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Subject: Digital Subscriber Line (xDSL) FAQ v20010108

This article was archived around: 21 Jun 2009 16:51:01 GMT

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Archive-name: datacomm/xdsl-faq Last-modified: January 08, 2008 Version: 20001001 URL: http://condor.depaul.edu/~jkristof/xdsl-faq.txt Copyright: (c) 1998-2001 John Kristoff Maintainer: John Kristoff <jtk@depaul.edu> 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 Questions --------- 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 <jtk@depaul.edu>. 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 ftp://rtfm.mit.edu. The latest version can always be found at: http://condor.depaul.edu/~jkristof/xdsl-faq.txt [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 <gla [at] avana.net> Brandon Applegate <brandon [at] one.net> Dave Burstein <daveb [at] dslprime.com> Danny Briere <dbriere [at] telechoice.com> John Brothers <John.Brothers.johnbr [at] nt.com> Luke Diamond <lgd [at] virata.com> Niall Gillespie <nafg [at] virata.com> Dave Hannon <dhannon [at] jps.net> Chris Hansen <galaxy5 [at] oro.net> Jeff Huber <huber [at] home.net> John Kristoff <jkristof [at] depaul.edu> Jonathon C McLendon <mclejc [at] aur.alcatel.com> Michael Sabo <msabo [at] rhythms.net> Bob Schreibmaier <k3ph [at] dxis.monroe.pa.us> Bryan Sheppeck <bsheppeck [at] accesslan.com> Craig Spannring <cts [at] bangkok.office.cdsnet.net> Michael Stroh <stroh [at] mpinet.net> Edward Vielmetti <emv [at] umich.edu> John M. Wobus <jmwobus [at] MailBox.Syr.Edu> [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 jtk@depaul.edu 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) <http://www.itu.int> G.992.1 (G.dmt) standards information G.992.2 (G.lite) standards information From American National Standards Institute (ANSI) <http://www.ansi.org> ANSI TI.413-1998 ($175.00 US) Asymmetric Digital Subscriber Line (ADSL) Metallic Interface From Universal ADSL Working Group <http://www.uawg.org> [site down] G.lite standards information From the Standards Committee T1-Telecommunications <http://www.t1.org> Many xDSL standards Relevant documents are from the T1E1.4 (Digital Subscriber Loop Access) working group From European Telecommunications Standards Institute (ETSI) <http://www.etsi.org> ADSL, VDSL and SDSL standards From the Internet Engineering Task Force (IETF) <http://www.ietf.org> ADSL MIB working group <http://www.ietf.org/html-charters/adslmib-charter.html> [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. ISDN ---- ISDN is a telephone company technology that provides digital service typically in increments of 64Kb/s channels. ISDN has been around for many years, but it's popularity only recently began to increase due to the limitations of analog modems and the rise of Internet usage. ISDN requires the phone company to install services within their phone switches to support this digitally switched connection service. Roll out of this service initially got off to a slow start and was stalled by high costs, lack of standards and low acceptance rates by consumers. xDSL and other new high speed technologies have in many cases "leapfrogged" the ISDN market. T1 -- A T1 (E1 is the European near equivalent) line is a 1.544 Mb/s pulse code modulated (PCM) system compromised of 24 time division multiplexed (TDM) channels of 64 Kb/s each. A T1 defines a copper copper wire interface specification for transmission between a customer and provider. Not to be confused with a DS1, which is the digital signaling rate of the underlying carrier. Many people however use these terms interchangeably. T1/E1 lines have been used in voice and data networks throughout the world where highly available, high capacity networks needed to be built. In fact, DS1 (or T1) is just one step in hierarchy of systems with higher speeds (e.g. T3/DS3). In many cases, T1 lines have been installed for end users who require dedicated high speed bandwidth between their home and work (or Internet). T1/E1 cabling requirements are more stringent than that of xDSL with the setup costs reflecting the differences in the service. Still a popular solution for many organizations and individuals, typically you will find that this service is considerably more expensive for an end user than xDSL or cable modems. However, the service level for T1 lines is usually very high. Voiceband Modems ---------------- Voiceband modems (or just modems for short) use a telephone network as is. That is, there are no special provisions that are required to use modems in today's telephone networks. Modems allow digital data to flow over the telephone company's traditional telephone network by performing a digital to analog conversion for transmission onto the network and vice versa on the receiving end. The only requirement for modems is that each end of the call must have a compatible modem. In essence, this makes modem connections the most ubiquitous form of data communications available today. However, modems are limited by the telephone company's voice bandwidth service. Current voiceband modem technology is struggling to achieve rates of only 56Kb/s. With only a bandwidth of about 3,000 Hz, there is a extremely finite limit on the amount of data that can be encoded and sent reliably through this network. User requirements far outstrip what modems can obtain today. Wireless -------- There are a number of different wireless schemes proposed, planned and implemented throughout the world. Wireless access technology takes shape in a number of different forms such as via a satellite TV service provider or a cellular phone network. Wireless systems can provide ubiquitous access to a large number of subscribers in a relatively large area. Bandwidth can range from a few kilobits a second to many megabits and be either symmetrical or asymmetrical. Like all other technologies, there can be deployment issues which may include spectrum licensing, interference, line of sight requirements, noise problems or bandwidth limitations. xDSL ---- xDSL is technology backed by telephone companies to provide next generation high bandwidth services to the home and business using the existing telephone cabling infrastructure. xDSL to the home over existing phone lines promises bandwidths up to 9Mb/s or more, but distance limitations and line quality conditions can reduce what will actually be achievable. xDSL technologies will use a greater range of frequencies over the telephone cable than the traditional telephone services have used. This in turn allows for greater bandwidth with which to send and receive information. xDSL technology is still in the early stages of development with standards and products just getting under way. Driving this market is the competition from competing access providers and the pursuit of your Internet access dollar. [2.5]Should I get xDSL? That depends on a number of answers to questions which you'll need to ask yourself. First and foremost you need to determine if DSL is even available in your area. You may not have a choice. By reading this FAQ, you can hopefully learn enough about xDSL and how to get more information to make an informed decision. Although there are merits to all competing technologies, we make no recommendation in this FAQ to specify which one is right for you. 3.0 General xDSL information [3.1] How does xDSL work? xDSL utilizes more of the bandwidth on copper phone lines than what is currently used for plain old telephone service (POTS). By utilizing frequencies above the telephone bandwidth (300Hz to 3,200Hz), xDSL can encode more data to achieve higher data rates than would otherwise be