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DSL and Cable Modem Network
By Matthew Castelli.

Digital Subscriber Line
Cable Access Technologies

DSL and cable modem network access are two alternative ways to connect to a network service provider without the use of more expensive dedicated service, such as Frac-T1/T1. DSL and cable modem networks achieve the same result of providing dedicated access to a network service, often the Internet, but each do so using differing technologies. This chapter discusses what DSL and cable modem technologies do and how they do it.

Digital Subscriber Line

Digital subscriber line (DSL) technology is a modem technology using existing twisted-pair telephone lines to carry high-bandwidth applications, such as multimedia and video. The term xDSL covers a number of DSL technologies, such as Asymmetrical Digital Subscriber Line (ADSL), Symmetrical Digital Subscriber Line (SDSL), Hi-Speed Digital Subscriber Line (HDSL), HDSL-2 (HDSLv2), ITU DSL standard (G.SHDSL), ISDN Digital Subscriber Line (IDSL), and Very-High-Data-Rate Digital Subscriber Line (VDSL).
xDSL services are dedicated point-to-point network access over twisted-pair copper wire on the local loop (last mile) between a network service provider's (NSP) central office (CO) and the customer site. xDSL also can be deployed in intra-building and intra-campus environments, as illustrated in Figure 7-1.

xDSL offers two chief benefits over dial-up service:

Dial-up service is limited to 53.3 Kbps, whereas xDSL service can enable up to 6.122 Mbps.
Dial-up service is initiated "on-demand" by the end-user, but xDSL service is a dedicated connection, meaning that it is "always on."

Intra-Building and Intra-Campus/Inter-Building

The following sections discuss ADSL. ADSL is often deployed in the small office/home office (SOHO) environment and is the traditional DSL service for residential deployment. The asymmetry is ideal in these environments because the majority of upstream bandwidth is consumed by Internet requests; for example, users navigating through web sites. These upstream requests are small compared to the downstream response, such as the web site fulfilling the user's request.


ADSL technology makes more bandwidth available downstream, from a NSP central office (CO) to the customer site, than it makes available upstream, from the customer site to the CO. Figure 7-2 illustrates an example of an ADSL connection.

ADSL Connection

ADSL Architecture

ADSL circuits connect ADSL modems on each end of a twisted-pair telephone line, creating three data channels: 

A high-speed downstream channelRanges from 1.5 to 9 Mbps.
A low-speed upstream channelRanges from 16 to 640 Kbps.
A basic telephone service channelThe basic telephone service channel is split off from the digital modem by filters or plain old telephone service (POTS) splitters, providing uninterrupted basic telephone service. 

The upstream and downstream bandwidth ranges depend upon the distance between the customer site and the DSL provider's CO; the greater the distance, the lower the bandwidth capacity.

illustrates the architecture of an ADSL network.

ADSL Architecture

ADSL architecture is made up of the following components:

Transport SystemProvides the carrier backbone transmission interface for the DSLAM system. This device can provide service specific interfaces such as T1/E1, T3/E3, OC-1/3, and STS-1/3.
Local Access NetworkUses the local carrier Inter-CO network as a foundation, providing connectivity between multiple service providers and multiple services users, often with Frame Relay or ATM switches. 
Digital Subscriber Line Access Multiplexer (DSLAM)Concentrates data traffic from multiple DSL loops onto the backbone network for connection to the rest of the network.
DSL Transceiver Unit-Remote (xTU-R)The customer site equipment for service connection to the DSL loop.
POTS SplittersOptional device at both CO and service user locations, enabling the copper loop to be used for simultaneous DSL and transmission and single line telephone service. POTS splitters come in two configurations:

      o Single splitter version for mounting at the residence 
      o Multiple splitter version for mass termination at the CO 

POTS splitters are either passive or active. Active splitters require an external power source, and passive splitters require no power and often have a higher mean time between failure (MTBF) than the active splitter. Passive splitters enable lifeline services, such as 911, in the event of a DSLAM or xTU-R power loss; active splitters require backup power.

ADSL Data Rates

Downstream bandwidth depends on a number of factors:

Length of the copper line
Wire gauge of the copper line
Presence of bridged taps
Presence of cross-coupled interference

Bridged taps are any cable pair spliced into the main pair. Many unused bridged taps remain from the early days when party lines were the norm and two or more taps were made on every line. Bridged taps cause undesirable reflection that can distort the high-frequency signals in modern transmission technologies.
Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Ignoring bridged taps, ADSL performs as shown in Table 7-1.

