MSO and vary between 256 Kbps and 6 Mbps. Despite the many variations of the DOCSIS specifications, it is important to remember that cross-version compatibility has been maintained across all versions of DOCSIS, with the devices falling back to the highest supported version in common between both endpoints, i.e. the CM and CMTS. The DOCSIS specifications operate at Layer 1 and Layer 2 of the OSI Model.
The DOCSIS Physical Layer
The DOCSIS Physical layer allows for bidirectional communication between the CM and the CMTS by using low frequencies to send the information from the CM to the CMTS (upstream) and higher frequencies to send the data from the CMTS to the CM (downstream) as illustrated in the following diagram:
Upstream: 5 – 42 MHz
CMTS CM
Downstream: 50 - 860 MHz Coaxial Cable
DOCSIS specifies different bandwidths for each channel. The channel widths are 200 KHz, 400 KHz, 800 KHz, 1.6 MHz, 3.2 MHz, and 6.4 MHz. Additionally, all versions of DOCSIS specify that 64-level or 256-level Quadrature Amplitude Modulation (QAM) be used for modulation of downstream data. These are commonly referred to as 64-QAM or 256-QAM. DOCSIS upstream
modulation uses either Quadrature Phase-Shift Keying (QPSK) or 16-level QAM (16-QAM) be used for upstream modulation. Furthermore, DOCSIS 2.0 and 3.0 specify 32-QAM, 64-QAM and 128-QAM which are also available for upstream use.
The DOCSIS Physical Layer is further broken down into the Downstream Convergence Layer and the Physical Media Dependent sub-layer. The Downstream Convergence Layer conforms to the MPEG-2 standard. At this layer, data received by the cable modem is encapsulated into 188- byte MPEG-2 frames, which allows the data to be multiplexed with other MPEG streams on the same carrier on the downstream path. The PMD sub-layer is used for downstream and upstream data transmission. The DOCSIS upstream PMD sub-layer uses the Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) burst-access mechanisms. Additionally, the PMD also uses the QPSK and 16 QAM modulation formats.
The DOCSIS Data Link Layer
The DOCSIS Data Link Layer is broken up into three different sub-layers, which are the Logical Link Control (LLC) sub-layer, the Link Security sub-layer, and the Media Access Control (MAC) sub-layer, as illustrated in the following diagram:
Security Sub-Layer
LLC Sub-Layer Data Link Layer
MAC Sub-Layer
The LLC sub-layer ensures that DOCSIS conforms to Ethernet standards, and the Link Security sub-layer protects user data. Because the cable network is shared, there has to be a method to protect user data from malicious users. DOCSIS has therefore defined the Baseline Privacy Interface (BPI) for this function, which uses the Cipher Block Chaining (CBC) mode of the Data Encryption Standard (DES) algorithm to encrypt data in both upstream and downstream frames. CMs use the protocol to obtain authorization and encryption keys from the CMTS, and to support periodic reauthorization and changing the keys.
The key management protocol uses RSA, which a public-key encryption algorithm, and the Electronic Codebook (ECB) mode of DES to secure key exchanges between the CM and CMTS. CMs must have factory-installed RSA private and public key pairs, or provide an internal algorithm to generate such key pairs dynamically. This layer is also commonly referred to as the Data Link Encryption sub-layer. Although the terms CBC, ECB, DES, and RSA have been used in this section, they will not be discussed in this chapter as they are pertain to cryptography, which is a core requirements of the ISCW certification and will be described, in detail, later in this guide. For the time being, the task at hand should simply be that you are familiar with the uses of the various sub-layers.
The Media Access Control (MAC) sub-layer controls CM access to the return (downstream) path. In the downstream direction, the CMs listen to all traffic flows from the CMTS to which they are registered, and accept messages targeted to them. Traffic in the upstream direction consists of data transmitted from several CMs intended for the common CMTS. Hence, a multiple access scheme, such as TDMA, is used in order to facilitate this bi-directional flow. In order to transmit data in the upstream direction, the CM must first request bandwidth authorization from the CMTS, listen to the downstream bandwidth allocation message (MAP), and then transmit the desired data if allowed to do so. This sub-layer also provides Quality of Service (QoS) and other features, which are beyond the scope of the ISCW requirements.
The final section we are going to learn about as far as cable networks are concerned is the provisioning of cable modems. It is important to know that you are not expected to perform any actual CMTS or CM configurations in the current ISCW certification exam; however, you are expected to be familiar with the steps a cable modem transitions through until it is fully operational. These seven (7) steps are outlined and described in the following section.
Cable Modem Provisioning
In the first step, the cable modem (CM) scans for the downstream (DS) frequency. There are approximately twenty (20) frequency tables in the modem for scanning purposes in the US; however, going into detail on all of them is beyond the scope of the ISCW requirements. By default, 53 MHz is the starting frequency for Cisco CMs. The CM locks on to the digital carrier center frequency and looks for the hexadecimal 1FFE MPEG-2 packet identifier (PID), which signifies DOCSIS. Additionally, it is important to remember that the CM might also have EuroDOCSIS and therefore use special frequencies that are not used in the US.
Once the DS frequency scan is complete, the CM continues to the second step and waits for all upstream channel descriptors (UCDs), which are used for frequency, modulation profile, channel width, and other information. If the CM receives the wrong UCD, it times out and it tries the next UCD until it finally connects. Some modems might actually listen to an upstream channel change (UCC) command sent by the CMTS on the DS frequency which is used to advise the CM of the UCD it should be using. The latest versions of Cisco CMs have essentially three scanning algorithms:
1. Scan NTSC 2. Scan selective European center (EuroDOCSIS) frequencies 3. Scan for a DOCSIS DS at every frequency that is divisible by 250 kHz or 1 MHz
The third stage entails establishing the Layer 1 and Layer 2 connection between the CM and the CMTS. The US level starts at approximately 6 dBmV and increments by 3 dB until it hits the CMTS within –25 to +25 dBmV. The modem uses a temporary Service ID (SID) of 0. Once in range, the modem is told to power adjust to its required level: usually, this is 0 dBmV CMTS input, but it can be set between -10 and +25 dBmV). This finalizes Ranging 1 (R1, init(r1)), and then Ranging 2 (R2, init(r2)) commences by fine-tuning the modem in 1 dB increments. The CMTS can track in 0.25 dB increments, but the modem can only change in 1 dB increments. Init(r1) is in contention time, so collisions could occur. Modems attempt to initialize during the cable insertion interval. Once init(r2) is reached, the modem gets another temporary SID that it usually keeps after full registration. Init(r2) and other provisioning steps are done during reserved times, based on the modem’s SID. Once ranging completes, the CMTS and CM are synchronized. The acronym dBvm simply stands for Decibel referenced to millivolts.
The fourth step is IP address assignment. After the modem and the CMTS are synchronized with levels and timing, the modem obtains its IP address through DHCP. Most CM systems set up a non-routable address space for the modems, such as the 192.168.0.0/24 subnet, which can be allocated to clients, and use a public addressing network for CPE, to allow Internet access.
In the fifth step, the modem obtains the DOCSIS configuration via TFTP. The DOCSIS configuration file is a binary file that contains the parameters required for the modem to come online. This configuration file will vary by provider. The DOCSIS file contains important information such as maximum upstream and downstream speeds, radio frequency information, SNMP management information, and authentication information, amongst many other things.
The sixth step is very brief. During this step, the modem registers QoS with the CMTS. Quality of Service (QoS) for networks is an industry-wide set of standards and mechanisms for ensuring high-quality performance for critical applications. QoS is very important, especially if the MSO is also providing the subscriber with Voice over IP (VoIP) services.
In the seventh step, the modem initializes IP services. The modem downloads the configuration file and configures routing and other IP services, such as NAT, so that one or many subscriber devices can access the Internet at the same time. The modem receives the following information from the provider DHCP server:
. The IP Address and Subnet Mask . The Default Gateway . The address of the TFTP Server . The name of the DOCSIS configuration file . The address of the Time of Day (ToD) server
. The address of the Syslog server . The DNS domain information
These steps are summarized in the following diagram:
Back Office Services
HFC CM CMTS
Step 1: Scan for DS Frequency
Step 2: Receive UCDs
Step 3: Establish Layer 1 and Layer 2
Step 4: Receive IP Addressing
Step 5: Receive DOCSIS File
Step 6: Register QoS
Step 7: Initialization Services
The Public Switched Telephone Network
Although going into detail on the PSTN is beyond the scope of the ISCW, it is important to have a basic understanding of PSTN operation and the components the PSTN is comprised on in order to fully understand the implementation and operation of DSL, which runs on the PSTN.
The Public Switched Telephone Network (PSTN) is the network of the world's public circuit- switched telephone networks. The PSTN is also referred to as Plain Old Telephone Service or Post Office Telephone Service (POTS). Although the PSTN was originally developed as a network of fixed-line analog telephone systems, it is now almost entirely a digital network. The PSTN is largely governed by technical standards created by the ITU-T, and uses E.164 addresses, which are telephone numbers.
The local telephone provider services neighborhoods via a local central office (CO). The CO is the place where the telephone company terminates lines and switches calls between different locations. On the network side, the CO is typically connected to a regional CO that connects to the digital WAN, allowing the local telephone provider to provide local, national long distance, and even international long distance service to subscribers.
On the customer facing side, the telephone company may employ the use of remote switching facilities between the house and the CO. While most signals received from the house are analog signals, some remote switching facilities have the ability to convert those received analog signals into digital signals before forwarding them on to the local CO.
The portion of the PSTN that connects the home to the local CO is referred to as the local loop or last mile. This part of the PSTN almost always uses analog signals; although some remote switching facilities can convert the analog signals from the house into digital signals before forwarding them on to the local CO. At the end of the local loop is the network interface unit (NIU) or network interface device (NID). The NIU is a CPE device that serves as the demarcation point between the carrier's local loop and the customer's premises wiring. The following diagram illustrates the PSTN components described in this section:
Digital Digital WAN
Regional CO
Digital
Remote Switching Analog or Digital Facility Local CO Analog
NIU
Local Loop
On the PSTN, voice communications use the frequency band of 300 Hz – 3 KHz, which leaves higher, inaudible frequencies unused and available for carrying data. By taking advantage of this space, telephone companies are able to provide various types of DSL services to subscribers over the existing PSTN infrastructure.
Digital Subscriber Line
A family of DSL types has been developed to provide high-speed Internet services over the existing telephone line infrastructure. The idea behind DSL technologies is to use a wider frequency band for communicating data over the existing twisted pair lines (telephone lines) at the same time as voiceband. This means that a second frequency band, above the voiceband, must be defined to perform data modulation. This two different frequency bands for voice and data are illustrated in the following diagram:
30 KHz – 1.1 MHz
300 Hz – 3 KHz Data (Modulation)
Voice Amplitude (PSTN)
300 Hz 3 KHz 30 KHz 1.1 MHz Frequency
In the diagram above, the frequency range used for voice is from 300 Hz to 3 KHz. The frequency range used for data ranges from 30 KHz to 1.1 MHz. The frequency band used for data is further segmented into two more bands depending on the modulation technique implemented. DSL modulation techniques will be described in detail later in this chapter.
DSL Network Architecture
Before we delve into detail on the specific type of DSL implementations and standards, it is imperative to have a solid understanding of the components that are used in DSL networks.
DSL modems contain receptacles to connect both to the telephone line of the subscriber and computer, or other network connectivity device such as a wireless router, for example. In DSL terminology, a DSL modem at the customer premise equipment (CPE) end of the local loop, i.e. at the subscriber’s house, is referred to as an xDSL Transmission Unit-Remote (xTU-R). The acronym xTU-R is used to refer to either a DSL modem or DSL-capable router. The x in xTU-R is replaced by the actual DSL variant. For example, for Asymmetric DSL (ADSL), which is the most common type of DSL service, an ATU-R would be used.
The primary function of the DSL modem or ATU-R is to modulate outgoing signals from the telephone and computer and demodulate incoming DSL signals from the telephone provider network. When the DSL modem or ATU-R receives the outgoing signal, it forwards the modulated signal across the local loop towards the CO. Within the CO, a POTS splitter is used to separate the data signal from any voice signals that are also carried on the line.