possible in the restricted frequency range of a POTS network. In order to utilize the frequencies above the voice audio spectrum, xDSL equipment must be installed on both ends and the copper wire in between must be able to sustain the higher frequencies for the entire route. This means that bandwidth limiting devices such as loading coils must be removed or avoided. [3.2] What are the various types of xDSL? There are several forms of xDSL, each designed around specific goals and needs of the marketplace. Some forms of xDSL are proprietary, some are simply theoretical models and some are widely used standards. They may best be categorized within the modulation methods used to encode data. Below is a brief summary of some of the known types of xDSL technologies. ADSL Asymmetric Digital Subscriber Line (ADSL) is the most popular form of xDSL technology. The key to ADSL is that the upstream and downstream bandwidth is asymmetric, or uneven. In practice, the bandwidth from the provider to the user (downstream) will be the higher speed path. This is in part due to the limitation of the telephone cabling system and the desire to accommodate the typical Internet usage pattern where the majority of data is being sent to the user (programs, graphics, sounds and video) with minimal upload capacity required (keystrokes and mouse clicks). Downstream speeds typically range from 768 Kb/s to 9 Mb/s Upstream speeds typically range from 64Kb/s to 1.5Mb/s. ADSL Lite (see G.lite) CDSL Consumer Digital Subscriber Line (CDSL) is a proprietary technology trademarked by Rockwell International. CiDSL Globespan's proprietary, splitterless Consumer-installable Digital Subscriber Line (CiDSL). EtherLoop EtherLoop is currently a proprietary technology from Nortel, short for Ethernet Local Loop. EtherLoop uses the advanced signal modulation techniques of DSL and combines them with the half-duplex "burst" packet nature of Ethernet. EtherLoop modems will only generate hi-frequency signals when there is something to send. The rest of the time, they will use only a low-frequency (ISDN-speed) management signal. EtherLoop can measure the ambient noise between packets. This will allow the ability to avoid interference on a packet-by-packet basis by shifting frequencies as necessary. Since EtherLoop will be 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. G.lite A lower data rate version of Asymmetric Digital Subscriber Line (ADSL) was been proposed as an extension to ANSI standard T1.413 by the UAWG (Universal ADSL Working Group) led by Microsoft, Intel, and Compaq. This is known as G.992.2 in the ITU standards committee. It uses the same modulation scheme as ADSL (DMT), but eliminates the POTS splitter at the customer premises. As a result, the ADSL signal is carried over all of the house wiring which results in lower available bandwidth due to greater noise impairments. Often a misnomer, this technology is not splitterless per se. Instead of requiring a splitter at customer premises, the splitting of the signal is done at the local CO. G.shdsl G.shdsl is a ITU standard which offers a rich set of features (e.g. rate adaptive) and offers greater reach than many current standards. G.shdsl also allows for the negotiation of a number of framing protocols including ATM, T1, E1, ISDN and IP. G.shdsl is touted as being able to replace T1, E1, HDSL, SDSL HDSL2, ISDN and IDSL technologies. HDSL High Bit-rate Digital Subscriber Line (HDSL) is generally used as a substitute for T1/E1. HDSL is becoming popular as a way to provide full-duplex symmetric data communication at rates up to 1.544 Mb/s (2.048 Mb/s in Europe) over moderate distances via conventional telephone twisted-pair wires. Traditional T1 (E1 in Europe) requires repeaters every 6000 ft. to boost the signal strength. HDSL has a longer range than T1/E1 without the use of repeaters to allow transmission over distances up to 12,000 feet. It uses pulse amplitude modulation (PAM) on a 4-wire loop. HDSL2 High Bit-rate Digital Subscriber Line 2 was designed to transport T1 signaling at 1.544 Mb/s over a single copper pair. HDSL2 uses overlapped phase Trellis-code interlocked spectrum (OPTIS). IDSL ISDN based DSL developed originally by Ascend Communications. IDSL uses 2B1Q line coding and typically supports data transfer rates of 128 Kb/s. Many end users have had to suffice with IDSL service when full speed ADSL was not available in their area. This technology is similar to ISDN, but uses the full bandwidth of two 64 Kb/s bearer channels plus one 16 Kb/s delta channel. MDSL Usually this stands for multi-rate Digital Subscriber Line (MDSL). It depends on the context of the acronym as to its meaning. It is either a proprietary scheme for SDSL or simply a generic alternative to the more common ADSL name In the former case, you may see the acronym MSDSL. There is also another proprietary scheme which stands for medium-bit-rate DSL. Confused yet? RADSL Rate Adaptive Digital Subscriber Line (RADSL) is any rate adaptive xDSL modem, but may specifically refer to a proprietary modulation standard designed by Globespan Semiconductor. It uses carrierless amplitude and phase modulation (CAP). T1.413 standard DMT modems are also technically RADSL, but generally not referred to as such. The uplink rate depends on the downlink rate, which is a function of line conditions and signal to noise ratio (SNR). SDSL Symmetric Digital Subscriber Line (SDSL) is a 2-wire implementation of HDSL. Supports T1/E1 on a single pair to a distance of 11,000 ft. The name has become more generic over time to refer to symmetric service at a variety of rates over a single loop. UDSL Universal DSL. See G.lite. VDSL Very High Bit-rate Digital Subscriber Line (VDSL) is proposed for shorter local loops, perhaps up to 3000 ft. Data rates exceed 10 Mb/s. [3.3] How much does xDSL cost? It varies. xDSL service availability is still in the early stages, but pricing in some areas has been very aggressive. Prices can change overnight and differ significantly depending on the service provider and surrounding area. Local tariffs and government regulations may also play a role in determining end user cost. To complicate matters further, some providers are claiming to offer free xDSL service. In many of these cases however, it requires you to be subjected to directed marketing or to make long term commitments to their service. You should first determine what your needs and tolerances are. Do you want static IP addresses? How fast do you want to go? What level of service do you require? Do you need multiple email addresses? ...and so on. Your answers to these types of questions will help you narrow down your choices. To find out more about how much xDSL service may cost, check with the service providers listed in section [8.8] or ask in the newsgroup(s) or mailing list(s) for the most up to date information. [3.4] Is xDSL available in my area? To find out, you can check a number of sources. First, you can check with your local telephone company to see if they are providing xDSL services. Second, check around with your local Internet Service Providers (ISPs). Thirdly, try the competitive local exchange companies (CLECs) in your area. A good resource for CLECs is at <http://www.clec.com>. Fourth, try perusing some of the resources listed in section [8.8] of this FAQ. Also ask around in the xDSL newsgroup(s) or mailing list(s). Lastly, there are some sites which claim to tell you if DSL is available in your area simply by filling in a online form. Unfortunately you cannot rely upon these sites for 100% accuracy. Even if you're told xDSL is available in your area, you still might be not able