Table 7-1 ADSL Rates (Ignoring Bridged Taps)

Rate (Mbps Wire Gauge (AWG) Distance (feet) Wire Size (mm) Distance (km)
1.5 or 2 24 18,000 0.5 5.5
1.5 or 2 26 15,000 0.4 4.6
6.1 24 12,000 0.5 3.7
6.1 26 9000 0.4 2.7

Customer sites beyond the previously listed distances can be reached with fiber-based digital loop carrier (DLC) systems, as illustrated 

ADSL with and Without Fiber-based DLC

xDSL service will not work over fiber-to-the-curb (FTTC) implementations. FTTC is the installation of optical fiber to within a thousand feet of the home or office. Fiber-to-the-home (FTTH) is the installation of optical fiber from the carrier directly into the home or office.

ADSL Standards and Associations

The American National Standards Institute (ANSI) Working Group T1E1.4 approved an ADSL standard at rates up to 6.1 Mbps (DMT/ANSI Standard T1.413). The European Technical Standards Institute (ETSI) contributed an annex to T1.413 reflecting European requirements including a single terminal interface at the premise side of the access circuit. 

The ATM Forum and the Digital Audio-Visual Council (DAVIC) have both recognized ADSL as a physical layer transmission protocol for unshielded twisted pair (UTP) media.

UTP is a popular type of cable consisting of two unshielded wires twisted around each other. Because UTP cabling is cost efficient, it is used extensively for local-area networks (LANs) and telephone connections. UTP cabling does not offer the high bandwidth or protection from interference that is found with coaxial or fiber optic cables; however, UTP is less expensive and easier to work with than coaxial or fiber-optic.

Other xDSL Technologies

There are several xDSL implementations in addition to ADSL. These are as follows:

Single-lined digital subscriber line (SDSL)A rate-adaptive version of Hi-speed digital subscriber line (HDSL) which like HDSL is symmetric. SDSL enables equal bandwidth downstream from a network service provider CO to the customer site as upstream from the customer site to the CO. SDSL supports data only (maximum of 1.544 Mbps) on a single line and does not support analog calls.
High-data-rate digital subscriber line (HDSL)Developed by Bellcore, high bit-rate DSL (HDSL)/T1/E1 technologies have been standardized by ANSI in the United States and by ETSI in Europe. HDSL is a more cost-efficient method of installing T1 service to a customer site than traditional dedicated DS1 service. 
HDSL 2Standard enabling 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 by which different vendors' equipment can interoperate.
G.SHDSL (ITU HDSL Standard)A standards-based, multirate version of HDSL-2, which offers symmetrical service.
Integrated services digital network (ISDN) digital subscriber line (IDSL)A cross between ISDN and xDSL, using a single-wire pair to transmit full-duplex data at 128 kbps. 
Very-high-data-rate digital subscriber line (VDSL)Transmits high-speed data over short reaches of twisted-pair copper telephone lines, with a range of speeds depending on actual line length. The maximum downstream rate under consideration is between 51 and 55 Mbps over lines up to 1000 feet (300 m). Downstream speeds as low as 13 Mbps over lengths beyond 4000 feet (1500 m) also are in consideration.


DSL and cable modem network access are two alternate ways to connect to an NSP without the use of more expensive dedicated service. DSL technology is a modem technology using existing twisted-pair telephone lines capable of carrying high-bandwidth applications.

There are several forms of xDSL, each designed around specific goals and needs of the marketplace. Each of these is summarized in Table 7-2.

Table 7-2 DSL Service Summary

DSL Type Description Data Rate Downstream; Upstream Distance Limit Application
ADSL Asymmetric digital subscriber line 1.544 to 6.1 Mbps downstream;
16 to 640 Kbps upstream
1.544 Mbps at 18,000 feet;
2.048 Mbps at 16,000 feet;
6.312 Mbps at 12,000 feet;
8.448 Mbps at 9,000 feet
Used for Internet and web access, motion video, video on demand, remote LAN access.
HDSL High-data-rate digital subscriber line 1.544 Mbps duplex on two twisted-pair lines; 2.048 Mbps duplex on three twisted-pair lines 12,000 feet on 24-gauge wire T1/E1 service between server and phone company or within a company; WAN, LAN, server access.
SDSL Single-line digital subscriber line 1.544 Mbps duplex (U.S. and Canada); 
2.048 Mbps (Europe) on a single duplex line downstream and upstream
12,000 feet on 24-gauge wire Same as for HDSL but requiring only one line of twisted-pair.
VDSL Very-high digital subscriber line 12.9 to 52.8 Mbps downstream;
1.5 to 2.3 Mbps upstream;
1.6 Mbps to 2.3 Mbps downstream
4500 feet at 12.96 Mbps;
3000 feet at 25.82 Mbps;
1000 feet at 51.84 Mbps
ATM networks; Fiber to the Neighborhood.


DSL and cable modem network access is not available in all parts of the country or even to every house and business within a city. Before planning on deploying either of these services, it is imperative to discuss these plans with the local DSL/Cable NSP. In the event these services are not available for connectivity, you need to consider the more traditional Frac-T1/T3, ISDN, or dial-up services. 

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