Once the signals have been separated, the data request is sent to a Digital Subscriber Line Access Multiplexer (DSLAM), which exists for the sole purpose of terminating the CO side of the DSL link. A DSLAM is a single chassis that contains multiple subscriber-facing DSL modems which are referred to as xTU-C devices. xTU-C stands for ADSL Transmission Unit- Central, which indicates that this is a DSL modem in the telephone providers CO. As is the case with xTU-R devices, the x in xTU-C is replaced by the DSL variant. For example, an ATU-C would be used for ADSL. ADSL and SDSL will be described in detail later in this chapter.
Once the DSLAM has received the data, it typically uses an integrated Asynchronous Transfer Mode (ATM) switch to send the data across the providers ATM network until it ends up on an aggregation router on the provider’s network that connects to the Internet. ATM is standard for cell relay wherein information for multiple service types, such as voice, video, and data, are transmitted in small, fixed-size 53-byte cells via connection-oriented Virtual Circuits. ATM will be described later in this chapter. The return path for the data follows the same logic. At the customer premise, another splitter is used to separate the voice and data signals. The different components used in DSL networks are illustrated in the following diagram:
To Digital To WAN Internet
Central Office
Phone Voice Data DSLAM Switch Splitter 0 0 0
0 0
Local Loop
NIU Customer Premise
0 0 0 Splitter 0 0
DSL Modem (ATU-R)
NOTE: In the DSL networks of today, microfilters replace splitters at the customer premise. A microfilter is a passive low-pass filter that is connected to the subscriber’s telephone wall jack. Microfilters only allow frequencies in the 0 – 4 KHz to pass through to connected analog devices, such as analog telephones, modems, and fax machines. Microfilters maintain voice quality on analog devices when DSL and voice are co-existent on the same telephone line.
Now that we have an understanding of how DSL works, and how it leverages the existing telephone infrastructure, it is important to understand why DSL is not implemented everywhere, especially since almost every home has some kind of fixed telephone line. The following section lists and describes the issues that limit complete DSL deployment.
DSL Deployment Issues
There are many issues that hinder the widespread deployment of DSL, which include:
. DSLAM Costs . Radio Interference . Distance Limitations . Wire Gauge . Loading Coils . Crosstalk . Bridged Taps . Pair Gain . Fiber-To-The-Premises
In order to provide DSL service, telephone companies need to purchase DSLAMs and then retrofit their central offices to accommodate the new DSLAMs and required cabling. These activities are expensive and the telephone company, like most businesses, needs to verify the return on investment (ROI) before performing such activities in areas they service.
AM radio frequencies can interfere with DSL signal quality and cause significant throughput reductions in DSL service. Sometimes, the cabling is so bad that DSL service cannot be supported. This is typically a problem in houses that have old or poor-quality twisted or untwisted pair cabling.
One of the biggest hindrances to greater DSL deployment is distance limitations. The existing telephone infrastructure typically uses twisted pair cabling on the local loop. Unfortunately, twisted-pair cabling rapidly attenuates frequencies much higher than one MHz. While the use of repeaters can be implemented to increase distances at which the signals travel, the downside is that the greater the distance, the slower the DSL throughput.
Variations in wire thickness (wire gauge) can affect DSL throughput rates due to impedance mismatches. Impedance is the resistance that contributes to controlling an electrical signal. Impedance is measured in ohms. Impedance mismatches in the local loop cause echo, which results in noise on the line, which ultimately affects DSL throughput rates and quality.
Loading coils, or load coils, are inductors that cancel noise during voice calls, as well as to extend the range of the local loop for voice services. The problem is that they also cut off any frequencies above 4 kHz. Because DSL technologies work at higher frequencies, loading coils must be removed in order to support DSL. However, this is not always feasible, more so because about 15 to 20 percent of the local loops in the U.S. contain loading coils.
Crosstalk is a type of interference caused by signals travelling on nearby wire pairs infringing on another pairs signal. Crosstalk also occurs when there is a frequency overlap between channels. Two types of cross talk pose potential problems for DSL systems are Near End Crosstalk (NEXT) and Far End Cross Talk (FEXT). NEXT occurs when a receiving station overhears a signal being sent by a transmitting station at the same end of a neighboring line. FEXT occurs when a receiving station overhears a signal sent by a transmitting station at the opposite end of a neighboring line. NEXT poses more of a problem for DSL than FEXT, particularly for downstream transmissions. This is because there are more copper pairs closer together at the CO end of a loop running DSL than at the user's end of a loop running DSL.
A bridged tap, also called a bridge tap, is a non-terminated and unused wire pair connected to a primary local loop. Unlike traditional T1 and E1 services, for which all bridged taps must be removed, which is a major contributing factor to the high cost of those services, DSL can tolerate bridged taps. Nevertheless, bridged taps can impair signal transmission, and depending on how many bridged taps are on a local loop, their presence can significantly limit the rate for DSL service. Additionally, multiple bridged taps can degrade a line beyond use for ADSL.
Pair gain is a method of transmitting multiple POTS signals over the twisted pairs traditionally used for a single subscriber line in telephone systems. Pair gain has the effect of creating additional subscriber lines. However, analog pair gain is detrimental to high speed dial-up modem connections, does not support 56 Kbps throughput and is incompatible with DSL systems. While recent digital pair gain systems restored 56 Kbps and DSL capabilities by performing the functions of a DSLAM at the pair gain device, some telephone networks have legacy telephone networks that use analog pair gain. Replacing these is expensive.
Finally, Fiber-To-The-Premises (FTTP) or Fiber-To-The-Home (FTTH) is when a telephone company connects residential users to its network via fiber optic cabling. While this increases the range of services and potential throughput available to customers it prohibits DSL deployment because the signals cannot pass the analog to digital to analog conversion that occurs when a portion of the telephone circuit traverses the fiber optic cable in transit. Two classic examples of FTTH services are AT&T U-verse and Verizon FiOS.
Now that we are aware of the factors that hinder greater DSL deployment, we are going to learn about the different flavors of DSL technology. There are several varieties of DSL, which are collectively referred to as xDSL varieties. The x is xDSL is replaced by the variety name. xDSL speeds are typically determined by the distance that the subscriber is from the CO. DSL achieves its highest maximum the closer it is to the CO. All DSL variants have distance limitations. In other words, all DSL variants can work only within a certain distance from the CO. DSL types can be divided into two broad categories, which are symmetrical DSL (SDSL) and asymmetrical DSL (ADSL).
Symmetric Digital Subscriber Line
SDSL provides equal capacity for data traveling both upstream and downstream. Symmetrical transmission is most suited to users who both upload and download significant amounts of data. The following section describes SDSL variants.
Symmetric DSL (SDSL) is a DSL variant that runs over one pair of copper wires and supports data only. SDSL does not support analog calls because of the fact that it takes over the entire bandwidth of the line. SDSL is a proprietary technology that was never standardized. SDSL provides up to T1 (1.544 Mbps) and E1 (2.048 Mbps) downstream and upstream throughput. SDSL has a distance limit of 10,000 feet. SDSL is also typically offered in the following speeds:
Type Upstream Speed Downstream Speed SDSL-192 192 Kbps 192 Kbps SDSL-384 384 Kbps 384 Kbps SDSL-768 768 Kbps 768 Kbps SDSL-1.1 1.1 Mbps 1.1 Mbps
Single-Pair High-speed DSL or Single-Line High bit-rate DSL (SHDSL) is an industry standard for SHDSL defined in ITU-T recommendation G.991.2. SHDSL is also referred to as G.SHDL. SHDSL uses frequencies that overlap with those used by traditional POTS telephone services to provide symmetric data rates, which means SHDSL does not support analog calls. SHDSL provides symmetric throughput of 2.3 Mbps; however, an optional extended SHDSL mode makes it possible to allow even greater symmetric throughput speeds. SHDSL has a distance limit of 26,000 feet.
High bit-rate (HDSL) is the most mature of the xDSL approaches. HDSL can be used either at the T1 or E1 rates. This DSL variant is commonly used in place of traditional T1 circuits. As is the case with SHDSL and SDSL, HDSL does not allow for standard telephone service over the copper. HDSL has a distance limitation of 12,000 feet.
HDSL2 is the 2nd generation of HDSL with a 6dB Noise Margin. HDLS2 provides symmetric service at T1 speeds using a single-wire pair rather than the two pairs of HDSL service. HDSL-2 also was developed as a standard, allowing for interoperability between different vendors' equipment. HDSL2 employs a line coding technique known as trellis-coded pulse amplitude modulation (TC-PAM), also known as trellis-coded modulation (TCM) and has a distance limitation of 22,000 feet.
ISDN DSL (IDSL) uses ISDN-based technology to provide a data communication channel across existing copper telephone lines at a rate of 1.44 Kbps. IDSL uses a single-wire pair for symmetric speeds. While IDSL has a distance limitation of 18,000 feet, local telephone providers can increase this limitation to 45,000 feet using repeaters. Repeaters, which were described earlier in this chapter, are used to boost digital signals in the same manner that amplifiers are used to boost analog signals.
Asymmetric Digital Subscriber Line
ADSL is the most common type of DSL service. Unlike SDSL which provides the same upstream and downstream speeds (symmetric), Asymmetric DSL provides different downstream and upstream speeds. In asymmetrical connections, the downstream throughput is higher than the downstream throughput. Additionally, it is important to know that unlike SDSL variants, ADSL variants allow for voice and data to be sent simultaneously over the existing telephone line. This is because ADSL operates at higher frequencies than PSTN/POTS so they can coexist on the same media. The following section describes ADSL variants.
Asymmetric DSL (ADSL) supports speeds of 1.5 to 8 Mbps, depending on line quality, distance, and wire gauge. Upstream rates range between 16 Kbps and 1 Mbps. The table that follows lists the distance limitations for ADSL based on wire gauge and data rate:
Data rate Wire gauge Wire size Distance Distance (Mbps) (AWG) (mm) (feet) (kilometers) 1.5 or 2 24 0.5 18,000 5.5 1.5 or 2 26 0.4 15,000 4.6 6.1 24 0.5 12,000 3.7 6.1 26 0.4 9,000 2.7
G.Lite ADSL is also referred to as splitterless ADSL, because it can allow voice and data to coexist on the existing telephone line without using a splitter. The idea was to trade the potential for bandwidth greater than T1 speeds in order to enable "splitterless" installation, meaning that if the functionality of splitting off the analog voice could be built into the ADSL modem, then it wouldn't be necessary to dispatch a technician for installation. Although now standardized in ITU G.992.2, it is unclear if it will ever be widely deployed, since service providers are currently holding trials with another splitterless variation on ADSL that is faster (ADSL2). G.Lite ADSL has a distance limitation of up to 25,000 feet.
ADSL2 extends the capability of basic ADSL in data rates to 12 Mbps downstream and 3.5 Mbps upstream, with a mandatory capability of ADSL2 transceivers of 8 Mbps downstream and 800 Kbps upstream. However, actual speeds may reduce depending on line quality and the distance from the subscriber to the CO. ADSL2 is standardized in ITU G.992.3 and is also referred to as G.DMT.bis. Splitterless ADSL2, on the other hand, is standardized in ITU G.992.4 and has the
data rate mandatory capability reduced to 1.536 Mbps downstream and 512 Kbps upstream. It is also referred to as G.lite.bis. ADSL2 has a distance limitation of about 20,000 feet.
ADSL2+ or ADSL2Plus is standardized in ITU G.992.5. ADSL2+ extends the capability of basic ADSL by doubling the number of downstream bits. The data rates can be as high as 24 Mbps downstream and 1.4 Mbps upstream depending on the distance from the CO to the home of the subscriber. ADSL2+ also allows port bonding, where multiple ports are physically provisioned to the subscriber and the total bandwidth is equal to the sum of all provisioned ports. For example, if two lines capable of 24 Mbps were bonded the end result would be a connection capable of 48 Mbps. ADSL2+ port bonding is also known as G.998.x or G.Bond. Not all vendor DSLAMs support port bonding and it is important to know that speeds vary depending on distance. ADSL2+ has a distance limitation of about 20,000 feet.
Rate-adaptive DSL (RADSL) has the same transmission limits as ADSL, but it automatically adjusts transmission speed according to the length and quality of the local line. While this is considered the defining characteristic of RADSL, it should be noted that standard ADSL also allows the DSL modem to adapt speeds of data transfer. With Rate-adaptive DSL, connection speed is established when the line syncs up and varies between 600 Kbps and 7 Mbps downstream, and between 128 Kbps and 1 Mbps upstream. Rate-adaptive DSL has a distance limitation of 18,000 feet.