to get it. Often providers will need to perform a "qualification test" to determine if they can send and receive a signal within their parameters. Long local loops and poor cabling plants are common reasons for failing a loop qualification test. [3.5] Why are some variations of xDSL asymmetric? It is primarily due to near-end crosstalk (NEXT). The large bundle of wire at the CO is heavily susceptible to crosstalk, particularly with regards to the signal that travels from the far end (the end user). At the far end, there are fewer problems with NEXT so bandwidth is greater from the CO to the user. High bit rates, or in this case, higher frequencies suffer a greater amount of attenuation. The reason that the upstream speed in ADSL is generally much less than the downstream rate is due to this fact. When the high frequencies have attenuated at the CO end, they are very susceptible to all the other signals in the binder group due to EMI. In the downstream direction, the high frequencies still attenuate, but at the customer end, they have a better chance of avoiding crosstalk since most subscribers will not have large bundles of cables running into their premises. [3.6] What does a POTS splitter do and when do I need one? A POTS splitter uses a low pass filter to separate the low end frequencies of the telephone audio spectrum from the higher frequencies of the xDSL signals. The splitter should be a passive device, not requiring power so that "life-line" voice service can be provided as has been in the past. This splitter allows for the traditional voice service that consumers are accustomed to. A splitter is required at both the customer premises and at the far end (CO). xDSL that does not use a POTS splitter on customer premises is termed "splitter-less xDSL". However, there really is no such thing as splitter-less xDSL. The splitter function in these cases is just performed at the provider, generally the CO. Whether a POTS splitter is required or not depends on the xDSL service being provided. [3.7] What test equipment is available for xDSL? Agilent Technologies <http://www.agilent.com> Handheld testers for field technicians Aware Veritas product line <http://www.aware.com> Physical line testing and qualification of standards compliance Fluke One Touch <http://www.fluke.com/nettools/> Performs simple asymmetric bandwidth testing. Harris <http://www.harris.com> TS1000 handheld tester for field technicians TTC <http://www.ttc.com> Various testing equipment 4.0 Basic Data Communications [4.1] What is analog? A good starting point in order to understand analog communications is to first take in the picture below. + .'^'. + / \ + / \ + / \ ++++++++++++++\++++++++++++ + \ / + \ / + \ / + `._.' Although my artistic ability leaves much to be desired, this wave form is a depiction of a simple analog signal. The key to the analog signal is that it is *continuous*. In other words, notice how the wave slowly rises, peaks, slowly descends, bottoms out and slowly climbs again. Taken as a simple example, imagine many forms of this wave signal. Some of the waves are closer together than others, some may have more height, still others may actually start their peaks and descents in entirely different places! Encoding data can be done based on these various kinds of wave changes. One of the important considerations in analog communications is the ability to decode these continuous wave forms. With the introduction of noise, or other signal disturbance, decoding a analog signal properly can be difficult. This is why we turn to the digital communications system (see next question). [4.2] What is digital? Again, with a picture let us look a simplistic view of a digital signal. + .--------. .-- + | | | + | | | + | | | ++++|++++++++|++++++++|+++ + | | | + | | | + | | | + --' `--------' Compared to the picture of the analog signal above, there is a major difference in this wave form. The transition from the peak of the wave to the bottom of the wave is *discrete*. In this case, the only way to represent data is by using the high or low point of the wave. For example, the high point may represent a "on" signal and the low point may represent a "off" signal. In the world of computers, this is also known as a binary numbering system consisting of only two digits. By using a digital signaling system in this fashion, it makes encoding and decoding data very simple. Generally, it will be very easy to determine where the peaks and valleys are, even with some signal loss or disturbance. Digital methods are used as long as frequency response (bandwidth) is not a limitation. Analog methods are used only because multiple signal levels must be exploited to communicate a higher data rate of digital values in lieu of having adequate bandwidth. A digital signaling system often has an analog component. Strictly speaking, this means the a digital wave isn't as sharp cornered as the picture shows above. The corners will likely be slightly rounded and even more so as the signal travels over some distance. For our purposes, this definition should give you a basic idea of how a digitally encoded system works. [4.3] What is modulation? Modulation is a prescribed method of encoding digital (or analog) signals onto a waveform (the carrier signal). Once encoded, the original signal may be recovered by an inverse process called demodulation. Modulation is performed to adapt the signal to a different frequency range than that of the original signal. Here's how it flows: bits -> modulator -> audio -> phone network -> audio -> demodulator -> bits Hence the name MODEM short for modulator/demodulator. The modem is necessary because the phone network transmits audio, not data bits. The modem is for compatibility with existing equipment. [4.4] What is attenuation? Attenuation is signal loss due to the diminishing availability of signal energy, or signal power. As a analog or digital signal traverses across a medium, it fades. High attenuation may lead to the inability to recover the signal on the far end. Signal repeaters may be used on the transmission path to periodically boost the signal strength. Baseband transmission is extremely limited to attenuation. Broadband much less so. In addition, wireless communications is much less susceptible to attenuation that is wireline communications such as xDSL or cable modems. [4.5] What is crosstalk? Crosstalk refers to the interference between channels. In the xDSL world, the interference between nearby cables can have a negative impact on the performance of the affected cable(s). Have you ever been on the phone and heard some other conversation, not yours, in the background? If so, you have experienced the effect of crosstalk. Near-end crosstalk (NEXT) occurs when the transmitter sends a signal and a nearby transceiver at the same end of link, through capacitive and inductive coupling, "hears" the signal. Far-end crosstalk (FEXT) occurs when the transmitter sends a signal and a transceiver at the far end of the link, through capacitive and inductive coupling, "hears" the signal. FEXT will be of more concern in an asymmetrical system such as ADSL than symmetrical systems like HDSL. This is because strong signals originating from the near end, can interfere with the weaker signals originating at the far end. [4.6] What is the effect of noise? Noise may be defined as the combination of unwanted interfering signal sources whether it comes from crosstalk, radio frequency interference, distortion, or random signals created by thermal energy. Noise impairs the detection of the smallest analog levels which may be resolved within the demodulator. The noise level along with