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Digital Subscriber Line (xDSL) FAQ v20010108


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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

See reader questions & answers on this topic! - Help others by sharing your knowledge
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--

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Last Update March 27 2014 @ 02:11 PM