Very High bit-rate DSL (VDSL or VHDSL) is the fastest DSL technology, with downstream rates of 13-52 Mbps and upstream rates of 1.5-2.3 Mbps over a single wire pair. VDSL can also operate in symmetric mode at 26 Mbps. VDSL is standardized in ITU G.993.1 and has a distance limitation of only 4,500 feet, which is considered a very short local loop. VDSL was principally developed for the transport of ATM at high speed over a short distance.
Very High bit-rate DSL 2 (VDSL or VHDSL) is an enhancement of VDSL and is standardized in ITU-T G.993.2. VDSL2 allows for the the transmission of asymmetric and symmetric aggregate data rates up to 200 Mbps on twisted pairs using a bandwidth up to 30 MHz. ADSL-like long reach performance is one of the key advantages of VDSL2. Long Reach VDSL 2 (LR-VDSL2) enabled systems are capable of supporting speeds of around 1-4 Mbps downstream over distances of 16,000 feet, gradually increasing the bit rate up to symmetric 100 Mbps as loop- length shortens.
Other Digital Subscriber Line Technologies
While the previous two sections have described the most common SDSL and ADSL types, the following section lists and describes other emerging and interesting DSL technologies.
Uni-DSL (UDSL) is a DSL technology developed by Texas Instruments which would provide at least 200 Mbps in aggregate on the downstream and upstream paths. UDSL is backwards compatible with all DMT standards (ADSL, ADSL2, ADSL2+, VDSL and VDSL2). DMT
modulation will be described later in this chapter. UDSL means One DSL for universal service. With UDSL, providers will be able to provide all DMT-based services from one line card or home gateway, making deployment easier and more affordable.
Multi-rate Symmetric DSL (MSDSL) is simply and SDSL type that is capable of more than one transfer rate. The transfer rates are typically based on the service type, price, or both, depending on the provider. MSDSL is capable of up to 2 Mbps of symmetric throughput and has a maximum distance of 29,000 ft.
Power line Digital Subscriber Line (PDSL) is a system for carrying data on a conductor also used for electric power transmission. PDSL is more commonly referred to as power line communication or power line carrier (PLC) because it operates by sending high-speed broadband transmissions (Broadband over Power Lines or BPL) over U.S. overhead electric power lines. BPL was developed by Penn State engineers and uses PLC by sending and receiving information bearing signals over power lines to provide access to the Internet. Current trials run at DSL comparable rates of 2 or 3 Mbps. However, it is worth mentioning that in computer simulated testing, speeds close to 1 Gbps have been attained.
CDSL (Consumer Digital Subscriber Line) is a version of DSL service, trademarked by Rockwell Corp. that is somewhat slower than Asymmetric DSL (ADSL). CDSL provides up to 1 Mbps downstream and has the advantage that a splitter does not need to be installed at the subscriber end of the local loop.
EtherLoop is currently a proprietary technology from Nortel Networks that stands for Ethernet Local Loop. EtherLoop uses the advanced signal modulation techniques of DSL and combines them with the half-duplex nature of Ethernet. EtherLoop modems will only generate hi- frequency signals when data needs to be sent. The rest of the time, they will use only a low- frequency management signal. Because EtherLoop is half-duplex, it is capable of generating the same bandwidth rate in either the upstream or downstream direction, but not simultaneously. Nortel is initially planning for speeds ranging between 1.5Mb/s and 10Mb/s depending on line quality and distance limitations.
Finally, Consumer-installable Digital Subscriber Line (CiDSL) is a proprietary, splitterless DSL variant created by Globespan.
Digital Loop Carrier
As stated earlier in this chapter, one of the greatest hindrances to DSL deployment is distance limitations. However, in order to reach locations that couldn't otherwise be reached, a technique called Digital Loop Carrier (DLC) is being used by some providers. DLC is to DSL what HFC is to cable networks. In DLC, a fiber optic cable carries the aggregated traffic back to the local central office (CO).
Residences and businesses that are more than 18,000 feet from the CO are sometimes served by remote terminals, which are fed by fiber optic cable from the CO. Copper wire then runs from the remote terminals to the residence or business over a short loop. As we learned earlier in this chapter, DSL cannot run over fiber optic cable. However, providers have a couple of options for making DSL available to users served by DLC.
The first option is that providers can use Integrated Services Digital Network DSL (IDSL), to deliver DSL services through a DLC remote terminal using an installed base of Integrated Services Digital Network (ISDN) modems.
The second option is that providers can install DSLAMs within these remote terminals to provide ADSL or G.Lite services from the remote terminal to these DLC customers. Going into further detail on DLC implementation and operation is beyond the scope of the ISCW.
Digital Subscriber Line Modulation
DSL flavors use various modulation techniques. These techniques include:
. 2 Binary, 1 Quaternary . Quadrature Amplitude Modulation . Carrierless Amplitude and Phase Modulation . Discrete Multi Tone Modulation
NOTE: You are not expected to demonstrate advanced knowledge on DSL modulation techniques. However, you are expected to be familiar with these techniques.
2 Binary 1 Quaternary (2B1Q) modulation exhibits similar transmission characteristics to analog signals when transmitted over copper loops. The spectrum of signals transmitted by 2B1Q extends from zero to over 1.5MHz because of the multi-level bit modulation technique used. This limits the use of this baseband technique to short distances as the bandwidth for 12,000 feet of 24-gauge twisted pair is only 400Khz. 2B1Q is not at all suitable for very high bit-rate systems. Many implementations of HDSL use 2B1Q.
Quadrature Amplitude Modulation (QAM) is an ADSL modulation technique. QAM is a way of fitting information onto a limited frequency line. In the case of ADSL it is copper wire. QAM
can split a single signal into 16 by using both phase and amplitude modulation. QAM uses a combination of sine and cosine waves at different phases to each other to produce these signals in quadrature. QAM uses four different amplitudes for each of the waves, which results in 16 different signal types being generated using all possible pairs of the amplitudes.
Carrierless Amplitude and Phase Modulation (CAP) is a version of suppressed carrier QAM. By definition, CAP is an ADSL single-carrier modulation technique that divides the available space into three distinct bands as follows:
1. The frequency band (range) for POTS transmissions is from 0 to 4 KHz 2. The frequency band (range) for upstream data traffic is from 25 KHz to 160 KHz 3. The frequency band (range) for downstream data traffic is 240 KHz to 1.1 MHz
Like 2B1Q, CAP supports more than one bit per frequency cycle. CAP allows anywhere from 2 to 9 bits per frequency cycle. CAP-based DSL transceivers can transmit the same amount of information as a 2B1Q DSL transceiver using a lower range of the frequency spectrum, resulting in less signal attenuation and, thus, greater loop reach. Although many ADSL systems are based on CAP, the American National Standards Institute (ANSI) and the International Telecommunications Union (ITU) both selected DMT modulation as the standard for ADSL.
Discrete Multi Tone Modulation (DMT) is an ADSL modulation technique that divides its copper communications channel into separate (or discrete) sub-channels or tones. With DMT, there are usually 256 sub-channels for downstream data and 32 channels for upstream data. At startup, modems that use DMT run a test to determine the carrying capacity of each sub- channel. These modems then break down the incoming data into bits and distribute the bits among the sub-channels. Each sub-channel carries from 0 to 15 bits per hertz (Hz), depending upon the sub-channel's ability to carry the transmission. For example, low-frequency sub- channels typically carry more bits per hertz than high-frequency sub-channels because low- frequency sub-channels are less affected by attenuation.
In addition to these, two other line codes have been proposed for VDSL. A line code, which is also a digital baseband modulation, is a code chosen for use within a communications system for baseband transmission purposes. Line coding is used for digital data transport. These two line codes are DWMT and SLC.
Discrete Wavelet Multi-tone (DWMT) is a multicarrier system using wavelet transforms to create and demodulate individual carriers. Although DWMT uses FDM for upstream multiplexing, it also allows TDMA. Simple Line Code (SLC) is a version of 4-level baseband signaling that filters the based band and restores it at the receiver. SLC would most likely use TDMA for upstream multiplexing, although FDM is possible. Going into any further detail pertaining to DWMT and SLC is beyond the scope of the ISCW certification requirements.
Sending Data over ADSL Networks
Because ADSL is the most common DSL implementation, the remainder of this chapter will be restricted to ADSL operation and configuration. DSL provides Layer 1 connectivity to the provider’s network. This connectivity is established between the xTU-R (which is the DSL capable modem or DSL-capable router) and the xTU-C, which is the DSL modem housed in the DSLAM in the CO. As previously stated, the ATU-C and ATU-R are the main endpoint components in an ADSL data service network.
On the provider side, the DSLAM typically has connectivity to the provider network via an ATM network; with ATM being the Layer 2 technology used. On the subscriber side, several methods can be used. These methods are:
. Long Range Ethernet . PPP over Ethernet . PPP over ATM . Routed Bridged Encapsulation . Multiprotocol Encapsulation over ATM . Service Selection Gateway
Of the technologies listed, it is important to know that only PPP over Ethernet (PPPoE), and PPP over ATM (PPPoA are mandatory requirements of the ISCW. These technologies will be described in detail later in this chapter. The remaining technologies are described briefly in the following section.
The Cisco Long Range Ethernet (LRE) solution leverages VDSL technology to dramatically extend Ethernet services over existing Category 1, 2, or 3 twisted pair cabling at speeds from 5 to 15 Mbps (full duplex) and distances up to 5,000 feet. The Cisco LRE technology delivers broadband service on the same lines as Plain Old Telephone Service (POTS), digital telephone, and ISDN traffic. In addition, Cisco LRE supports modes compatible with ADSL technologies, allowing providers to provision LRE to buildings where broadband services already exist.
Routed Bridged Encapsulation (RBE) is the process by which a stub-bridged segment is terminated on a point-to-point routed interface. The router routes on an Ethernet header carried over a point-to-point protocol, such as PPP, RFC 1483 ATM, or RFC 1490 Frame Relay. RBE was developed to address the known RFC1483 bridging issues, including broadcast storms and security. Except for the fact that it operates exclusively over ATM, the RBE feature functions identically to half-bridging. Additional scalability, performance, and security can be achieved by using the unique characteristics of xDSL subscribers.
Multiprotocol Encapsulation over ATM was originally standardized in RFC 1483, which was then rendered obsolete by RFC 2684. Multiprotocol Encapsulation over ATM describes two different methods for carrying connectionless network interconnect traffic over an ATM network, which are routed protocol data units (PDUs) and bridged PDUs. Routing allows multiplexing of multiple protocols over a single ATM virtual circuit (VC). The protocol of a carried PDU is identified by prefixing the PDU with an IEEE 802.2 Logical Link Control (LLC) header. Bridging performs higher-layer protocol multiplexing implicitly by ATM VCs.
The Cisco Service Selection Gateway (SSG) is a switching solution for Service Providers who offer Intranet, Extranet, and Internet connections to subscribers using broadband access technology such as digital subscriber lines (DSL), cable modems, or wireless LAN. SSG works in conjunction with the Cisco Subscriber Edge Services Manager (SESM), a software toolkit that can reside on Windows, UNIX, and Linux servers. Together with the SESM, SSG provides subscriber authentication, service selection, service connection and accounting capabilities to subscribers of Internet services. The SSG-SESM solution, which is collectively known as Subscriber Access and Management (SAM), also provides account self-care portal for subscribers and branding, advertisement abilities for service providers. Subscribers interact with a SESM-based web application using a standard Internet browser. Current deployments include DSL, Public Wireless LAN (PWLAN), and Mobile Wireless solutions.
Point-to-Point Protocol
This section provides a basic overview of the PPP protocol, which is used in ADSL networks. However, it is important to know and keep in mind that PPP on its own is not a requirement of the ISCW certification. However, because of its integral use in ADSL implementation, it is important to have a basic understanding of PPP operation.
PPP operates in full-duplex mode. Unlike Cisco’s HDLC, PPP is an open-standard protocol meaning it can be used to connect two routers from different vendors. PPP works by encapsulating upper layer protocols for transmission across the link and offers the following advantages over HDLC:
. Compression . Dynamic addressing . Authentication . Link configuration and negotiation . Error detection . Multilink capability . Multiple protocol support
PPP uses Link Control Protocol (LCP) to manage the Data Link connection. LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. PPP uses the Network Control Protocol (NCP) support and allow multiple Layer 3 protocols to use the same communications link.