the maximum clip level of an analog signal path set the available amplitude dynamic range. The maximum data rate of a modem is limited by the available frequency range (bandwidth) and signal-to-noise ratio (SNR) which is amplitude dynamic range. If more of either is available, more bits may be transferred per second. The information carrying limit was discussed theoretically by Claude Shannon and is known as Shannon's limit, or information theory. Because modems run close to Shannon's limit today, no further advances will be made to traditional telephone line modems other than incremental improvement of V.90. The frequency range of the audio channel is very limited at about 4 kHz. V.34+ modems are limited to a maximum data rate of 33.6Kb/s by an SNR of about 36 dB caused mostly by network PCM quantization noise. While V.90 improves the SNR by utilizing the network PCM levels directly, it is still subject to Shannon's limit. xDSL modems take advantage of the spectrum above the telephone audio channel. While operating with somewhat less amplitude dynamic range they increase data rates by greatly increasing the frequency range of the communication signal (from about 10 kHz to over 1.0mHz). To do this they require the installation of special equipment at the central office and customer premise. 5.0 The Local Loop [5.1] What is the local loop? A pair of wires, moderately twisted for the entire length between the telephone company's end office and the user premises (the common telephone set) form a loop, so it is referred to as the local loop. This loop provides a user with access to the global telecommunications infrastructure that is installed all over the world. The local loop has been historically designed to provide voice grade audio service. The circuit is powered from the central office with 48V (open circuit voltage) limited in current to a value somewhat higher than 20mA. This current is used for signaling phone access, burning off moisture, breaking through metallic oxides caused by corrosion, and powering a carbon microphone. The original telephone equipment contained no active electronics. The actual wiring of the local loop may be considered to be a lossy transmission line. xDSL uses whatever frequencies will propagate on this line for purposes of digital data transmission. T1 modulation (alternate mark inversion) has been doing this for years. xDSL extends the capability by using modern technology to increase the data rates and distances spanned. [5.2] What is a bridge tap? A bridge tap is an accidental connection of another local loop to the primary local loop. Generally it behaves as an open circuit at DC, but becomes a transmission line stub with adverse effects at high frequency. It is generally harmful to xDSL connections and should be removed. Extra phone wiring within one's house is a combination of short bridge taps. A POTS splitter isolates the house wiring and provides a direct path for the xDSL signal to pass unimpaired to the ATU-R modem. [5.3] What are loading coils? Loading coils are used to extend the range of a local loop for voice grade communications. They are inductors added in series with the phone line which compensate for the parallel capacitance of the line. They benefit the frequencies in the high end of the voice spectrum at the expense of the frequencies above 3.6kHz. Thus, loading coils prevent xDSL connections. [5.4] What are echo suppressors and echo cancellers? These are active devices used by the phone company to suppress the reflection of an analog signal or positive feedback (singing) on the phone network. The effect of the echo on a voice connection is undesirable. Imagine that as you spoke into the phone's microphone, there was a short delay and you hear your own voice back over the earpiece. A soft echo that comes back fast enough is not bothersome to the average person. A more delayed echo is annoying. A echo suppressor works by allowing only one direction to transmit at a time so as to entirely eliminate the effect of an echo. An echo suppressor is able to switch between each end very rapidly, typically within 5msec. Network echo suppressors make full-duplex communication impossible. However, modems can deactivate these devices by sending the 2100 Hz answer tone at the beginning of the connection. An echo canceller subtracts a locally generated replica of the predicted echo based on the signal propagating in the forward direction. Echo cancellers do allow full-duplex operation and are generally preferred over echo suppressors in voice calls. But when network echo cancellers compete with echo cancellers within the modem they are problematic. Typically they reduce data rates to 9.6Kb/s or lower. Network echo cancellers are deactivated by placing 180 degree phase reversals every 450msec on answer tone. As long as carrier is maintained, they are supposed to remain deactivated. xDSL is not affected by network echo suppressors/cancellers because they are part of the CODEC signal processing. [5.5] What is a CODEC? CODEC is an abbreviation for coder/decoder. Specifically it converts a voice grade analog signal to u-law or A-law encoded samples at an 8 kHz sampling rate. xDSL bypasses the CODECs at the central office by separating the xDSL signal and voice frequencies in a POTS splitter. The voice signal is passed to a CODEC while the xDSL signal terminates in a DSLAM, the xDSL equivalent of a CODEC. [5.6] How do I determine how far I am from my CO? You can call your service provider and ask them for the address of your local CO. Using a map, you get an approximate distance from your residence to the CO. However, these are very rudimentary measurements because you can never be sure exactly what route your line takes between the two points. It may not be a direct route. If you're interested in whether you will qualify for high speed broadband service or if you're just wondering what the potential speed you could attain may be, there are other factors to consider (i.e. wire gauge, element continuity, environments, etc.) In a nutshell, just knowing the where the CO is, may not tell you much at all. [5.7] What do people mean by a "truck roll"? Anytime a service technician needs to be dispatched in order to install, configure or troubleshoot a line installation, it is referred to as a "truck roll". The significance of this term implies a real cost to the service provider whenever a technician's time is required. The term derives from the scene of a technician driving the familiar "company truck" and pulling up to the curb of your premises with the intention to install, configure or troubleshoot a line. [5.8] What is dry copper? Dry copper refers to twisted pairs that are not connected to a telephone switch, battery or anything else between customer locations. They are merely cross-connected in between. The term "dry" actually originated over 100 years ago, when batteries were first used to power telephones. A dry pair had no power applied to it from the CO and a "wet" one did. Some folks have been able to implement xDSL via dry copper connection between two sites. By simply placing xDSL modems at each end of the dry copper connection, a xDSL may be possible with little intervention from the perspective of the CO. However, this is a risky method of deploying xDSL, especially asymmetrical versions. The problems occur when there is interference between the dry copper xDSL lines and other lines nearby, such as T1 and POTS. Typically dry copper has been used for low speed alarm circuits. By implementing xDSL service over dry copper, you run the risk of future problems. You may disrupt service at the CO and hence, the CO's customers. Unless you have specifically contracted for this method of xDSL service in advance, beware. [5.9] What are binder groups and why are they important? A binder group is just a bunch of wires. More correctly in the telco world, a collection of twisted pair wires will share a common "sheath". The implementation of services within a binder group needs to be considered so that the effect of interference between services does not degrade nearby signals. Interference between wire pairs in a binder group can be a major issue in xDSL deployment. 