Home > Articles > Cisco Network Technology > General Networking > DSL and Cable Modem Networks

DSL and Cable Modem Networks

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Mar 14, 2003.

Chapter Description

Before committing to a DSL or cable-modem, learn exactly what these technologies consist of and what kind of service they can provide for you.

DSL and Cable Modem Networks

This chapter covers the following topics:

  • 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."

Figure 7-1 Figure 7-1 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.

Figure 7-2 Figure 7-2 ADSL Connection

The asymmetry of ADSL, combined with always-on access (which eliminates call setup), makes ADSL another solution for Internet/intranet surfing, video-on-demand, and remote LAN access because users of these applications often download more data than they upload.

ADSL Architecture

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

  • A high-speed downstream channel—Ranges from 1.5 to 9 Mbps.

  • A low-speed upstream channel—Ranges from 16 to 640 Kbps.

  • A basic telephone service channel—The 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.

Figure 7-3 illustrates the architecture of an ADSL network.

Figure 7-3 Figure 7-3 ADSL Architecture

ADSL architecture is made up of the following components:

  • Transport System—Provides 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 Network—Uses 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 Splitters—Optional 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:

    • Single splitter version for mounting at the residence

    • 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





1.5 or 2















Customer sites beyond the previously listed distances can be reached with fiber-based digital loop carrier (DLC) systems, as illustrated in Figure 7-4.

Figure 7-4 Figure 7-4 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 2—Standard 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.

2. Cable Access Technologies | Next Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020