Data Link Layer configuration and negation is the first stage and must be complete before any Layer 3 information can be exchanged. This stage is said to be complete when a configuration- acknowledgment frame (Configure-ACK) has been sent and received. Additionally, two other PPP messages types, which are the Configure-NACK and Configure-Reject, may also be sent during this phase if the values received by the peer during negotiation are unacceptable, or if there are some unknown values present.
Link quality determination is an optional stage in which the link is tested to determine whether or not the link quality is sufficient enough to bring up network layer protocols. Network Layer protocol negotiation is the third stage and involves Layer 3 protocol configuration. PPP uses NCP to configure the Layer 3 protocols. Assuming that IP is the Layer 3 protocol, NCP uses IP Control Protocol (IPCP) to manage the use of IP over the PPP communications link.
Link termination is the final phase and involves the termination of the communications link due to user requests or other events, such as a physical link failure.
Asynchronous Transfer Mode
As is the case with PPP, ATM by itself is not a requirement of the ISCW. However, because of its integral use in ADSL implementation, it is important to have a basic understanding of ATM.
Asynchronous Transfer Mode (ATM) is an International Telecommunication Union- Telecommunications Standards Section (ITU-T) standard for cell relay wherein information for multiple service types, such as voice, video, or data, is conveyed in small, fixed-size 53 byte cells via connection-oriented Virtual Circuits (VCs).
ATM provides two kinds of virtual connection services, permanent and switched. Permanent virtual connections (PVCs) are manually set up and remain up until manually torn down. Switched Virtual Circuits (SVCs) are dynamically established when data needs to be transferred. SVCs are beyond the scope of this topic. The two main types of ATM PVCs are:
. Permanent Virtual Channel Connections (PVCCs), which are specified by a Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI)
. Permanent Virtual Path Connections (PVPCs), which are specified by a VPI only
Both PVCCs and PVPCs can support point-to-point and point-to-multipoint connections. The VCI is a unique identifier which indicates a particular virtual circuit on a network. It is a 16-bit field in the header of an ATM cell. VPI refers to an 8-bit (user-to-network packet) or 12-bit (network-network packet) field within the header of an Asynchronous Transfer Mode packet. The VPI, together with the VCI, is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination.
From the standpoint of the ATM switch router, virtual connections can be further characterized as transit or terminating connections. Transit connections are switched from the ingress to the egress of the connection, while terminating connections terminate at the ATM switch router. For the most part, terminating connections are used for management and signaling purposes; however, the endpoint of a normal data connection can also be considered as terminating.
The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an Adaptation Layer in order to support information transfer protocols, which are not based on ATM. The ATM Adaptation Layer defines how to segment and reassemble higher-layer packets into ATM cells, and how to handle various transmission aspects in the ATM layer. Several ATM Adaptation Layer protocols (AALs) have been defined by the ITU-T. These are:
. AAL 1 . AAL 2 . AAL 3/4 . AAL 5
Of these protocols, only AAL 5 is relevant to DSL technology. ATM AAL 5 was introduced to:
. Reduce protocol processing overhead . Reduce transmission overhead . Ensure adaptability to existing transport protocols
AAL 5 is used to send variable-length packets up to 65,535 bytes in size across an ATM network. Each AAL 5 packet is divided into a number of ATM cells and reassembled into a packet before delivery to the receiving host, in a process is known as Segmentation and Reassembly (SAR). By default AAL 5 SNAP encapsulation is used for ATM PVCs.
In AAL 5 SNAP (Subnetwork Access Protocol) encapsulated PVCs, LLC (Logical Link Control) SNAP encapsulation is used to identify the protocol of packets transmitted across the ATM PVC. However, this encapsulation method adds bandwidth usage with the transmission of frames, which can affect voice quality. To address this issue, ATM AAL 5 MUX encapsulation can be used. AAL 5 MUX reduces SNAP encapsulation bandwidth usage by using multiplexed encapsulation to reduce the number of ATM cells needed to carry voice packets. ATM AAL 5 MUX in a VoIP environment results in improved throughput and bandwidth usage.
PPP over Ethernet
PPP over Ethernet (PPPoE) provides the ability to connect a network of hosts over a simple bridging access device to a remote access concentrator or aggregation concentrator. Each host uses its own PPP stack, thus presenting the user with a familiar user interface. Access control, billing, and type of service can be done on a per-user, rather than a per-site, basis. By default, PPPoE runs on top of ATM AAL 5 SNAP; however, PPPoE can also be configured to use ATM AAL 5 MUX encapsulation.
As specified in RFC 2516, PPPoE has two distinct stages: a discovery stage and a PPP session stage. When a host initiates a PPPoE session, it must first perform discovery to identify which server can meet the client's request, then identify the Ethernet MAC address of the peer and establish a PPPoE session id. While PPP defines a peer-to-peer relationship, discovery is inherently a client-server relationship.
At a very high level, during the discovery process, a host (referred to as the client) discovers one or more access concentrators and selects one. When discovery completes successfully, both the host and the selected access concentrator have the information in order to build their point-to- point connection over Ethernet. After a PPP session is established, both the host and the access concentrator must allocate the resources for a PPP virtual interface, although it should be noted that this is probably not the case for all implementations.
The discovery phase has 4 steps. These steps are depicted in the following diagram and described in detail following the diagram:
The PPPoE session is established between the PPPoE Client and the provider Aggregator
ATM To Core IP or ATM Ethernet Local Backhaul Loop Connection Network Network Host with Cisco DSL- Cisco 6400 PPPoE Client capable Router Cisco 6160 Aggregator (ATU-R) DSLAM
STEP 1: Broadcast Initiation Packet
STEP 2: Unicast Discovery Offer
STEP 3: Unicast Discovery Request
STEP 4: Unicast Session Confirmation
Referencing the diagram illustrated above, in the first step of the discovery phase, the PPPoE client broadcasts a PPPoE Active Discovery Initiating (PADI) packet. The PADI consists of one tag that indicates what service type it requests.
In the second step, one or more aggregators respond to the PADI packet via a PPPoE Active Discovery Offer (PADO) packet. The PADO packet is Unicast to the PPPoE client. If the access concentrator or aggregator cannot serve the PADI, it must not respond with a PADO. Because the PADI was broadcast, the host can receive more than one PADO, in the same manner that a DHCP client can receive a response from more than one DHCP server. It is up to the client to decide which concentrator it will use.
In the third step of the discovery phase, the PPPoE client looks through the PADO packets it receives (if more than one is received) and chooses one. The choice is based on the services offered by each access concentrator. The PPPoE client then sends a PPPoE Active Discovery Request (PADR) packet to the access concentrator it chooses. The destination address field is set to the Unicast Ethernet address of the access concentrator or aggregator that sends the PADO. At this stage, the request moves on to the session phase.
In the fourth and final step of the discovery phase, the selected aggregator sends a confirmation packet. As previously stated, when the access concentrator receives a PADR packet, it prepares to begin a PPP session and generates a unique session id for the PPPoE session and replies to the host with a PPPoE Active Discovery Session-Confirmation (PADS) packet. The destination address of the PADS packet is the Unicast Ethernet address of the host that sends the PADR.
Once the PPPoE session begins, PPP data is sent as in any other PPP encapsulation. A PPPoE Active Discovery Terminate (PADT) packet can be sent by either the PPPoE client or the access concentrator any time after a session is established to indicate that a PPP over Ethernet session has been terminated.
The conversation between the PPPoE client and the aggregator takes place using Ethernet frames. PPP frames are encapsulated in PPPoE session frames, which have Ethernet frame type 0x8864. The following diagram illustrates the PPPoE session frame:
0 7 15 23 31
Access Concentrator (Aggregator) Ethernet MAC Address (first four bytes)
Access Concentrator (Aggregator) Ethernet PPPoE Ethernet MAC Address MAC Address (last two bytes) (last two bytes)
PPPoE Ethernet MAC Address (last four bytes)
Ethernet Frame Type Version Type Code
Session ID Length
PPP Payload
Because at this stage you should be familiar with what a MAC address is, only the fields in bold font will be described. The information contained in each field is as follows:
. Ethernet Frame Type
This 2-byte field specifies the Ethernet frame type for PPP frames encapsulated in PPPoE session frames. This field contains the value 0x8864.
. Version
This is a 4-bit field that is used to specify the version for this specification of PPPoE. This field must also always be set to 1 (0x1).
. Type
This is a 4-bit field that is used to specify the frame type for this specification of PPPoE. This field must be set to 1 (0x1). This is not the same as the type field in Ethernet packets.
. Code
This 8-bit or 1-byte field is used in the discovery phase to identify the packet type, but is always set to zero (0x0) in the session phase.
. Session ID
This 16-bit or 2-byte field contains the session ID assigned during discovery. This value is fixed for a given session and must never be 0xFFFF (which is reserved for future use).
. Length
This 16-bit or 2-byte field contains the length of the PPP payload, not including the Ethernet or PPPoE headers.
. PPP Payload
This is a variable length field that contains the PPP data. The PPP data begins with the PPP protocol field. Note that no flag sequences are included. The PPP data is not byte-stuffed with the escape sequence, and does not include the PPP Frame Check Sequence (FCS), which is omitted because Ethernet frames have their own FCS, and there is no point in duplicating it.
Once the session phase begins, PPP data may be sent. At this stage, all Ethernet packets are Unicast between the aggregation router and the PPPoE client.
PPPoE introduces an interesting and unique problem. The maximum Ethernet frame is 1518 bytes long. 14 bytes are consumed by the header and 4 by the FCS, leaving 1500 bytes for the payload. For this reason, the Maximum Transmission Unit (MTU) of an Ethernet interface is usually 1500 bytes. This is the largest IP datagram which can be transmitted over the interface without fragmentation. PPPoE, however, adds another 6 bytes of overhead, and the PPP protocol field consumes 2 bytes, leaving 1492 bytes for the IP datagram. Because of this, RFC 2516 specifies the Maximum-Receive-Unit (MRU) option for PPPoE that must not be negotiated to a larger size than 1492. The MTU of PPPoE interfaces must therefore be set to 1492 bytes.
While PPPoE is typically implemented using Ethernet interfaces (allowing the PPP packets to be encapsulated in the Ethernet frames) it can also be implemented using Asynchronous Transfer Mode (ATM) interfaces. In such cases, the PPPoE packets are encapsulated in ATM cells.
The PPPoE over ATM (PPPoEoA) AAL 5 MUX feature enables PPPoE over AAL5-multiplexed (AAL 5 MUX) PVCs, which reduces LLC and SNAP encapsulation bandwidth usage and thereby improving bandwidth usage for the PVC. While going into detail on ATM cell formats is beyond the scope of the ISCW, ensure that you have a basic understanding of this concept and how the encapsulation is different from that applicable with Ethernet interfaces. PPPoEoA is standardized in RFC 1483/2648.
PPPoE has numerous advantages. These advantages are:
1. Per session authentication based on PAP or CHAP. This is the greatest advantage of PPPoE as authentication overcomes the security hole in a bridging architecture.
2. Per session accounting is possible, which allows the service provider to charge the subscriber based on session time for various services offered. The service provider can also require a minimal access charge.
3. You can use PPPoE on current CPE installations that cannot be upgraded to PPP or that do not have the ability to run PPPoA, which extends the PPP session over the bridged Ethernet LAN to the PC.
4. PPPoE preserves the point-to-point session used by Internet Service Providers (ISPs) in the current dialup model. PPPoE is the only protocol capable to run point-to-point over Ethernet without the requirement of an intermediate IP stack.
5. The Network Access Provider (NAP) or Network Service Provider (NSP) can provide secure access to a corporate gateway without the management of end-to-end permanent virtual circuits (PVCs) and without the use of Layer 3 routing and/or Layer 2 Tunneling Protocol (L2TP) tunnels. This makes the business model of the sale of wholesale services and virtual private networks (VPNs) scalable. L2TP is beyond the scope of this guide.
6. PPPoE can provide a host (PC) access to multiple destinations at a given time. In other words, PPPoE allows you to have multiple PPPoE sessions per PVC.