6.0 Encoding and modulation [6.1] What is QAM? Quadrature amplitude modulation (QAM) is a method for encoding data on a single carrier frequency. The modulation encodes data (or bits) as discrete phase plus amplitude changes of a carrier tone. The phase vectors are arranged in a pattern of points called a constellation from which the transmitted point is selected based on the data to be sent. The modem sends the symbols as abrupt changes in phase and amplitude, but only as what emerges from a sharp cutoff filter which carefully limits the bandwidth. The transmitted signal occupies slightly more than 1/2 the modulation rate either side of the carrier frequency. The excess bandwidth, perhaps as much as 10%, is required for recovering symbol timing within the remote receiver. The receiver has to pick which point was transmitted with great reliability. It may employ adaptive equalization or other methods to reduce intersymbol interference to levels which are acceptable for discriminating the received point. The background noise level of the receiver limits the number of distinct constellation points which may be reliably determined, and hence limits the data rate for a given symbol rate. QAM has become the dominate modulation for high speed voice band modems. Examples are V.22bis, V.27, V.29, V.32bis, V.34. About every 2/3 of a carrier cycle the phase or amplitude is changed to a new value. This signaling rate is known as the baud (or symbol) rate. The highest QAM baud rate in use today for telephone line modems is 10/7 of 2400 Hz or about 3429 baud on a 1920 Hz carrier in V.34. By encoding something between 9 to 10 bits per baud a final data rate of 33.6Kb/s is developed. To encode this number of bits, over 1000 different phase/amplitude values must be resolved by the receiver. This is a nontrivial process involving adaptive equalizers, trellis coding, and other highly sophisticated signal processing. Transmit path: scrambler -> symbol generator -> 3x upsample (S1,0,0,S2,0,0,S3,...) -> complex transmit baseband FIR filter -> e^jwt carrier modulation -> scale real signal output -> DAC converter The baseband filter is about 3 dB down at 1/2 symbol rate, so for 3429 baud the signal out of the filter extends from -1715 Hz to +1715Hz. This is shifted by the positive 1920 Hz carrier to +205Hz to +3635Hz. One can see that this just fits in the frequency spectrum of the voice band telephone network. This filter, the analog electronics and the phone channel smear any given symbol over a 10msec period of the signal (about 32 symbols). The scrambler is very important. It randomizes the signal so an adaptive equalizer in the remote modem can build the inverse channel response (including the transmit filter). The smearing (or intersymbol interference) is largely eliminated by dynamically adjusting adaptive equalizer coefficients with the goal of minimizing least square error in the received points. The major adaptation is done during the training phase, although the feedback loops remain active throughout the connection. Other impairments to be solved are gain normalization, timing recovery, carrier offset frequency, phase jitter removal, and echo cancellation. [6.2] What is PCM? Pulse code modulation (PCM) is used in the phone network to reduce the data rate required for voice grade audio to less than 64Kb/s. It uses either u-law (North America) or A-law (Europe) as the compression method. Any given 8 kHz analog audio sample is converted to 4 bits of mantissa, 3 bits of exponent, and a sign bit. This code has a characteristic that quantization noise is proportional to signal amplitude and does not become objectionable to the average telephone user. For a conventional modem this noise floor limits the available dynamic range to about 36 dB which sets the maximum data rate. The least significant bit of the mantissa may be periodically stolen for signaling within the phone network (called robbed-bit signaling) further increasing the noise. The 8-bit codes are processed through the telephone switching network in fixed time slots. There exists an ever increasing hierarchy of data rates to support this. A DS0 is a 64Kb/s time slot. 24 DS0s become a DS1. 4 DS1s become a DS2 (now obsolete). 7 DS2s become a DS3, etc. The physical layer of a DS1 (T1) may be remodulated as alternate mark inversion for passing over a wire pair as a method to concentrate local loops. Repeaters regenerate the signal every 6000-9000'. These signals may coexist with xDSL in the same wire bundle. [6.3] What is PAM? Pulse amplitude modulation (PAM) is the physical layer of an ISDN or HDSL connection. The modulation consists of sending discrete amplitude levels (symmetric about 0 volts) at a regular rate. Both use the two binary one quaternary (2B1Q) line code. Four analog voltages (called quaternary symbols) are used to represent the four possible combinations of two bits. These symbols are assigned the names +3, +1, -1, and -3. So each amplitude level being held for one symbol time communicates two bits. The following diagram is typical of the 2B1Q waveform at the transmitter: +3 = 2.64V + .--. .--. .-- + | | | | | +1 = 0.88V + | `--. .--' | .--. | ++++|+++++|++|+++++|++++++++|++|++++++++|++++ -1 = -0.88V + --' | | | .-----' `--. | + | | | | | | -3 = -2.64V + `--' `--' `-----' One might assume this is a digital signal relative to the definition in [4.2], but by the time the signal has reached the receiver these discrete levels have diffused into each other because of phone line induced amplitude and phase distortion. This is called intersymbol interference. Therefore an adaptive equalizer must be used to restore the levels to values which may be discriminated for recovering the data. The symbol timing is recovered by examining the squared signal energy for a tone at the modulation rate. Transitions between levels cause the instantaneous power to dip on average provided there is adequate excess bandwidth. PAM differs from the other modulations in that it is baseband modulation and does not use a carrier. Some versions of HDSL increase the number of levels to 16 which communicates four bits per symbol in the same bandwidth. [6.4] What is V.90? V.90 is actually a variant of PAM. It has 256 PCM levels from which to choose a more limited set. The spacing between levels is set by the u-law or A-law characteristic described in [6.2]. The inner levels become more closely spaced so some of these must be excluded for reasons of limited signal-to-noise ratio. In addition, outer codes are excluded to keep transmit power on the local loop below -12dBm, a formal limit established by the FCC. V.90 includes a spectral shaping algorithm to prevent sending signal at DC. V.90 bypasses the problems associated with a conventional modem. It recognizes that with enough signal processing the original PCM samples sent by the phone company may be resolved as individual levels using a 16-bit A/D converter on the receiving end. Audio is sent through the digital network as 8-bit u-law or A-law samples. Of course, the telco D/A converter, reconstruction filter, and phone network