Home > Articles > Networking

Introduction to DSL

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

1.3 The Family of DSL Technologies

Several species of DSL have resulted from the evolution of technology and the market it serves. The earliest form of DSL, 144 kb/s basic rate ISDN, was first used for ISDN service in 1986, and then was later applied to packet mode ISDN DSL (IDSL), and local transport of multiple voice calls on a pair of wires (DAML: digital added main line). Basic rate ISDN borrowed from earlier voice band modem technology (V.34), and T1/E1 digital transmission technology (ITU Rec. G.951, G.952).

As shown in Figure 1.4, DSL transmission standards have evolved from 14.4 kb/s voice-band modems in the 1970s to 52 Mb/s VDSL in the year 2001. This has been an evolution, with each generation of technology borrowing from the prior generation.

Figure 4Figure 1.4 Evolution of DSL Technology (Note: Dates indicate publication of relevant standards.)

High bit-rate DSL (HDSL) was introduced into service in 1992 for 1.5 Mb/s (using two pairs of wires) and 2 Mb/s (using two or three pairs of wires) symmetric transmission on local lines. HDSL greatly reduced the cost and installation time required to provide service by reducing the need for midspan repeaters and simplifying the line engineering effort. HDSL is widely used for private line services, and links to remote network nodes such as digital loop carrier remote terminals and wireless cell sites. In 2000, HDSL2 was introduced to accomplish the same bit-rate and line reach as HDSL but using one pair of wires instead of the two pairs required for HDSL. Both HDSL and HDSL2 operate over CSA (carrier serving area) length lines consisting of up to 12 kft2 of 24 AWG wire, 9 kft of 26 AWG wire, or a proportionate length of mixed wire gauges. HDSL2 is spectrally compatible with other services in the same cable within the CSA line lengths but may not be spectrally compatible if a midspan repeater is used to serve longer lines. HDSL4, using trellis-coded pulse amplitude modulation (TC-PAM) for two pairs of wires, achieves spectral compatibility for 1.5 Mb/s transport on longer loops. By reaching up to 11 kft on 26 AWG lines without repeaters, HDSL4 further reduces the need for repeaters. The complementary pair of technologies—HDSL2 (for CSA lines) and HDSL4 (for longer lines)—provide a lower cost and spectrally compatible means to provide symmetric 1.5 Mb/s for nearly all lines. Chapters 4 and 6 discuss HDSL2 and HDSL4, respectively. Chapter 6 also addresses the symmetric SHDSL technology.

Asymmetric DSL (ADSL) service was introduced in 1995 and employed the following new technology aspects:

  • Higher downstream bit rates are achieved via transmission asymmetry, using a wider bandwidth for downstream transmission and a narrower bandwidth for upstream transmission.

  • Near-end crosstalk is reduced by partial or full separation of the upstream and downstream frequency bands.

  • Simultaneous transport of POTS and data is achieved by transmitting data in a frequency band above voice telephony.3

  • Use of advanced transmission techniques (trellis coding, Reed-Solomon codes with interleaving, and DMT modulation).

  • Rate-adaptive transmission that adjusts to the highest bit rate allowed by the unique conditions for each line.

ADSL is widely used for applications benefiting from the bit-rate asymmetry, for example, high-speed Internet access and workstation access for small business offices and home work offices (SOHO). ADSL supports downstream bit-rates up to 8 Mb/s and upstream bit-rates up to 900 kb/s on short lines (less than 6 kft) with moderate line noise. However, to assure service to more lines with more noise, ADSL service is most often provided at bit-rates of 2 Mb/s or less downstream and 128 kb/s or less upstream. At mid-year 2002, there were 26 million ADSLs in service worldwide, with approximately 80 percent of the lines serving residential customers and 20 percent of the lines serving business customers. The early deployments of ADSL employed a splitter at both ends of the line to combine the 0–3.2 kHz analog voice signal with the ADSL signals in a higher frequency band. The development of the "G.lite" (ITU Rec. G.992.2) standard introduced the concept of enabling customer self-installation without a splitter at the customer end of the line. This reduced the labor cost to install the service. Field trials of G.lite demonstrated that an in-line filter must be inserted in series with most types of telephone sets to prevent problems for both the voice transmission as well as the digital transmission. Subsequently, it was determined that the in-line filters permitted effective operation of the full-rate ADSL (T1.413 and ITU Rec G.992.1), whereas G.lite is restricted to about 1.5 Mb/s downstream. As a result, the large majority of current ADSL installations use full-rate ADSL self-installed by the customer, placing an in-line filter by every telephone in their premises. Because ITU Recs. G.922.1 and G.992.2 were derived from the earlier T1.413 standard, all these ADSL standards are very similar, and most ADSL equipment supports all three standards. Chapter 3 discusses ADSL in more detail.

Single-pair high-bit-rate DSL (SHDSL) products were available by the end of 2000 based on the ITU Rec. G.991.2 standard. Like the nonstandard 2B1Q SDSL systems, SHDSL supports symmetric transmission at bit-rates from 192 kb/s to 2.32 Mb/s while providing at least 2,000 feet greater line reach than SDSL. Furthermore, the SHDSL specifications provided for the use of multiple pairs of wires and midspan repeaters to achieve greater bit-rates and line lengths. SHDSL uses trellis coded pulse amplitude modulation (TC-PAM), which is also used for HDSL2 and HDSL4. Chapter 6 discusses SHDSL in more detail.

Prestandard very high-bit-rate DSL (VDSL) systems were used in field trials in 2000. VDSL supports asymmetric bit-rates as high as 52 Mb/s downstream or symmetric bit-rates as high as 26 Mb/s. The key distinction of VDSL is its limitation to very short loops, as short as 1,000 feet for the highest bit-rates or up to about 4,000 feet for moderate data rates. The very short line length operation depends on shortening the copper line by placing an optical network unit (ONU) close to the customer site and then connecting one or more ONUs to the network with a fiber. Like ADSL, VDSL is a rate adaptive system that provides for simultaneous transmission of data and an analog voice signal. Chapter 7 discusses VDSL in more detail.

The term xDSL applies to most or all types of DSL technology. Chapter 5 discusses ITU G.994.1 (g.handshake), which DSL transceivers use to negotiate a common operating mode.

Figure 1.5 shows the upstream and downstream rates supported by the various DSL technologies with the symmetric technologies (ISDN, SHDSL, HDSL) residing along a line of symmetry, and the rate adaptive technologies (ADSL, VDSL) covering a broad range of bit-rates with the corresponding maximum line lengths indicated.

Figure 1.5Figure 1.5 DSL Data Rates

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT 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 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@informit.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 InformIT. 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.informit.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