7. The NSP can oversubscribe by the deployment of idle and session time-outs with the help of an industry standard Remote Authentication Dial-In User Service (RADIUS) server for each subscriber. The RADIUS security protocol will be described in detail later in this guide.
8. You can use PPP with the Service Selection Gateway (SSG) feature.
In contrast, however, PPPoE has the following disadvantages:
1. You must install PPPoE client software on all hosts (computers) that connect to the Ethernet segment. This means that the access provider must maintain the CPE and the client software on these hosts.
2. Since PPPoE implementation uses RFC 1483 bridging, it is susceptible to broadcast storms and possible denial-of-service attacks.
PPP over Asynchronous Transfer Mode (ATM)
PPPoA is specified in RFC 2364. Though PPPoE is used in most countries, the U.K and some parts of the U.S use PPPoA to provide ADSL service. PPPoA is a network protocol for encapsulating PPP frames in ATM Adaptation Layer 5 (AAL5). While PPPoA uses AAL 5 SNAP for encapsulation, by default, the most common encapsulation type used is AAL 5 MUX.
The network architecture of PPPoA is similar to that of PPPoE. However, the process for establishing connectivity is slightly different. For PPPoA, when the CPE (ATU-R) is first powered on, it starts sending PPP LCP configuration requests to the aggregation router. The aggregation server, with the PVCs configured, also sends out the PPP LCP configuration request on a Virtual Access Interface that associated with the PVC. When the CPE and aggregation server (router) see each other’s configuration requests, they both acknowledge the configuration requests and the PPP LCP state is opened.
For the authentication stage, the CPE sends the authentication request to the aggregation router or server. The router, depending on its configuration, either authenticates the user based on the domain name (if supplied), or the username using its local database or RADIUS servers. If the request from the subscriber is in the form of 'username@domainname', the aggregation server will try to create a tunnel to the destination, if one is not already there.
After the tunnel is created, the aggregation router forwards the PPP requests from the subscriber to the destination. The destination, in turn, authenticates the user and assigns an IP address. If the request from the subscriber does not include the domain name, the user is authenticated by the local database. If SSG is configured on the aggregation router, the user can access the default network as specified and can get an option to select different services.
As is the case with PPPoE, PPPoA has several advantages, which are:
1. Per session authentication based on PAP or CHAP. This is the greatest advantage of PPPoA as authentication overcomes the security hole in a bridging architecture.
2. Per session accounting is possible, which allows the service provider to charge the subscriber based on session time for various services offered. Per session accounting enables a service provider to offer a minimum access level for minimal charge and then charge subscribers for additional services used.
3. IP address conservation at the CPE. This allows the service provider to assign only one IP address for a CPE, with the CPE configured for network address translation (NAT). All users behind one CPE can use a single IP address to reach different destinations. IP management overhead for the Network Access Provider/Network Services Provider (NAP/NSP) for each individual user is reduced while conserving IP addresses. Additionally, the service provider can provide a small subnet of IP addresses to overcome the limitations of port address translation (PAT) and NAT.
4. NAPs and NSPs provide secure access to corporate gateways without managing end-to-end PVCs and using Layer 3 routing or Layer 2 Forwarding (L2F) and Layer 2 Tunneling Protocol (L2TP) tunnels. Hence, these entities can scale their business models for selling wholesale services.
5. Troubleshooting individual subscribers. The NSP can easily identify which subscribers are on or off based on active PPP sessions, rather than troubleshooting entire groups as is the case with bridging architecture.
6. The NSP can oversubscribe by deploying idle and session timeouts using an industry standard Remote Authentication Dial-In User Service (RADIUS) server for each subscriber.
7. Highly scalable as we can terminate a very high number of PPP sessions on an aggregation router. Authentication, authorization, and accounting can be handled for each user using external RADIUS servers.
8. It allows for the optimal use of features on the Service Selection Gateway (SSG).
On the downside, PPPoA has the following disadvantages:
1. Only a single session per CPE on one virtual channel (VC). Since the username and password are configured on the CPE, all users behind the CPE for that particular VC can access only one set of services . Users cannot select different sets of services, although using multiple VCs and establishing different PPP sessions on different VCs is possible.
2. Increased complexity of the CPE setup. Help desk personnel at the service provider need to be more knowledgeable. Since the username and password are configured on the CPE, the subscriber or the CPE vendor will need to make setup changes. Using multiple VCs increases configuration complexity.
3. The service provider needs to maintain a database of usernames and passwords for all subscribers. If tunnels or proxy services are used, then the authentication can be done on the basis of the domain name and the user authentication is done at the corporate gateway. This reduces the size of the database that the service provider has to maintain.
4. If a single IP address is provided to the CPE and NAT or PAT is implemented, certain applications such as IPTV, which embed IP information in the payload, will not work. Additionally, if an IP subnet feature is used, an IP address has to be reserved for the CPE.
Configuring PPPoE and PPPoA for DSL
This section describes the configuration steps required to configure a Cisco DSL-enabled router as either a PPPoE client or a PPPoA client. In most cases the Cisco DSL-enabled router is also configured for additional services, such as DHCP and NAT, to allow hosts that reside on the LAN access to the Internet; however, those configuration tasks will not be demonstrated in this chapter as they are beyond the scope of the ISCW certification requirements.
NOTE: You are not expected to perform any configuration tasks or troubleshooting on the DSLAM or aggregator. Assume that the necessary configuration in place is operational.
Configuring PPPoE
The PPPoE configuration and validation tasks in this section will be based on the following network topology diagram which shows the PPPoE client and the aggregator:
E0/0 ATM0/0 DSL Network R1 150.1.1.1
Cisco PPPoE Cisco 6400 Client Router Aggregator
The first configuration task when configuring PPPoE on a router with an Ethernet-based interface is to enable PPPoE on the desired interface. Two commands are required to accomplish this task. The first command is the pppoe enable interface configuration command. This command is effectively enables PPPoE on the specified interface.
The second command required is the pppoe-client dial-pool-number [number] command. This command is used to configure a PPP over Ethernet (PPPoE) client and to specify dial-on-demand routing (DDR) functionality. Dial-On-Demand Routing (DDR) is a technique whereby a router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adaptor or modem. The [number] specified at the end of this command binds the interface to a
logical dialer interface (which must be manually configured) which will then have the PVC automatically provisioned across it. These first two configuration steps performed as follows:
R1(config)#interface ethernet 0/0 R1(config-if)#pppoe enable R1(config-if)#pppoe-client dial-pool-number 55 R1(config-if)#exit R1(config)#exit
The second configuration step is to configure the dialer interface. The dialer interface is simply a logical dial-on-demand (DDR) interface. This is performed by issuing the interface dialer [number] global configuration command. The [number] at the end of this command should be the same as that which was configured using the pppoe-client dial-pool-number [number] command. The following configuration tasks must then be performed on the dialer interface:
1. The interface must be configured to receive IP addressing dynamically via the ip address negotiated interface configuration command. However, in the rare case that the provider assigns you with a static IP address, the ip address [address] [mask] command can be issued on the interface with the assigned IP address
2. Because PPPoE must comply with RFC 2516, the dialer interface MTU must be set to 1492 bytes. This is performed by issuing the ip mtu 1492 interface configuration command
3. In order to bind the interface to the dialer interface to the Ethernet interface, the dialer pool [number] interface configuration command must be issued. The [number] should match the dialer interface number, as well as the number specified in the pppoe-client dial-pool-number [number] command
4. Finally, PPP encapsulation must be specified on the dialer interface. This is performed by issuing the encapsulation ppp interface configuration command
These configuration tasks are illustrated in the following output on R1:
R1(config)#interface dialer 55 R1(config-if)#ip address negotiated R1(config-if)#ip mtu 1492 R1(config-if)#dialer pool 55 R1(config-if)#encapsulation ppp R1(config-if)#no shutdown R1(config-if)#exit R1(config)#exit
As illustrated in the output above, the no shutdown command is issued on the dialer interface. Although technically not required, it is good practice to always issue this command.
The third configuration step is to configure a default static route on the DSL router. Keep in mind that because the IP address is dynamically assigned, the default route must point to the dialer interface that was previously configured. This is illustrated below on R1:
R1(config)#ip route 0.0.0.0 0.0.0.0 dialer 55
The next steps are optional. These steps may include none, all, or some of the following tasks:
. Configuring PPP authentication methods and credentials on the dialer interface . Configuring NAT or PAT on router to provide hosts Internet access . Configuring DHCP services to provide client machines with IP addressing
The final configuration on R1 is illustrated below, with the PPPoE configuration in bold font:
R1#show running-config Building configuration...
Current configuration : 2849 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! no logging console ! no aaa new-model ip cef ! ! ! ! ip domain name howtonetwork.net ! multilink bundle-name authenticated ! ! ! ! username admin privilege 15 secret 5 $1$LE94$AOOi72zla5fPxRcubcQU1. archive log config hidekeys ! ! ! !
ip ssh time-out 30 ip ssh authentication-retries 2 ! ! ! interface Ethernet0/0 no ip address speed auto pppoe enable group global pppoe-client dial-pool-number 55 ! interface Serial0/0 no ip address shutdown ! interface Dialer55 ip address negotiated ip mtu 1492 encapsulation ppp dialer pool 55 dialer-group 55 ! ip forward-protocol nd ip route 0.0.0.0 0.0.0.0 Dialer55 ! ! ip http server ip http authentication local ip http secure-server ! ! ! ! ! control-plane ! ! ! line con 0 line aux 0 line vty 0 4 privilege level 15 password cisco login ! ! end
NOTE: If you have been paying attention, you will notice that an additional command that was not manually configured has been added to the dialer interface configuration. This command, (dialer-group 55), is automatically added to the interface, and the [number] specified is the same as the one specified in the dialer pool [number] command.
This command is used to control access by configuring an interface to belong to a specific dialing group. Dial-on-demand interfaces, such as dialer interfaces, need 'interesting traffic' to initiate a connection. Going into detail on DDR is beyond the scope of the ISCW.
Configuring PPPoE over ATM – PPPoEoA (AAL 5 SNAP Encapsulation)
The PPPoE configuration and validation tasks in this section will be based on the following network topology diagram which shows the PPPoE client and the aggregator:
ATM0/0 ATM0/1 DSL Network ATM PVC 2/200 150.1.1.1/24
Cisco PPPoE Cisco 6400 Client Router Aggregator
As stated earlier in this chapter, by default, PPPoE runs on top of AAL 5 SNAP. This means that when configuring ATM PVCs, no explicit encapsulation commands are required. This follows the same logic as when configuring Serial interfaces; because HDLC is the default encapsulation type, there is no need to explicitly configure the encapsulation hdlc command. Similary, with with PPPoE on ATM interfaces, there is no need to explicity configure the encapsulation aal5snap configuration command for the PVC.
The first configuration task when configuring PPPoE on a router with an ATM interface using AAL 5 SNAP encapsulation is to configure the ATM interface to use the proper modulation method by issuing the dsl operating-mode {adsl2 [annex a | annex m] | adsl2+ [annex a | annex m] | ansi-dmt | auto | itu-dmt} interface configuration command. The options that are supported and provided by this command are described in the following table:
Keyword Description adsl2 Configures operation in ADSL2 operating mode—ITU G.992.3 Annex A, Annex L, and Annex M. If an Annex operating mode is not chosen, Annex A, Annex L, and Annex M will all be enabled. The final mode will be decided by negotiation with the DSL access multiplexer (DSLAM). adsl2+ Configures operation in ADSL2+ mode—ITU G.992.5 Annex A and AnnexM. If an Annex A operating mode is not chosen, both Annex and Annex M will be enabled. The final mode will be decided by negotiation with DSLAM. (Optional) If the annex option is not specified, both Annex A and Annex M will be enabled. The final mode will be decided by negotiation with the
Digital Synchronous Line Access Multiplexer (DSLAM). ansi-dmt Configures a router to operate in ANSI full-rate mode—ANSI T1.413. auto Default setting. Configures the router so that the DSLAM automatically picks the DSL operating mode, in the sequence described in the "Usage Guidelines" section. All supported modes are enabled. itu-dmt Configures operation in ITU G.992.1 Annex A full-rate mode.