blur the levels into one continuous signal, so it's up to the receiver to reconstruct what was sent. An additional problem is recovering symbol (i.e. PCM sample) timing information which must be inferred from the residue of modulation at a frequency around 4 kHz. By just selecting a limited set of codes with say 64 levels, 6 bits per 8 kHz symbol may be sent for a data rate of 48Kb/s. More levels, more data, but a maximum of about 53.3Kb/s is a practical limit. [6.5] What is CAP? Carrierless amplitude and phase (CAP) modulation is a proprietary standard implemented by Globespan Semiconductor. While the name specifies that the modulation is "carrierless" an actual carrier is imposed by the transmit band shaping filter through which the outbound symbols are filtered. Hence CAP is algorithmically identical to QAM. The upstream symbol rate is 136K baud on a 113.2KHz carrier, while the downstream symbol rate is 340K baud on a 435.5KHz carrier, 680K baud on a 631KHz carrier, or 952K baud on a 787.5KHz carrier. This allows the modem to be symbol rate adaptive to varying line conditions (see RADSL). The QAM modulation is also rate adaptive by varying the number of bits per symbol. One advantage CAP claims to have is a lower peak-to-average signal power ratio relative to DMT. This means that the drivers and receivers may operate at lower power than DMT because they are not required to have the peak signal capacity that is required in the DMT circuitry. This is mitigated by the infrequency of the really high signal peaks in DMT which may be just considered to be another form of noise if they happen to clip. CAP's principle advantage is its installed base of modems. It is actively being deployed in many trial markets and is available from several manufacturers. [6.6] What is DMT? Discrete multitone (DMT) modulation is a method by which the usable frequency range is separated into 256 frequency bands (or channels) of 4.3125KHz each. These are intimately connected to the FFT (fast Fourier transform) algorithm which DMT uses as its modulator and demodulator. The FFT is not perfect in separating the frequencies into individual bands, but it does well enough, and it generates spectra which are fully separable on the receiving end. By dividing the frequency spectrum into multiple channels DMT is thought to perform better in the presence of interference sources such as AM radio transmitters. It is also better able to focus its transmit power on those portions of the spectrum in which it is profitable to send data. The assignment of channels is less flexible, but typical settings might be channels 6-31 for upstream (24KHz-136KHz), 32-250 for downstream (136KHz-1.1MHz). The modulation used on any given frequency channel is QAM. Channels 16 and 64 are reserved for pilot tones which are used to recover timing. The number of bits per symbol within each channel may be independently selected allowing the modem to be rate adaptive. The use of the FFT is considered to be somewhat substandard to other orthogonal transformations such as the discrete wavelet transform which do a better job of isolating the individual frequency spectra. The FFT is chosen for its computational efficiency. While DMT is off to a slow start in the marketplace, it is expected to dominate for two reasons: it is thought to perform better for technical reasons and there is an ANSI standard behind it (not to mention Intel/Microsoft support). 7.0 Setup and Configuration [7.1] What hardware does my home computer need? Although it depends on your provider and the equipment they use, typically you will need a 10BASE-T adapter with which to connect to the external DSL device. Typically the customer DSL device is implemented as a bridge, router or both. Often your provider will give you an adapter that is to be installed in your PC in the form of a ISA/PCI card. Often these are actually ATM based adapters. If you have a laptop or any special requirements, you often need to purchase a separate router/bridge device in order to interface to your provider's DSL network. Some DSL CPE manufacturers will provide a plethora of connectivity options, including an external router or USB interface to an end user PC. [7.2] How does the DSL line encapsulate my data? Most often, the CPE portion of the xDSL equipment provides an ATM PVC between it and the provider network. Between the CPE and the the end user equipment the data link layer is most often a 10BASE-T connection. In this case, you will find RFC 2684 being used to define the standard mechanism for putting IP data over the ATM PVC. PPP over ATM or Classical IP over ATM can also be used. If the CPE device is not an external device, such as a PCI card installed in a PC, the framing is usually ATM end-to-end. Regardless, the datalink encapsulation type means little to the end user, although a 10BASE-T interface is probably much more flexible for most users. If you're wondering why ATM is so prevalent in xDSL networking it is simply due to the fact that the organizations providing xDSL service, telco's, have a large investment in ATM based backbone equipment. By leveraging this investment, ATM all the way to the edge of their networks (your home) makes sense. Yes, there is some overhead in packaging your IP datagrams into individual 53 byte cells, but it is probably not as significant as you think. In the end, the link layer technology is not what users should be generally concerned unless one is significantly cheaper and/or faster than another. [7.3] Can I use my 28.8K/56K modem with my xDSL line? Theoretically yes. However, most DSL providers have been installing separate DSL circuits to the remote user without using a splitter to separate out the voiceband bandwidth. If a splitter was used, you could use a traditional POTS modem over the the voiceband frequency spectrum of your phone line as you always did. In most cases however, the line is dedicated for DSL. [7.4] What's up with static versus dynamic IP addresses? Depending on the DSL provider, you may either be assigned one or more static IP addresses for your end hosts or you may be required to use the dynamic host configuration protocol (DHCP) to obtain a valid IP address while you are connected to the Internet. Static addresses are generally preferred by end users, because they make it easier to maintain always-on connections and host services (e.g. run a web server, game server or ftp server). Some providers who require the use of DHCP seem to do so in order to discourage hosting such always-on services. They do this by periodically changing your IP address through the DHCP mechanism. Of course, this also breaks any non- hosting session such as simply browsing the web. A temporary side benefit is that they may also have limited IP address space and anticipate that users will not maintain always on connections, thus saving IP address allocation requirements. Unfortunately, those providers who make static IP addresses available may do so at a premium price. [7.5] How do I share multiple hosts on my DSL line? To do so is largely dependent on how your DSL line is terminated to your CPE. If your provider gives you an Ethernet interface and multiple static IP addresses (many do), then you can simply use a 10BASE-T hub to connect a number of stations as you have IP addresses. After that, it gets more complex. If you have at least one valid IP address, you will need some type of gateway, proxy and/or network address translation (NAT) device. Matters are complicated further if your provider does not issue static IP addresses, which causes your gateway to require re-configuration each time the external IP address