Digital Subscriber Line (xDSL) FAQ v20010108

Archive-name: datacomm/xdsl-faq Last-modified: January 08, 2008 Version: 20001001 URL:

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(c) 1998-2001 John Kristoff Maintainer: John Kristoff Frequency: Monthly

comp.dcom.xdsl Frequently Asked Questions

----------------------------------------- This document is provided as is without any express or implied warranties. While every effort has been taken to ensure the accuracy of the information contained in this document, the author(s) assume no responsibility for errors, omissions, or damages resulting from the use of the information contained herein. The contents of this document reflect opinions only and not necessarily of the employer of the author(s).

Note: This FAQ is best viewed using a mono-spaced font such as Courier to ensure that any ASCII charts and graphics will be displayed properly.

Recent Changes


20010108 many updates from previous version (finally! :-)

FAQ Table of Contents


1.0 FAQ Administration [1.1] What is this FAQ about? [1.2] Who maintains this FAQ? [1.3] Where can this FAQ be found? [1.4] Who provides information to this FAQ? [1.5] Can I post this FAQ on my web page? [1.6] Who should I direct questions (and answers) to?

2.0 Introduction to xDSL

[2.1] What is xDSL? [2.2] How fast is xDSL? [2.3] Where are the xDSL standards documents? [2.4] How does xDSL compare to other technologies? [2.5] Should I get xDSL?

3.0 General xDSL information

[3.1] How does xDSL work? [3.2] What are the various types of xDSL? [3.3] How much does xDSL cost? [3.4] Is xDSL available in my area? [3.5] Why are some variations of xDSL asymmetric? [3.6] What does a POTS splitter do and when do I need one? [3.7] What test equipment is available for xDSL? [3.8] What are the barriers to a xDSL installation? [3.9] What is a DSLAM? [3.10] How are people using xDSL technology?

4.0 Basic Data Communications

[4.1] What is analog? [4.2] What is digital? [4.3] What is modulation? [4.4] What is attenuation? [4.5] What is crosstalk? [4.6] What is the effect of noise?

5.0 The Local Loop

[5.1] What is the local loop? [5.2] What is a bridge tap? [5.3] What are loading coils? [5.4] What are echo suppressors and echo cancellers? [5.5] What is a CODEC? [5.6] How do I determine how far I am from my CO? [5.7] What do people mean by a "truck roll"? [5.8] What is dry copper? [5.9] What are binder groups and why are they important?

6.0 Encoding and modulation

[6.1] What is QAM? [6.2] What is PCM? [6.3] What is PAM? [6.4] What is V.90? [6.5] What is CAP? [6.6] What is DMT?

7.0 Setup and Troubleshooting

[7.1] What hardware does my home computer need? [7.2] How does the DSL line encapsulate my data? [7.3] Can I use my 28.8K/56K modem with my xDSL line? [7.4] What's up with static versus dynamic IP addresses? [7.5] How do I share multiple hosts on my DSL line? [7.6] How do I secure my systems from Internet attacks? [7.7] Can I have more than on xDSL line in my home? [7.8] How do I tune my xDSL line for maximum performance? [7.9] What differentiates one xDSL provider from another? [7.10] Does xDSL require a UPS in case of a power failure? [7.11] I'm rewiring my home, what cabling do I use for xDSL?

8.0 xDSL Resources

[8.1] What web sites maintain xDSL information? [8.2] Are there any xDSL mailing lists? [8.3] What Usenet newsgroups discuss xDSL? [8.4] Are there any books that cover xDSL? [8.5] What periodicals cover xDSL technology? [8.6] Are there industry conferences that cover xDSL technologies? [8.7] What companies make xDSL products? [8.8] Who are the xDSL service providers? [Appendix A] Acronym List



1.0 FAQ Administration [1.1] What is this FAQ?

This FAQ will attempt to explain the intricacies of Digital Subscriber Line technologies (xDSL) and answer some of the most common questions relating to xDSL services. Although this FAQ contains technical information, it is best used as an introduction to xDSL services. See section 8.0 for a comprehensive list of xDSL resources.

[1.2] Who maintains this FAQ?