In most cases, you will not know what modulation is being used. Therefore, Cisco recommends that you use the dsl operating-mode auto command if you are not sure what DMT technology your service provider uses. Issuing this command allows the router to automatically detect the correct modulation method to use. This recommended configuration is illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#dsl operating-mode auto R1(config-if)#exit R1(config)#exit
The second configuration task is to configure a point-to-point ATM subinterface. The ATM PVC information must be configured on this subinterface using the pvc [vpi/vci] interface configuration command. Additionally, the pppoe-client dial-pool-number [number] command must be configured to bind the ATM PVC to a dialer interface. Because the default encapsulation for PPPoE is AAL5SNAP, the encapsulation does not need to be manually configured under the PVC. These steps are illustrated below on R1:
R1(config)#interface atm 0/0.55 point-to-point R1(config-subif)#pvc 2/200 R1(config-if-atm-vc)# pppoe-client dial-pool-number 55 R1(config-if-atm-vc)#exit R1(config-subif)# exit R1(config-if)#exit R1(config)#exit
NOTE: In some configurations, the entire configuration (i.e. the PVC configuration) is performed on the physical ATM interface. Ensure that you remember that using a subinterface is simply another way of accomplishing the same task. Additionally, as an example, if you really wanted to manually specify AAL 5 SNAP, even though it is the default, the following configuration commands would be issued on the router:
R1(config)#interface atm 0/0.55 point-to-point R1(config-subif)#pvc 2/200 R1(config-if-atm-vc)#encapsulation aal5snap R1(config-if-atm-vc)# pppoe-client dial-pool-number 55 R1(config-if-atm-vc)#exit R1(config-subif)# exit R1(config-if)#exit R1(config)#exit
The third configuration task is to configure the dialer interface. The same steps performed when configuring PPPoE using Ethernet interfaces are applicable for PPPoE using ATM interfaces. This is illustrated below on R1:
R1(config)#interface dialer 55 R1(config-if)#ip address negotiated R1(config-if)#ip mtu 1492 R1(config-if)#dialer pool 55 R1(config-if)#encapsulation ppp R1(config-if)#no shutdown R1(config-if)#exit R1(config)#exit
The fourth configuration task is to configure a static default route on the DSL router. Again, it is important to keep in mind that because the IP address is dynamically assigned, the default route must point to the dialer interface previously configured. This is illustrated below on R1:
R1(config)#ip route 0.0.0.0 0.0.0.0 dialer 55
The next steps are optional. These steps may include none, all, or some of the following tasks:
. Configuring PPP authentication methods and credentials on the dialer interface . Configuring NAT or PAT on router to provide hosts Internet access . Configuring DHCP services to provide client machines with IP addressing
The final configuration on R1 is illustrated below, with the PPPoEoA configuration in bold font:
R1#show running-config Building configuration...
Current configuration : 2849 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! no logging console ! no aaa new-model ip cef ! ! ! !
ip domain name howtonetwork.net ! ! ! ! ! interface Ethernet0/0 no ip address duplex auto speed auto shutdown ! interface ATM0/0 no ip address bundle-enable dsl operating-mode auto hold-queue 224 in
! interface ATM0/0.1 point-to-point no ip address no atm ilmi-keepalive pvc 2/200 pppoe-client dial-pool-number 55 ! interface Dialer55 ip address negotiated ip mtu 1492 encapsulation ppp dialer pool 55 ! ip forward-protocol nd ip route 0.0.0.0 0.0.0.0 Dialer55 ! ! ! ! line con 0 line aux 0 line vty 0 4 privilege level 15 password cisco login ! ! end
NOTE: In the output above, several default commands may be added by the Cisco IOS software, depending on the version of IOS code the router is running. For example, the hold- queue 224 in command under the ATM0 interface and the no atm ilmi-keepalive command under the ATM0.1 subinterface are automatically added by Cisco IOS software.
Configuring PPPoE over ATM - PPPoEoA (AAL 5 MUX Encapsulation)
The PPPoE configuration and validation tasks in this section will be based on the following network topology diagram which shows the PPPoE client and the aggregator:
ATM0/0 ATM0/1 DSL Network ATM PVC 2/200 150.1.1.1/24
Cisco PPPoE Cisco 6400 Client Router Aggregator
The first configuration task when configuring PPPoE on a router with an ATM interface using AAL 5 MUX encapsulation is to configure the ATM interface to use the proper modulation method by issuing the dsl operating-mode {adsl2 [annex a | annex m] | adsl2+ [annex a | annex m] | ansi-dmt | auto | itu-dmt} interface configuration command.
In most cases, you will not know what modulation is being used. Therefore, Cisco recommends that you use the dsl operating-mode auto command if you are not sure what DMT technology your service provider uses. Issuing this command allows the router to automatically detect the correct modulation method to use. This recommended configuration is illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#dsl operating-mode auto R1(config-if)#exit R1(config)#exit
The second configuration task is to configure the PVC on the interface via the pvc [vpi/vci] interface configuration command. However, unlike AAL 5 SNAP encapsulation, an additional command is required to specify the ATM AAL 5 MUX encapsulation. This command is the encapsulation aal5mux ppp dialer PVC configuration command. This command is used to enable AAL 5 MUX. Additionally, this command is used to instruct the PVC to use dialer profile configuration. Finally, the pppoe-client dial-pool-number [number] command must be configured to bind the PVC to a dialer interface. These two steps are illustrated below on R1:
R1(config)#interface atm 0/0.55 point-to-point R1(config-subif)#pvc 2/200 R1(config-if-atm-vc)#encapsulation aal5mux ppp dialer R1(config-if-atm-vc)#pppoe-client dial-pool-number 55 R1(config-if-atm-vc)#exit R1(config-subif)# exit R1(config-if)#exit R1(config)#exit
The third configuration task is to configure the dialer interface. The same steps performed when configuring PPPoE using Ethernet interfaces are applicable for PPPoE using ATM interfaces. This is illustrated below on R1:
R1(config)#interface dialer 55 R1(config-if)#ip address negotiated R1(config-if)#ip mtu 1492 R1(config-if)#dialer pool 55 R1(config-if)#encapsulation ppp R1(config-if)#no shutdown R1(config-if)#exit R1(config)#exit
The fourth configuration task is to configure a static default route on the DSL router. Again, it is important to keep in mind that because the IP address is dynamically assigned, the default route must point to the dialer interface previously configured. This is illustrated below on R1:
R1(config)#ip route 0.0.0.0 0.0.0.0 dialer 55
The next steps are optional. These steps may include none, all, or some of the following tasks:
. Configuring PPP authentication methods and credentials on the dialer interface . Configuring NAT or PAT on router to provide hosts Internet access . Configuring DHCP services to provide client machines with IP addressing
The final configuration on R1 is illustrated below, with the PPPoEoA configuration in bold font:
R1#show running-config Building configuration...
Current configuration : 2849 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! no logging console ! no aaa new-model ip cef ! ! ! !
ip domain name howtonetwork.net ! ! ! ! ! interface Ethernet0/0 no ip address duplex auto speed auto shutdown ! interface ATM0/0 no ip address bundle-enable dsl operating-mode auto hold-queue 224 in ! interface ATM0/0.1 point-to-point no ip address no atm ilmi-keepalive pvc 2/200 encapsulation aal5mux ppp dialer pppoe-client dial-pool-number 55 ! ! interface Dialer55 ip address negotiated ip mtu 1492 encapsulation ppp dialer pool 55 ! ip forward-protocol nd ip route 0.0.0.0 0.0.0.0 Dialer55 ! ! ! ! ! line con 0 line aux 0 line vty 0 4 privilege level 15 password cisco login ! ! end Configuring PPPoA using dialer profiles (AAL 5 MUX Encapsulation)
Unlike PPPoE, PPPoA is only supported on ATM interfaces. The PPPoA configuration and validation tasks in this section will be based on the following network topology diagram which shows the PPPoA client and the aggregator:
ATM0/0 ATM0/1 DSL Network ATM PVC 2/200 150.1.1.1/24
Cisco PPPoE Cisco 6400 Client Router Aggregator
The first configuration task is to provision the ATM interface for the proper modulation method by issuing the dsl operating-mode {adsl2 [annex a | annex m] | adsl2+ [annex a | annex m] | ansi-dmt | auto | itu-dmt} interface configuration command. Cisco recommends using the dsl operating-mode auto command as illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#dsl operating-mode auto R1(config-if)#exit R1(config)#exit
The second configuration task is to configure the PVC on the ATM interface via the pvc [vpi/vci] interface configuration command. However, unlike PPPoE, an additional command is required to specify the ATM AAL 5 MUX encapsulation. This command is the encapsulation aal5mux ppp dialer PVC configuration command. It is used to enable AAL5MUX encapsulation for the ATM PVC, instead of the default AAL 5 SNAP encapsulation, and instruct the PVC to use dialer profile configuration. These two steps are illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#pvc 2/200 R1(config-if-atm-vc)#encapsulation aal5mux ppp dialer R1config-if-atm-vc)#no shut R1(config-if-atm-vc)#end
The third configuration task is to bind the PVC to the dialer interface by using the dialer pool-member [number] PVC configuration command. The [number] specified in this command must be the same as the dialer interface that will be configured for the ADSL service. The configuration steps to complete this task are illustrated in the following output on R1:
R1(config)#interface atm 0/0 R1(config-if)#pvc 2/200 R1(config-if-atm-vc)#dialer pool-member 55 R1config-if-atm-vc)#exit R1(config-if)#exit R1(config)#exit
The fourth configuration task is to configure the dialer interface. The same configuration commands as those used for PPPoE are used; however, it is imperative to remember that the ip mtu 1492 command is not used for PPPoA. This is a common mistake that is made. Ensure that you are well aware of this. This configuration is performed as illustrated below on R1:
R1(config)#interface dialer 55 R1(config-if)#ip address negotiated R1(config-if)#dialer pool 55 Router(config-if)#encapsulation ppp Router(config-if)#exit Router(config)#exit
The fifth configuration task is to configure a static default route on the DSL router. Again, keep in mind that because the IP address is dynamically assigned, the default route must point to the dialer interface. This is illustrated below on R1:
R1(config)#ip route 0.0.0.0 0.0.0.0 dialer 55
The next steps are optional. These steps may include none, all, or some of the following tasks:
. Configuring PPP authentication methods and credentials on the dialer interface . Configuring NAT or PAT on router to provide hosts Internet access . Configuring DHCP services to provide client machines with IP addressing
The final configuration on R1 is illustrated below, with the PPPoA configuration in bold font:
R1#show running-config Building configuration...
Current configuration : 2849 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! no logging console ! no aaa new-model ip cef ! ! ! ! ip domain name howtonetwork.net !
! ! interface Ethernet0/0 no ip address duplex auto speed auto shutdown ! interface ATM0/0 no shut no ip address dsl operating-mode auto pvc 2/200 encapsulation aal5mux ppp dialer dialer pool-member 55 ! interface Dialer55 ip address negotiated encapsulation ppp dialer pool 55 ! ip forward-protocol nd ip route 0.0.0.0 0.0.0.0 Dialer55 ! ! ! ! line con 0 line aux 0 line vty 0 4 privilege level 15 password cisco login ! ! end
Configuring PPPoA using dialer profiles (AAL 5 SNAP Encapsulation)
The PPPoA configuration and validation tasks in this section will be based on the following network topology diagram which shows the PPPoA client and the aggregator:
ATM0/0 ATM0/1 DSL Network ATM PVC 2/200 150.1.1.1/24
Cisco PPPoE Cisco 6400 Client Router Aggregator
The first configuration task is to provision the ATM interface for the proper modulation method by issuing the dsl operating-mode {adsl2 [annex a | annex m] | adsl2+ [annex a | annex m] | ansi-dmt | auto | itu-dmt} interface configuration command. Cisco recommends using the dsl operating-mode auto command as illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#dsl operating-mode auto R1(config-if)#exit R1(config)#exit
The second configuration task is to configure the PVC on the ATM interface via the pvc [vpi/vci] interface configuration command. Unlike PPPoE, AAL 5 SNAP encapsulation must be manually configured, and the router instructed to use dialer profiles for the PVC. This is enabled via the encapsulation aal5snap ppp dialer PVC configuration command. These configuration for these two steps is illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#pvc 2/200 R1(config-if-atm-vc)#encapsulation aal5snap ppp dialer R1config-if-atm-vc)#no shut R1(config-if-atm-vc)#end
The third configuration task is to bind the PVC to the dialer interface by using the dialer pool-member [number] PVC configuration command. The [number] specified in this command must be the same as the dialer interface that will be configured for the ADSL service. The configuration steps to complete this task are illustrated in the following output on R1:
R1(config)#interface atm 0/0 R1(config-if)#pvc 2/200 R1(config-if-atm-vc)#dialer pool-member 55 R1config-if-atm-vc)#exit R1(config-if)#exit R1(config)#exit The fourth configuration task is to configure the dialer interface. The same configuration commands as those used for PPPoE are used; however, it is imperative to remember that the ip mtu 1492 command is not used for PPPoA. This is a common mistake that is made. Ensure that you are well aware of this. This configuration is performed as illustrated below on R1:
R1(config)#interface dialer 55 R1(config-if)#ip address negotiated R1(config-if)#dialer pool 55 Router(config-if)#encapsulation ppp Router(config-if)#exit Router(config)#exit
The fifth configuration task is to configure a static default route on the DSL router. Again, keep in mind that because the IP address is dynamically assigned, the default route must point to the dialer interface. This is illustrated below on R1:
R1(config)#ip route 0.0.0.0 0.0.0.0 dialer 55
The next steps are optional. These steps may include none, all, or some of the following tasks:
. Configuring PPP authentication methods and credentials on the dialer interface . Configuring NAT or PAT on router to provide hosts Internet access . Configuring DHCP services to provide client machines with IP addressing
The final configuration on R1 is illustrated below, with the PPPoA configuration in bold font:
R1#show running-config Building configuration...
