This is the Frequently Asked Questions (FAQ) section for
ADSL.
Q: Are there ADSL Standards.
A: Sort of. The U.S. Bell Operating Companies have standardized on
Discrete Multi-Tone (DMT) ANTs (ANSI T1.413) in their current rollout.
Most others should follow their lead in the states. There
are other types of ANTs, most notably Carrier-less Amplitude
Phase Modulation (CAP), which, of course, are incompatible
with each other.
A biased comparison from an DMT-based vendor on this subject
can be found at the
Aware.
Still, it provides the best detail on this issue I have seen so far.
A rather expensive copy of the ANSI standard can be ordered at:
American National Standards Institute
ANSI Home Page
Asymmetric Digital Subscriber Line (ADSL) Metallic Interface
ANSI TI.413-1995
Note: ANSI TI.413 Issue 2 was released September 26, 2025
Q: Can I use ATM to connect to ADSL ANT?
A: Yes, you can! Some ADSL ANT (at least the Alcatel
version) has a ATM Forum 25Mbps interface,
which connects to a PCI NIC card. However, I have not
yet heard of any Linux drivers for such cards.
Q: Why the heck does ADSL have all these bit rates (384/1.5/8M/20M/etc) options?
A: The basic problem is the 100 year old design of the copper loop.
It works great for analog phone, but it presents a real challenge
for a digital signal. Remember that the distance of
a loop is inversely proportional
to the data rate that it can carry. Rate-Adaptive technologies are great for
making a digital signal work in many situations, but it can't provide
a consistent bandwidth for all applications, especially for very long
(over 18 kilofeet) loops. The different bandwidth that you see
advertised reflect various marketing wars of vendors equipment,
and the Telco struggle to finalize on a ''standard'' set of
data rates. I think that the 384k/1.5Mbps will become the standard
for now. The high bit rates will only be available for special application
and/or situations, since they can only be provided on a small percentage
of the available loops.
Also, check out the next question on the loop imparments that cause
this to happen.
Q: What are all these loop imparments (bridge taps, loading coils)
that could disqualify my line from using ADSL? (thanks to Bruce Ediger)
Load coils: in-line inductances that improve voice-frequency transmission
characteristics of a telephone circuit. Essentially, a "load" steals
energy from high frequencies and gives it to lower frequencies. Typically
only used in very long (>9,000 ft) phone lines.
By "bridges" I assume you mean "bridged taps". In older neighborhoods,
the phone wiring will have been used by more than one customer. Perhaps
these customers lived at different (though near-by) addresses. The unconnected
"spur" of wiring is a "bridged tab" on the currently connected circuit.
Digital loop carriers: there's a bunch of systems for carrying more than
one voice transmission on a single pair of wires. You can shift the
frequencies up or down, or you can digitize the voice transmissions and
divide the telephone circuit by time or code or something. The more
general term is "pair gain".
These things cause different problems for high-frequency communication.
Loads will completely mess up things by filtering high frequencies and
passing low frequencies. They probably also change the "delay envelope",
allowing some frequencies to arrive before others. One byte's tones will
interfere with the next byte's.
Bridged taps act as shunt capacitances if they're long in relation to the
signals wavelength, and they'll actually act as band pass filters if
they're about 1/4 wavelength of the signal. That is, they'll pass
particular frequencies freely. Particular tones of a DMT modem might
get shunted back, rather than passed along to the receiving modem,
reducing bandwidth for that telephone line.
Pair gain, digital or analog, limit the bandwidth available to one transmission
in order to multiplex several on one wire. High and low tones of a DMT
transmission get filtered out by the apparatus.
The book "Subscriber Loop Signaling and Transmission Handbook", by Whitham
D. Reeve, , IEEE Press 1992, ISBN 0-87942-274-2 covers the math of how to
calculate the effect of line length, bridged tap, etc on the transmission
characteristics of a telephone line. It's pretty expensive, however.
Q: Do you have examples of ADSL ANTs?
A: Short Answer: Yes. Real Answer: The evolution of this technology
is moving too rapidly for anyone to keep up to date in a HOWTO.
A good source of ADSL ANTs is the
ADSL Forum Home Page .