changes. [7.6] How do I secure my systems from Internet attacks? Inherently, you are no more at risk with xDSL service than any other connection to the Internet, whether it be dial-up, cable modem or otherwise. However, you are probably more likely to be attacked due to the amount of the capacity you may have or due to the fact that you're always connected to the net. First and foremost, you must ensure that your end host(s) are secure by applying the latest patches to the OS and services you run. It is also highly recommended that you disable any services that you do not really need (e.g. web server, ftp server, port 139 on Windows machines, and so on). Although it is beyond on the scope of this FAQ to describe the process of securing your host, it is of utmost importance to leave few doors and windows open into your systems. You can use firewall toolkits and filtering software to help control access to your systems, but understand that they are limited as a network solution to a host problem. Email trojans for example can bypass most firewalls. It is recommended that you perform a "scan" on your DSL connected host(s) to see what services are open to outsiders. Understanding what an attacker may see and securing those services is your best defense in the long run. [7.7] Can I have more than on xDSL line in my home? Yes, generally this is not a problem. The telephone company will provide as many lines, each on a separate copper pair into your house as you want. This may get expensive, but it has been done many times. [7.8] How do I tune my xDSL line for maximum performance? There is little you can do to really tune your DSL line. You're generally relying on the cabling plant installed in your house all the way through to the provider's network. Since many types of xDSL service can run over what is jokingly referred to as CAT-0 cable, your speed is typically determined at provisioning time. If you are rewiring your home, it of course does make sense to perform a high quality wiring installation. In some cases, you can tune your operating systems to achieve higher performance through software tweaking. Although most systems are generally very fast in their default installations, some parameters such as TCP window size may greatly affect overall performance. Two good resources for learning more about performance tuning are John Navas' Cable Modem/DSL Tuning Guide <http://Cable-DSL.home.att.net/> and the Pittsburg Supercomputing Center's Performance Tuning web page <http://www.psc.edu/networking/perf_tune.html>. [7.9] What differentiates one xDSL provider from another? It varies widely. Obviously service, cost, equipment and policies can vary widely from one provider to another. However, many DSL ISPs may use a common DSL cabling provider. Most of the providers need to interface with the traditional telcos, at least in the U.S. This means that the physical link is generally no different from one provider to another. However, the DSLAM, CPE and ISP network may vary greatly. You may have to really dig to see what differentiates one service from another. In addition, the quality of the ISP network infrastructure is a good thing to look it. A large ISP may have many high speed, redundant routes throughout the Internet for example. On the other hand, smaller ISPs may be able to provide more personalized service with options well suited for power users. [7.10] Does xDSL require a UPS in case of a power failure? Unlike POTS, xDSL service does not provide any voltage to keep your CPE device working in the case of a power failure. Although it may be feasible to provide some power to a laptop or similar device, there are currently no standards to do so. So yes, you will need to provide a UPS for both your CPE (if it is an external device) and your host(s). [7.11] I'm rewiring my home, what cabling do I use for xDSL? You do not need any special cabling. Although you can run xDSL service over high grade CAT-5 cabling, it is not necessary. However, it doesn't hurt either. 8.0 xDSL Resources [8.1] What web sites maintain xDSL information? 2wire <http://www.2wire.com> ADSL Forum <http://www.adsl.com> DSL Digest <http://www.dsldigest.com> DSL Experience <http://www.adslexperience.com> DSL Marketplace <http://www.dslmarketplace.com> DSL Prime <http://www.dslprime.com> DSL Reports <http://www.dslreports.com> everythingDSL <http://www.everythingDSL.com> John Navas' Cable Modem/DSL Tuning Guide <http://cable-dsl.home.att.net> Linux ADSL Mini-HOWTO <http://www.linuxdoc.org/HOWTO/mini/ADSL.html> OpenDSL <http://www.opendsl.org> Network World DSL Resources <http://www.nwfusion.com/dsl/> Randy Day's xDSL page <http://www.tuketu.com/dsl/xdsl.htm> Telechoice Inc. <http://www.xdsl.com> Universal ADSL Working Group <http://www.uawg.org> xDSL Resource <http://www.xdslresource.com> Older sites, information is relatively stale ADSL Deployment Worldwide <http://conk.com/world/dsl/> Avalon Trials <http://dsl.avalon.net> Dan Kegel's ADSL Page <http://alumni.caltech.edu/~dank/isdn/adsl.html> Jeremie Kass' Ameritech ADSL FAQ <http://www.rc.lsa.umich.edu/~jeremie/adsl/> [8.2] Are there any xDSL mailing lists? ADSL and related technologies for people in the UK Send an email to adsluk-subscribe@egroups.co.uk ISP DSL list Send an email to join-isp-dsl@lists.isp-lists.com Telechoice sponsored xDSL list Send an email to: <join-xdsl@lists.telechoice.com> [8.3] What Usenet newsgroups discuss xDSL? comp.dcom.xdsl comp.dcom.telecom.tech [8.4] Are there any books that cover xDSL? ADSL, Walter Goralski McGraw-Hill, ISBN: 0070246793 Adsl, Vdsl, and Multicarrier Modulation, John A. C. Bingham Wiley Series in Telecommunications and Signal Processing, ISBN: 0471290998 ADSL/VDSL Principles, Dennis Rauschmayer Macmillan, ISBN: 1578700159 Adsl: Standards, Implementation, and Architecture (Advanced and Emerging Communications Techniques), Charles K. Summers, CRC Pr, ISBN: 084939595X Analog Circuit Design: (X)Dsl and Other Communication Systems, W. Sansen, J. Huijsing, R. De Plassche, Kluwer Academic Pub., ISBN: 0792386221 Broadband Access Technologies: ADSL/VDSL, Cable Modems, Fiber, and LMDS Niel Ransom, Albert A. Azzam, McGraw-Hill, ISBN: 0071350608 Demystifying ATM/ADSL, Mike Busby Wordware, ISBN: 155622592X Digital Subscriber Lines: Toward, Above and Beyond ADSL, Walter Y. Chen Macmillan, ISBN: 1578700175 DSL: ADSL, RADSL, SDSL, HDSL, VDSL, Howard Hecht, John Freeman, Marlis Humphrey McGraw-Hill, ISBN: 0070277354 DSL: Simulation Techniques and Standards Development for Digital Subscriber Lines, Walter Y. Chen, Macmillan Technical Publishing, ISBN: 1578700175 DSL Bible, Mark Gray IDG Books Worldwide, ISBN: 0764547216 DSL For Dummies, David Angell DG Books Worldwide, ISBN: 0-7645-0475-4 Implementing ADSL, David Ginsburg Addison Wesley, ISBN: 0201657600 Introduction to ADSL, (CD-ROM computer based training) Hill Associates, Inc, ISBN: 0966409116 Practical Guide to DSL: High-speed Connections for Local Loop and Network, James Y. Bryce, CMP Books, ISBN: 1578200601 Remote Access Networks: PSTN, ISDN, ADSL, Internet and Wireless, Chander Dhawan McGraw-Hill, ISBN: 0-07-016774-5 Residential Broadband: An Insider's Guide to the Battle for the Last Mile, Kim Maxwell, John Wiley & Sons, ISBN: 0471251658 Residential Broadband Networks, Uyless Black Prentice Hall, ISBN: 0-13-956442-x The DSL Source Book, Paradyne Corporation available free from Paradyne: DSL-BOOK-1-0797 Understanding Telecommunications and Lightwave Systems, John G. Nellist IEEE Press, ISBN: 0-7803-1113-2 Understanding Digital Subscriber Line Technology, Thomas Starr, John M. Cioffi, Peter Silverman, Prentice Hall, ISBN: 0137805454 Video Dialtone Technology: Digital Video Over ADSL, HFC, FTTC and ATM Daniel Minoli, McGraw-Hill, ISBN: 0-07-042724-0 [8.5] What periodicals cover xDSL technology? Communications News <http://www.comnews.com> Data Communications <http://www.data.com> Electronic Engineering Times <http://techweb.cmp.com/eet/> IEEE Spectrum <http://www.institute.ieee.org> IEEE Communications Magazine <http://www.comsoc.org/pubs/commag/> Inter@ctive Week <http://www.zdnet.com/intweek/> internetTelephony <http://www.internettelephony.com> Network World <http://www.nwfusion.com> tele.com <http://www.teledotcom.com> [8.6] Are there industry conferences that cover xDSL technologies? Broadband Access ComForum <http://www.iec.org> DSLcon <http://www.dslcon.com> Networld+InterOp <http://www.interop.com> Comnet <http://comnetexpo.com> SuperComm <http://www.supercomm2001.com> [8.7] What companies make xDSL products? 