Current configuration : 2849 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! no logging console ! no aaa new-model ip cef ! ! ! ! ip domain name howtonetwork.net ! ! ! ! ! interface Ethernet0/0 no ip address
duplex auto speed auto shutdown ! interface ATM0/0 no ip address dsl operating-mode auto pvc 2/200 encapsulation aal5snap ppp dialer dialer pool-member 55 ! interface Dialer55 ip address negotiated encapsulation ppp dialer pool 55 ! ip forward-protocol nd ip route 0.0.0.0 0.0.0.0 Dialer55 ! ! ip http server ip http authentication local ip http secure-server ! ! ! ! ! control-plane ! ! ! line con 0 line aux 0 line vty 0 4 privilege level 15 password cisco login ! ! end
Configuring PPPoA via Virtual Templates (AAL 5 SNAP Encapsulation)
A virtual template is a logical interface, similar to a dialer interface, which may also be used when configuring PPPoA. In a manner similar to dialer interfaces, PPP authentication commands can be used on the virtual template, using credentials provided by the ISP. The configuration in this section is based on the following diagram:
ATM0/0 ATM0/1 DSL Network R1 ATM PVC 2/200 198.1.1.254
Cisco PPPoE Cisco 6400 Client Router Aggregator
The first configuration task when configuring PPPoA using a virtual template is to provision the ATM interface for the proper modulation method by issuing the dsl operating-mode {adsl2 [annex a | annex m] | adsl2+ [annex a | annex m] | ansi-dmt | auto | itu-dmt} interface configuration command. However, as stated in previous section Cisco recommends using the dsl operating-mode auto command as illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#dsl operating-mode auto R1(config-if)#exit R1(config)#exit
The second configuration task is to configure the virtual template. This is performed via the interface virtual-template [number] global configuration command. It is important to configure the virtual template before it is applied to the PVC. This is performed as follows:
R1(config)#interface virtual-template 5 R1(config-if)#ip address negotiated R1(config-if)#no shutdown R1(config-if)#exit R1(config)#exit
By default, there is no need to enter the encapsulation ppp interface configuration command as the virtual template interface uses PPP encapsulation by default, as is illustrated on R1:
R1#show interfaces virtual-template 5 Virtual-Template5 is down, line protocol is down Hardware is Virtual Template interface Interface is unnumbered. Using address of Ethernet0/0 (150.1.1.1) MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation PPP, LCP Closed, loopback not set Keepalive set (10 sec) DTR is pulsed for 5 seconds on reset Last input never, output never, output hang never Last clearing of "show interface" counters 02:20:02 Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo Output queue: 0/40 (size/max) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 0 packets output, 0 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 unknown protocol drops 0 output buffer failures, 0 output buffers swapped out 0 carrier transitions
The third configuration step is to configure the PPP authentication credentials on the virtual template interface (if required). It is imperative to remember that the ppp authentication [method] command must never be issued on the virtual template. The only PPP authentication commands that must be issued are the ppp chap hostname [name] and ppp chap password [password] commands to provide CHAP credentials or the ppp pap sent-username [name] password [password] command to provide PAP credentials.
For example, to configure the router to use the CHAP username iscwcertification and the CHAP password iscwpassword when asked to provide valid authentication credentials, the following commands would be implemented under the virtual template interface:
R1(config)#interface virtual-template 5 R1(config-if)#ppp chap hostname iscwcertification R1(config-if)#ppp chap password iscwpassword R1(config-if)#exit R1(config)#exit
The fourth configuration task is to configure the PVC on the interface via the pvc [vpi/vci] interface configuration command. Next, specify a VC class on the ATM interface. This class is used to set the circuit characteristics. The VC class is configured via the class-int [name] interface configuration command, as illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#pvc 2/200 R1(config-if-atm-vc)#exit R1(config-if)#class-int PPPOA-CLASS R1(config-if)#exit
The fifth configuration task is to configure the characteristics of the VC class configured in the fourth configuration task. This is performed by using the vc-class atm [name] global configuration command. The [name] used in the vc-class atm [name] global configuration command must be the same name used in the class-int [name] interface configuration command under the ATM interface.
Within the VC class, AAL 5 SNAP encapsulation is configured via the encapsulation aal5snap command and the protocol ppp virtual-template [number] command specifies PPP at the protocol (PPPoA) and binds the configuration to the specified virtual
template. In this case, the [number] specified in this command must match the virtual template interface number. This configuration is illustrated below on R1:
R1(config)#vc-class atm PPOA-CLASS R1(config-vc-class)#encapsulation aal5snap R1(config-vc-class)#protocol ppp virtual-template 5 R1(config-vc-class)#exit
The sixth configuration task is to configure a static default route on the DSL router. This step presents an interesting problem. In Cisco IOS, a static route cannot be configured to point to a virtual template. In this example (based on the network diagram), R1 will connect to the aggregator 198.1.1.254. Therefore, the static route must be configured to point to that IP address. Because of default PPP operation, a host route for this IP address (the 198.1.1.254 address on the aggregator) will be installed into the IP routing table as soon as R1 connects to the aggregator. The static default route will then be placed into the routing table.
R1(config)#ip route 0.0.0.0 0.0.0.0 198.1.1.254
The next steps are optional. These steps may include none, all, or some of the following tasks:
. Configuring PPP authentication methods and credentials on the dialer interface . Configuring NAT or PAT on router to provide hosts Internet access . Configuring DHCP services to provide client machines with IP addressing
The final configuration on R1 is illustrated below, with the PPPoA configuration in bold font:
R1#show running-config Building configuration...
Current configuration : 2849 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! no logging console ! no aaa new-model ip cef ! ! ! ! ip domain name howtonetwork.net
! ! ! ! ! interface Ethernet0/0 ip address 150.1.1.1 255.255.255.0 ! interface Virtual-Template5 ip address negotiated ppp chap hostname iscwcertification ppp chap password 0 iscwpassword ! interface ATM0/0 class-int PPPOA-CLASS dsl operating-mode auto ! pvc 2/200 ! vc-class PPPOA-CLASS encapsulation aal5snap protocol ppp Virtual-Template5 ! ip forward-protocol nd ip route 0.0.0.0 0.0.0.0 198.1.1.254 ! ! ! line con 0 line aux 0 line vty 0 4 privilege level 15 password cisco login ! ! end
NOTE: It is also important to know that subinterfaces can also be used with this same configuration; however, only multipoint subinterfaces can be used. This alternative option is illustrated in the following configuration snippet:
! interface Virtual-Template5 ip address negotiated ppp chap hostname iscwcertification ppp chap password 0 iscwpassword ! interface ATM0/0 no ip address
dsl operating-mode auto ! interface ATM0/0.1 multipoint class-int PPPOA-CLASS pvc 2/200 ! vc-class PPPOA-CLASS encapsulation aal5snap protocol ppp Virtual-Template5 !
Configuring PPPoA via Virtual Templates (AAL 5 MUX Encapsulation)
A virtual template is a logical interface, similar to a dialer interface, which may also be used when configuring PPPoA. In a manner similar to dialer interfaces, PPP authentication commands can be used on the virtual template, using credentials provided by the ISP. The configuration in this section is based on the following diagram:
ATM0/0 ATM0/1 DSL Network R1 ATM PVC 2/200 198.1.1.254
Cisco PPPoE Cisco 6400 Client Router Aggregator
The first configuration task when configuring PPPoA using a virtual template is to provision the ATM interface for the proper modulation method by issuing the dsl operating-mode {adsl2 [annex a | annex m] | adsl2+ [annex a | annex m] | ansi-dmt | auto | itu-dmt} interface configuration command. However, as stated in previous section Cisco recommends using the dsl operating-mode auto command as illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#dsl operating-mode auto R1(config-if)#exit R1(config)#exit The second configuration task is to configure the virtual template. This is performed via the interface virtual-template [number] global configuration command. It is important to configure the virtual template before it is applied to the PVC. This is performed as follows:
R1(config)#interface virtual-template 5 R1(config-if)#ip address negotiated R1(config-if)#no shutdown R1(config-if)#exit R1(config)#exit
The third configuration step is to configure the PPP authentication credentials on the virtual template interface (if required). It is imperative to remember that the ppp authentication [method] command must never be issued on the virtual template. The only PPP authentication commands that must be issued are the ppp chap hostname [name] and ppp chap password [password] commands to provide CHAP credentials or the ppp pap sent-username [name] password [password] command to provide PAP credentials.
The fourth configuration task is to configure the PVC on the interface via the pvc [vpi/vci] interface configuration command. Next, specify the AAL 5 MUX encapsulation type via the encapsulation aal5mux ppp virtual-template [number] PVC configuration command. The [number] specified for the virtual template must be the same as that which was configured in the second configuration task as illustrated below on R1:
R1(config)#interface atm 0/0 R1(config-if)#pvc 2/200 R1(config-if-atm-vc)#encapsulation aal5mux ppp virtual-template 5 R1(config-if-atm-vc)#exit R1(config-if)#exit
The fifth configuration task is to configure a static default route on the DSL router. This step presents an interesting problem. In Cisco IOS, a static route cannot be configured to point to a virtual template. In this example (based on the network diagram), R1 will connect to the aggregator 198.1.1.254. Therefore, the static route must be configured to point to that IP address. Because of default PPP operation, a host route for this IP address (the 198.1.1.254 address on the aggregator) will be installed into the IP routing table as soon as R1 connects to the aggregator. The static default route will then be placed into the routing table.
R1(config)#ip route 0.0.0.0 0.0.0.0 198.1.1.254
The next steps are optional. These steps may include none, all, or some of the following tasks:
. Configuring PPP authentication methods and credentials on the dialer interface . Configuring NAT or PAT on router to provide hosts Internet access . Configuring DHCP services to provide client machines with IP addressing The final configuration on R1 is illustrated below, with the PPPoA configuration in bold font:
R1#show running-config Building configuration...
Current configuration : 2849 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption !
hostname R1 ! boot-start-marker boot-end-marker ! no logging console ! no aaa new-model ip cef ! ! ! ! ip domain name howtonetwork.net ! ! ! ! ! interface Ethernet0/0 ip address 150.1.1.1 255.255.255.0 ! interface Virtual-Template5 ip address negotiated ppp chap hostname iscwcertification ppp chap password 0 iscwpassword ! interface ATM0/0 dsl operating-mode auto pvc 2/200 encapsulation aal5mux ppp Virtual-Template5 ! ip forward-protocol nd ip route 0.0.0.0 0.0.0.0 198.1.1.254 ! ! ! line con 0 line aux 0 line vty 0 4 privilege level 15 password cisco login ! ! end
NOTE: It is also important to know that subinterfaces can also be used with this same configuration; however, only multipoint subinterfaces can be used. This alternative option is illustrated in the following configuration snippet:
! interface Virtual-Template5 ip address negotiated ppp chap hostname iscwcertification
ppp chap password 0 iscwpassword ! interface ATM0/0 no ip address dsl operating-mode auto ! interface ATM0/0.1 multipoint pvc 2/200 encapsulation aal5mux ppp Virtual-Template5 !