Go to the Vendors pages to see what's happening.
However, I will provide a list of some of the current
technology as of June 1998.
ADSL Forum Home Page
A comprehensive web site created by the adsl vendors. Fairly
complete for reference information on ADSL.
Dan Kegels ADSL Page
A good general reference on xDSL - includes vendor, service provider,
and other links. This page was getting a little long in the tooth as
of 2Q98. Dan also maintains a super page on ISDN.
PacBell's ADSL Page
Pacific Bell is the local Telco and my provider of ADSL service.
Jeremie's Unofficial Ameritech ADSL FAQ
Telechoice xDSL News Page
ADSL Deployment 'round the World
Claims to have a complete list - looked accurate for my area - gives providers, prices, speeds,
etc.
Thanks to all those that contributed information to this HOWTO.
I have anti-spammed their email addresses for their safety (and mine!).
Remove the X's from their names.
B Ediger (Xbediger@csn.net) Great Description of loop impairment.
C Wiesner ( Xcraig@wkmn.com) List of many ADSL URLs.
J Leeuw ( Xjacco2@dds.nl) Many tips on ADSL, especially in Europe
A dictionary of some of the jargon I use in this FAQ.
2 wire Copper Loop
The two wire twisted pair from the Telco Central
Office that terminates at a customer location.
ADSL
Asymmetric Digital Subscriber Line
ANT
ADSL Network Termination (a.k.a. the ADSL modem)
ATM
Asynchronous Transfer Mode - provides high-speed packet switching
from 155 Mbps to (currently) 2Gbps. Used to provide backbone switching
for the Internet.
ATMF-25Mbps
ATM Forum Interface - 25Mbps speed, provided
by a PCI NIC card.. One of the interfaces used between the ANT and PC.
Central Office
Usually refers to one of two meanings -1) The
Telco Building that houses Telephone equipment 2) The Telco
Voice Switch that provides dial tone.
CPE
Customer Premises Equipment - The Telco term for
customer equipment (i.e. the stuff you are responsible for fixing).
Examples are CSU/DSU, modems, ANTs, and your phone.
DHCP
Dynamic Host Configuration Protocol - The IP protocol
used to set up dynamically assigned IP addresses.
DS0
The basic digital circuit for Telcos - offered at
56 kbps or 64kbps. Can support one analog voice channel.
DSLAM
Digital Subscriber Line Access Multiplexer -
The Telco equipment that concentrates and multiplexes the
DSL lines.
xDSL
Digital Subscriber Line - A term describing a family of
DSL services, including ADSL, SDSL, VDSL, etc.
HDC
See Section 2
ISDN
Innovations Subscribers Don't Need; I Still Don't kNow
or maybe Integrated Services Digital Network, a digital phone
service that uses a single copper pair to run 2B (64k) + 1D(16k) channels
that can be used for switched voice or data.
ISP
Internet Service Provider
NID
Network Interface Device - The housing used to
protect the ADSL splitter from the elements.
NIC
Network Interface Card - A PC card (PCI/ISA) that
supports the required network interface. Usually an Ethernet
10baseT or an ATMF-25Mbps Card..
POTS
Plain Old Telephone Service - The service that
provides a single analog voice line. (i.e. your phone line)
Recursion
See "Recursion"
SNI
Subscriber Network Interface - The Telco term for the
phone wiring housing on the side of your house. It designates
the point between the Telco side and the Inside Wire.
This is also called the Demarcation Point.
Splitter
The passive device (low-bandpass filter) at the SNI that
splits the ADSL signal into separate voice and data channels.
Splitterless
An ADSL installation that does not require
the Splitter. For higher speeds, a RJ11 filter is placed on every
extension phone jack where an analog phone is used, thus
providing the filtering at the jack, rather than at the
NID. For lower speeds, no filter is required.
SOHO
Small Office HOme
T1
a.k.a DS1 - A digital dedicated line at 1.544 Mbps, used
for both Voice (24 DS0s) or Data.
T3
a.k.a DS3 - A digital dedicated line at 44.736 Mbps, provides
for both Voice (672 DS0s or 28 DS1s) or Data