3Com <http://www.3com.com> AccessLan Communications <http://www.accesslan.com> ADC Telecommunications <http://www.adc.com> ADTRAN <http://www.adtran.com> AG Communications Systems <http://www.agcs.com> Atlantech <http://www.atl.co.uk> Alcatel Alsthom <http://www.alcatel.com> Amati Communications <http://www.amati.com> Analog Devices <http://www.analog.com> Applied Innovation <http://www.aiinet.com> Ascom <http://www.ascom.ch> Ariel <http://www.ariel.com> Ascend <http://www.ascend.com> AWARE <http://www.aware.com> Cayman Systems <http://www.cayman.com> Cisco Systems <http://www.cisco.com> Consultronics <http://www.consultronics.on.ca> Copper Moutain <http://www.coppermountain.com> Diamond Lane <http://www.dlcc.com> Digital Link <http://www.dl.com> ECI Telecomm <http://www.ecitele.com> Efficient Networks <http://www.efficient.com> elantec Semiconductor <http://www.elantec.com> Ericssvon <http://www.ericsson.com> FlowPoint <http://www.flowpoint.com> GlobeSpan Semiconductor <http://www.globespan.net> Harris Semiconductor <http://www.semi.harris.com> Hyundai <http://www.hei.co.kr> Integrated Telecom Express <http://www.intexinc.com> Italtel <http://www.italtel.it> Level One <http://www.level1.com> Lucent Technologies <http://www.lucent.com> Metalink <http://metalink.co.il> Midcom <http://www.midcom-inc.com> Motorola <http://www.mot.com> NEC <http://www.nec.com> NetSpeed <http://www.netspeed.com> Nokia <http://www.nokia.com> Orckit Communication <http://www.orckit.com> PairGain Technologies <http://www.pairgain.com> Paradyne <http://www.paradyne.com> Pliant Systems <http://www.pliantsystems.com> Promatory <http://www.promatory.com/> Pulsecom <http://www.pulse.com> Rockwell <http://www.rockwell.com> Schott <http://www.schottcorp.com> Siemens <http://www.siemens.ch> SourceCom <http://www.sourcecom.com> Sparnex <http://www.sparnex.com> Tadiran <http://www.tadiran.com> Telmax Communications <http://www.telmaxcom.com> Tut Systems <http://www.tutsys.com> Vertel <http://www.vertel.com> Virata <http://www.virata.com> WaiLAN <http://www.wailan.com> Westell <http://www.westell.com> [8.8] Who are the xDSL service providers? AccNet<http://www.accnetco.net> Ameritech <http://www.ameritech.com> Aspen Internet Exchange <http://www.aspn.net> BC Tel <http://www.bctel.net> Bell Atlantic <http://www.bellatlantic.com/adsl/> Bell Canada <http://www.bell.ca> BellSouth <http://www.bellsouth.net> Brainstorm <http://www.brainstorm.net> Concentric Network <http://www.concentric.net> Covad Communications <http://www.covad.com> Dakota Services Limited <http://www.dslnet.com> DSL.com <http://www.dsl.com> DNAI <http://www.dnai.net> GTE <http://www.gte.com> HarvardNet <http://www.harvardnet.com> InterAccess <http://www.interaccess.com> InternetCDS <http://www.internetcds.com> MegsInet <http://www.megsinet.net> MM Intenret <http://www.mminternet.com> NETinc <http://www.netinc.ca> NorthPoint Communications <http://www.northpointcom.com> OneNet Communications <http://www.one.net> Optimum Communications <http://www.occs.net> Pacific Bell <http://www.pacbell.com> Rhythms <http://www.rhythms.net> SaskTel <http://www.sasktel.com> Southwestern Bell <http://www.swbell.com> Telus PLAnet <http://www.telusplanet.net> Transport Logic <http://www.transport.com> TransBay.Net <http://www.transbay.net> U S West <http://www.uswest.com> UUNet <http://www.uu.net> Verio <http://www.verio.net> VistaNet <http://www.vsta.com> Vitts Networks <http://www.vitts.com> Web Wave <http://www.webwave.net> Winfire <http://www.winfire.com> WWISP <http://www.fast-net.net> [Appendix A] Acronym List ADSL - Asymmetric Digital Subscriber Line ANSI - American National Standards Institute ATM - Asynchronous Transfer Mode ATU-C - ADSL Termination Unit - Central Office ATU-R - ADSL Termination Unit - Remote AWG - American Wire Gauge BERT - Bit Error Rate Test bps - Bits Per Second BRI - Basic Rate Interface CAP - Carrierless Amplitude and Phase CATV - Cable TV CBR - Constant Bit Rate CCITT - Consultative Committee for International Telegraph and Telephone CLEC - Competitive Local Exchange Carrier CO - Central Office CODEC - Coder/Decoder CPE - Customer Premise (or Provided) Equipment CSU - Channel Service Unit DCE - Data Communication (or Circuit-Terminating) Equipment DHCP - Dynamic Host Configuration Protocol DLC - Digital Loop Carrier DMT - Discrete Multi-tone DSL - Digital Subscriber Line DSLAM - Digital Subscriber Line Access Multiplexer DSP - Digital Signal Processor DSU - Data Service Unit DTE - Data Terminal (or Termination) Equipment EMI - Electromagnetic Induction ETSI - European Telecommunications Standards Institute FCC - Federal Communications Commission FDM - Frequency Division Multiplexing FEXT - Far-end crosstalk FTTC - Fiber To The Curb FTTH - Fiber To The Home HDSL - High bit-rate Digital Subscriber Line HFC - Hybrid Fiber-Coax IEC - Inter-Exchange Carrier IEEE - Institute of Electrical and Electronics Engineers IETF - Internet Engineering Task Force ILEC - Incumbent Local Exchange Carrier IP - Internet Protocol ISDL - ISDN Digital Subscriber Line ISDN - Intergrated Services Digital Network ISO - International Organization for Standards ISP - Internet Service Provider ITU - International Telecommunications Union IXC - Inter-exchange Carrier Kb/s - Kilobits Per Second LADC - Local Area Data Circuit LADS - Local Area Data Service LAN - Local Area Network LATA - Local Access and Transport Area LEC - Local Exchange Carrier Mb/s - Megabits Per Second MDF - Main Distribution Frame MUX - Multiplexer MVL - Multiple Virtual Line NAP - Network Access Provider NAT - Network Address Translation NEBS - Network Equipment Building Standards NEXT - Near-end Crosstalk NIC - Network Interface Card NID - Network Interface Device OPTIS - Overlapped Phase Trellis-coded Interlocking Spectrum PBX - Public Branch Exchange PCM - Pulse Code Modulation POP - Point of Presence POTS - Plain Old Telephone Service PPP - Point to Point Protocol PRI - Primary Rate Interface PSTN - Public Switched Telephone Network PTT - Postal, Telegraph and Telephone PVC - Permanant Virtual Circuit QAM - Quadrature Amplitude Modulation QoS - Quality of Service RADSL - Rate Adaptive Digital Subscriber Line RBOC - Regional Bell Operating Company SDSL - Symmetric Digital Subscriber Line SNR - Signal-to-Noise Ratio SOHO - Small Office/Home Office SVC - Switched Virtual Circuit TCP - Transport Control Protocol TELCO - Telephone Company TDM - Time Division Multiplexing UBR - Unspecified Bit Rate UDSL - Unidirectional Digital Subscriber Line UTP - Unshielded Twisted Pair VBR - Variable Bit Rate VDSL - Very high bit-rate Digital Subscriber Line VoIP - Voice over Internet Protocol VPN - Virtual Private Network WAN - Wide Area Network xDSL - (generic) Digital Subscriber Line --END of comp.dcom.xdsl FAQ--