Although technically beyond the scope of the ISCW requirements, the following configuration snippet illustrates how Cisco proprietary PPP encapsulation can be used with virtual templates. Because the configuration steps are exactly the same as those used for AAL 5 MUX encapsulation only the relevant portions of the configuration will be illustrated.
As you can see, the only significant different is the use of the encapsulation aal5ciscoppp virtual-template [number] configuration command. The following configuration snippet illustrates Cisco proprietary PPP configuration for PPPoA using virtual templates:
! interface Virtual-Template5 ip address negotiated ppp chap hostname iscwcertification ppp chap password 0 iscwpassword ! interface ATM0/0 no ip address dsl operating-mode auto ! interface ATM0/0.1 multipoint pvc 2/200 encapsulation aal5ciscoppp Virtual-Template5 !
Alternatively, this configuration can also be performed using a VC class as illustrated below:
! interface Virtual-Template5 ip address negotiated ppp chap hostname iscwcertification ppp chap password 0 iscwpassword ! interface ATM0/0 no ip address dsl operating-mode auto ! interface ATM0/0.1 multipoint class-int PPPOA-CLASS pvc 2/200 !
vc-class PPPOA-CLASS encapsulation aal5ciscoppp Virtual-Template5 !
Chapter Summary
The following section is a summary of the major points you should be aware of in this chapter.
Data Transmission Basics
. Data transmission is the process of sending data or the progress of the sent data . Computers generate and interpret digital signals as electric current . Analog data signals are also generated as voltage which is represented as a wavy line . The amplitude is a measure of the signals (waves) strength at any given time . Frequency is the number of time the wave’s amplitude cycles . Frequency is expressed in cycles per seconds, or hertz (Hz) . Wavelength is the difference between the corresponding points on a wave’s cycle . The wavelength is inversely proportional to the frequency . Data modulation is used to modify analog signals to carry data over a communication path . Multiplexing allows multiple signals to travel simultaneously over a single medium . FDM assigns a unique frequency band to each individual communications sub-channel . Voice communications use the frequency band of 300 Hz – 3KHz . Baseband is a transmission form in which signals are sent through direct current (DC) . Broadband is a form of transmission in which signals are modulated as radio frequency (RF) . Noise is an undesirable influence that may distort or degrade a signal . Attenuation is the loss of the strength of a signal as it travels away from its source . To boost the strength of analog signals, an amplifier is used . To boost the strength of digital signals, a repeater is used . Cable networks used RG coaxial cable to guide radio frequencies in broadband transmission . Twisted pair cable consists of color-coded pairs of insulated copper wires . The two categories of twisted pair cabling are STP and UTP . Fiber contains one or several strands of glass or plastic fibers at its center (core) . Data is transmitted over fiber via pulsating light sent from a laser or LED . Two types of fiber optic cabling are MMF and SMF
Cable Technology
. Cable modems operate at Layer 1 and Layer 2 of the OSI Model . On the network side, cable modems support Ethernet . On the cable side, cable modems support DOCSIS . Cable companies use HFC networks to provide fiber and coax connections to the customer . Typical cable throughput varies anywhere between 1Mbps and 6Mbps for downloads
. Typical cable throughput varies anywhere between 128Kbps and 768Kbps for uploads . FDM is used in cable to combine multiple signals onto a carrier wave in a wide range of RFs . Downstream signals are carried on a 50 MHz to 860 MHz band . Upstream signals use a 5 MHz to 42 MHz band (U.S) or 5 MHz to 65 MHz band (UK) . Downstream data traffic shares a 6 MHz channel and one or more 6 MHz upstream channel . DOCSIS defines communications and operation support requirements for a data over cable . Modulation signaling standards used are NTSC, PAL, SECAM and MPEG-2/MPEG-4 . RF is a frequency of electromagnetic radiation within the range of about 3 Hz to 300 GHz . DOCSIS defines the interface standards for cable modems and supporting equipment . The DOCSIS architecture consists of three primary components:
1. A Cable Modem (CM) 2. A Cable Modem Termination System (CMTS) 3. Back Office Services
. There are 3 versions of DOCSIS: version 1.0 (revised as 1.1), version 2.0 and version 3.0 . The DOCSIS Physical layer allows bidirectional communication between the CM and CMTS . The DOCSIS Data Link Layer is broken up into the LLC, Security and MAC sub-layer
The Public Switched Telephone Network
. The local telephone provider services neighborhoods via a local central office (CO) . The local loop exists between the local CO and the NIU at the customer premises . On the PSTN, voice communications use the frequency band of 300 Hz – 3 KHz
Digital Subscriber Line
. The xTU-C and xTU-R are the main endpoint components in a DSL data service network . The xTU-R modulates outgoing signals and demodulates incoming signals . Splitters are used to separate the data signal from any voice signals . In modern networks, microfilters are used instead of splitters at the customer premise . Microfilters only allow frequencies in the 0 – 4 KHz to pass through to analog devices . A DLSAM is used to terminate the CO side of the DSL link . Issues that hinder the widespread deployment of DSL include the following:
1. DSLAM Costs 2. Radio Interference 3. Distance Limitations 4. Wire Gauge 5. Loading Coils 6. Crosstalk 7. Bridged Taps 8. Pair Gain
9. Fiber-To-The-Premises
. SDSL provides equal capacity for data traveling both upstream and downstream . SDSL is a DSL variant that runs over one pair of copper wires and supports data only . SDSL provides up to T1 and E1 downstream and upstream throughput . SDSL has a distance limit of 10,000 feet. . SDSL is also typically offered in the following speeds:
Type Upstream Speed Downstream Speed SDSL-192 192 Kbps 192 Kbps SDSL-384 384 Kbps 384 Kbps SDSL-768 768 Kbps 768 Kbps SDSL-1.1 1.1 Mbps 1.1 Mbps
. SHDSL is an industry standard for SHDSL defined in ITU-T recommendation G.991.2 . SHDSL provides symmetric throughput of 2.3 Mbps . SHDSL has a distance limit of 26,000 feet . HDSL can be used either at the T1 or E1 rates . HDSL is commonly used in place of traditional T1 circuits . HDSL has a distance limitation of 12,000 feet . IDSL uses ISDN-based technology to provide throughput speeds of 1.44 Kbps . ADSL is the most common type of DSL service . Asymmetric DSL provides different downstream and upstream speeds . ADSL allow for voice and data to coexist on the same telephone wires . ADSL operates at higher frequencies than PSTN/POTS . ADSL supports speeds of 1.5 to 8 Mbps downstream and 16 Kbps and 1 Mbps upstream . G.Lite ADSL is also referred to as splitterless ADSL . ADSL2 supports data rates up to 12 Mbps downstream and 3.5 Mbps upstream . ADSL2+ supports data rates up to 24 Mbps downstream and 1.4 Mbps upstream . RADSL adjusts transmission speed based on the length and quality of the local line . Very High bit-rate DSL (VDSL or VHDSL) is the fastest DSL technology . In DLC, a fiber optic cable carries the aggregated traffic back to the local central office (CO) . DSL flavors use various modulation techniques. These techniques include:
1. 2 Binary, 1 Quaternary 2. Quadrature Amplitude Modulation 3. Carrierless Amplitude and Phase Modulation 4. Discrete Multi Tone Modulation
CAP is an ADSL single-carrier modulation technique that divides space into three bands:
4. The frequency band (range) for POTS transmissions is from 0 to 4 KHz 5. The frequency band (range) for upstream data traffic is from 25 KHz to 160 KHz
6. The frequency band (range) for downstream data traffic is 240 KHz to 1.1 MHz
. DMT is an ADSL modulation technique which divides its channel into discrete tones
Sending Data over DSL Networks
. DSL provides Layer 1 connectivity to the provider’s network . The ATU-C and ATU-R are the main endpoint components in an ADSL network . On the provider side, the DSLAM connects to the provider network via an ATM network . On the subscriber side, several methods can be used. These methods are:
1. Long Range Ethernet 2. PPP over Ethernet 3. PPP over ATM 4. Routed Bridged Encapsulation 5. Multiprotocol Encapsulation over ATM 6. Service Selection Gateway
. PPP operates in full-duplex mode . PPP works by encapsulating upper layer protocols for transmission across the link . PPP offers the following advantages over HDLC:
1. Compression 2. Dynamic addressing 3. Authentication 4. Link configuration and negotiation 5. Error detection 6. Multilink capability 7. Multiple protocol support . PPP uses Link Control Protocol (LCP) to manage the Data Link connection . PPP uses the Network Control Protocol (NCP) support multiple Layer 3 protocols . ATM is an ITU-T standard for cell relay that uses fixed-size 53-byte cells . The two main types of ATM PVCs are:
1. Permanent Virtual Channel Connections (PVCCs) 2. Permanent Virtual Path Connections (PVPCs)
. AAL defines how to segment and reassemble higher-layer packets into ATM cells . The AAL protocols defined by the ITU are:
1. AAL 1 2. AAL 2 3. AAL 3/4
4. AAL 5
. ATM AAL 5 was introduced to: 1. Reduce protocol processing overhead 2. Reduce transmission overhead 3. Ensure adaptability to existing transport protocols
. PPPoE provides connectivity to a remote access concentrator . By default, PPPoE runs on top of AAL 5 SNAP . PPPoE has two distinct stages: a discovery stage and a PPP session stage . The PPPoE discovery phase contains 4 steps and the following messages are exchanged:
1. PADI – Step 1 (client) 2. PADO – Step 2 (aggregator) 3. PADR – Step 3 (client) 4. PADS – Step 4 (aggregator)
. The MTU of PPPoE interfaces must therefore be set to 1492 bytes . PPPoE can run over ATM interfaces, as defined in RFC 1483/2648 . PPPoE over ATM (PPPoEoA) retains the same MTU restrictions as PPPoE . PPPoA is specified in RFC 2364 . PPPoA uses both AAL 5 SNAP and AAL 5 MUX encapsulation
Commands used in this chapter
Command Description pppoe enable Enables PPPoE on an Ethernet interface pppoe-client Enables PPPoE and binds the interface to a logical dialer interface dial-pool-number [number] interface dialer Configures a dialer interface used for dial-on-demand routing (DDR) [number] ip address Configures the interface to negotiate its IP address negotiated ip mtu Configures an MTU value for an interface dialer pool Binds the interface to a dialer interface (DDR) [number] encapsulation Enables PPP encapsulation on an interface ppp ip route Configures a static route show running- Prints the current router configuration (RAM) config dsl operating- Specifies the DSL modulation to be used on an ATM interface mode pvc [vpi/vci] Configures an ATM PVC on an interface
encapsulation Configures an ATM PVC to use AAL 5 encapsulation (this is the default aal5snap encapsulation on PVCs) encapsulation Configures the ATM PVC to use AAL 5 MUX encapsulation, and aal5mux ppp instructs it to use dialer profiles (DDR) dialer dialer pool- Binds the ATM PVC to the specified dialer interface. This is applicable member 55 for PPPoA configurations encapsulation Configures the ATM PVC to use AAL 5 SNAP encapsulation, and aal5snap ppp instructs it to use dialer profiles (DDR) dialer interface Configures a virtual template interface virtual-template [number] ppp chap Configures the sent PPP CHAP hostname if authentication is required hostname [name] ppp chap Configures the sent PPP CHAP password if authentication is required password [password] ppp pap sent- Configures the sent PPP PAP hostname and password if authentication username [name] is required password [password] class-int [name] Binds an ATM VC class to an interface vc-class atm Configures an ATM VC class and specifies its characteristics [name] protocol ppp Instructs the ATM VC class to use the PPP virtual template specified virtual-template [number] encapsulation Configures an ATM PVC to use AAL 5 MUX encapsulation and binds it aal5mux ppp to the specified PPP virtual interface virtual-template [number] encapsulation Configures an ATM PVC to use AAL 5 Cisco PPP (proprietary) aal5ciscoppp encapsulation and binds it to the specified PPP virtual interface virtual-template [number]