This FAQ is maintained by John Kristoff . Additions, comments, corrections and contributions are highly encouraged.

[1.3] Where can this FAQ be found?

This FAQ will be posted to the comp.dcom.xdsl newsgroup once a month and be archived to The latest version can always be found at:

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[1.4] Who provides information to the FAQ?

In many cases, the FAQ questions and answers are summarized from the comp.dcom.xdsl newsgroup, mailing list(s) and web sites. Much of the FAQ information is gathered through the direct or indirect contributions from numerous individuals. It's been difficult to keep track everyone's contributions. However a few people have been especially helpful, they include:

Gary Abbott Brandon Applegate Dave Burstein Danny Briere John Brothers Luke Diamond Niall Gillespie Dave Hannon Chris Hansen Jeff Huber John Kristoff Jonathon C McLendon Michael Sabo Bob Schreibmaier Bryan Sheppeck Craig Spannring Michael Stroh Edward Vielmetti John M. Wobus

[1.5] Can I post this FAQ on my web page?

Since this FAQ can change regularly, a copy of the FAQ on your web page could be out of date in a very short time. A more appropriate method would be to set a hyperlink to the URL found in the secondary header of this FAQ. Please send e-mail to John Kristoff at if you plan on adding a link to this FAQ on your web page.

[1.6] Who should I direct my questions (and answers) to?

If you have questions specifically about the FAQ or questions that you think should be added to the FAQ, please address them to the FAQ maintainer listed above. If you have questions about any other xDSL related question not covered in this FAQ, please do NOT send your questions directly to the FAQ maintainer.

For questions not answered by this FAQ, it is requested that you pose your query to the appropriate mailing lists, newsgroups, providers or vendors. Submitting your questions to the FAQ maintainer directly is not likely to generate a response. If possible, the question will be presented in a future version of this FAQ.

2.0 Introduction to xDSL [2.1] What is xDSL?

xDSL is a generic abbreviation for the many flavors of DSL or Digital Subscriber Line technology. DSL refers to the technology used between a customer's premises and the telephone company, enabling more bandwidth over the already installed copper cabling than users have traditionally had.

[2.2] How fast is xDSL?

The short answer is "it depends". Typically speeds start at about 128Kb/s and go up to 1.5Mb/s for most home users. Some installations may go as fast as 50Mb/s or more depending primarily on the equipment used, distances involved, cabling quality, encoding techniques, frequency spectrum available and even to some degree, end system configurations. Be aware that some xDSL is sold as asymmetric or "rate-adaptive". It is best to consult the providers in your area as to the access rates available in your area. Speeds can vary from provider to provider even if they are all servicing your area from the same central office.

[2.3] Where are the xDSL standards?

From International Telecommunication Union (ITU) G.992.1 (G.dmt) standards information G.992.2 (G.lite) standards information

From American National Standards Institute (ANSI) ANSI TI.413-1998 ($175.00 US) Asymmetric Digital Subscriber Line (ADSL) Metallic Interface

From Universal ADSL Working Group [site down] G.lite standards information

From the Standards Committee T1-Telecommunications Many xDSL standards Relevant documents are from the T1E1.4 (Digital Subscriber Loop Access) working group

From European Telecommunications Standards Institute (ETSI)

ADSL, VDSL and SDSL standards

From the Internet Engineering Task Force (IETF) ADSL MIB working group

[2.4] How does xDSL compare to other technologies?

Cable Modems

------------ Cable modems are devices that attach to the cable TV network connection in a home. This broadband technology is being driven by the cable companies to provide services beyond traditional broadcast cable TV such as Internet access. Along with xDSL, it is still in the early stages of development. There are a number of challenges faced by this industry, including return path capabilities, customer service issues and standards. However, potential bandwidth estimates range upwards of 30Mb/s from the service provider to subscriber. Cable networks are inherently different in design than telephone networks. Cable networks are broadcast oriented, with each subscriber in an area receiving the same signals as all others in that area. xDSL is circuit oriented so that each connection is independent of all others. Cable networks are inherently hierarchical in nature and thus require two paths, one for downstream and one for upstream. This requires either a second cable plant for upstream or a second frequency band allocated onto the existing system.


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