ADSL HOWTO for Linux Systems

ADSL HOWTO for Linux Systems
David Fannin, [email protected]
v0.92, 10 April 1999

How to setup a Linux System to work with Asymmetric Digital Subscriber
Loop (ADSL), a new high-speed digital access line technology available
from Telcos. ADSL is one of a family of digital subscriber line (DSL)
technologies available to residential and business customers using
existing copper loops, providing speeds from 384kbps to 1.5Mbps. This
document provides an introduction to ADSL, and information on how to
install, configuration and turn up service.
______________________________________________________________________

Table of Contents

1. Legal Stuff

2. Introduction

3. ADSL Overview

3.1 What is ADSL?
3.2 ADSL Applications
3.3 What is xDSL/DSL?
3.4 Why so many speeds?

4. How it works.

4.1 CPE: ADSL ANT and NIC
4.2 Splitter or Splitterless Design
4.3 DSLAM
4.4 ISP connection

5. Ordering Service

5.1 Home Requirements
5.2 Telco Options
5.3 ISP Options

6. Wiring it up

6.1 Wire the Splitter/NID (at the SNI)
6.2 Wire the DSL Jack (at the computer location)
6.3 Install the ANT (at the computer location)

7. Configuring Linux

7.1 Install and Connect the NIC card
7.2 Configure the Ethernet Interface
7.3 Setting up a Router
7.4 Setting up a Firewall/Masquerading

8. Appendix

8.1 FAQs
8.2 Links
8.3 Credits
8.4 Glossary

______________________________________________________________________

1. Legal Stuff

ADSL HOWTO for Linux Systems

Copyright (C)1998,1999 David Fannin.

This document is free; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your
option) any later version.

This document is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.

You can get a copy of the GNU GPL at at
<http://www.gnu.org/copyleft/gpl.html>.

2. Introduction

This document addresses the ordering, installation and configuration
of ADSL service for use on Linux Systems.

ADSL or Asymmetric Digital Subscriber Loop is a high-speed Internet
access technology that uses a commonly available telephone copper loop
(same wire as your phone service). Designed to run on up to 80% of
the telephone available in the United States, and utilizing line-
adaptive modulation, ADSL provides data speeds from 384kbps to 1.5
Mbps, normally using different speeds for the upstream and downstream
channel (hence the " Asymmetric" ). ADSL provides a direct, dedicated
pipe to an ISP.

ADSL was designed for SOHO applications, such as a small business with
5-30 systems, or the high-end Linux user that has wants bandwidth and
has some money to spend. Applications range from low-bandwidth
server interconnection to streaming video applications. ADSL is
designed and priced somewhere in between ISDN and T1 service,
providing near T1 speeds without the cost/complexity/availability
issues of T1. Since ADSL is a dedicated service, it avoids the delays
and use charges inherent in ISDN service.

This HOW-TO starts with a description of ADSL services and it many
variants, and a block diagram of the components that make up the
service. Next, a list of requirements for ordering ADSL are provided.
Also included is guide on ADSL wiring and configuring your Linux
System. I have also included an Appendix with a FAQ, a listing of
interesting Links,and a Glossary.

I welcome any and all comments on this document; They can be sent to
[email protected] .

The latest version of this FAQ is available from
<http://www.sushisoft.com/adsl/>.

3. ADSL Overview

3.1. What is ADSL?

Asymmetric Digital Subscriber Line (ADSL) is:

� A Telephone Loop Technology that uses existing phone lines

� Provides high-speed data and analog voice (Data over Voice)

� Dedicated digital line for an IP connection

� Data rates (North America) combinations of :
Upstream/downstream
256 kbps/256 kbps
384 kbps/128 kbps
384 kbps/384 kbps
384 kbps/1.5 Mbps
and many others

� Wide range of CPE options, including Ethernet 10baseT Interfaces.

� Dedicated ISP connection (static or dynamic addresses)

� Can support an IP subnet (from 1 to 254 IP addresses, depending on
ISP)

� Priced lower than dedicated private line (T1) connections

3.2. ADSL Applications

ADSL was designed to provide a dedicated, high-speed data connection
for Internet/Intranet Access, using existing copper phone lines. This
allows ADSL to work on over 60-80% of the phone lines existing in the
U.S. without modification. Additionally, ADSL provides speeds
approaching T1 (1.5Mbps), which are much greater than analog modems
(56kbps) or ISDN (128kbps) services provided over the same type of
line. ADSL is usually priced to be much less other dedicated digital
services, and is expected to priced somewhere between T1 and ISDN
services (including the ISDN usage charges).

The Telcos see ADSL as a competitive offering to the Cable Company's
Cable Modems, and as such, are expected to provide competitive
pricing/configuration offerings. Although Cable Modems are
advertised as having 10-30Mbps bandwidth, they use a shared
transmission medium with many other users on the same line, and
therefore performance varies, perhaps greatly, with the amount of
traffic and other users.

ADSL is positioned for Home and Small Office (SOHO) applications that
require high-speed Internet Access. Since it also provides dedicated
access, It can be used for interconnecting low-bandwidth servers to
the Internet, and would provide a great access solution for 5-20 PCs
in an Office location. It is also a great solution for those Linux
power users that just want high speed access from home:-).

3.3. What is xDSL/DSL?

Digital Subscriber Line (DSL) provides a dedicated digital circuit
from your home to the Telcos central office, using analog telephone
line. DSL also provides a separate channel for voice phone
conversations, which means analog calls (voice, fax. etc.) can be
carried at the same time high-speed data is flowing across the line.
DSL uses the frequency spectrum between 0kHz-4kHz for Analog Voice,
and 4kHz-2.2MHz for data. xDSL is a generic acronym for a family of
dedicated services, where the " x"stands for:

� ADSL Asymmetric Digital Subscriber Line: 1.5
Mbps-384kbps/384-128kbps

� HDSL High-bit-rate Digital Subscriber Line: 1.5 Mbps/1.5 Mbps
(4Wire)

� SDSL Single-line Digital Subscriber Line: 1.5 Mbps/1.5 Mbps
(2Wire)

� VDSL Very high Digital Subscriber Line: 13 Mbps-52 Mbps/1.5
Mbps- 2.3 Mbps.

� IDSL ISDN Digital Subscriber Line: 128 Kbps/128 Kbps.

� RADSL Rate Adaptive Digital Subscriber Line: 384kbps/128kbps

� UDSL Universal Digital Subscriber Line:
1.0Mbps-384kbps/384kbps-128kbps
also called " splitterless" DSL or DSL-Lite, as it doesn't require
a splitter.

where Xbps/Ybps is X=Downstream Bit rate, Y=Upstream Bit rate

3.4. Why so many speeds?

ADSL has to work over existing phone lines, which were designed 100
years ago, and were never designed for digital services (See the FAQ
answers for more information). Also, ADSL is a new service, and all
the providers are trying to find the right price/feature combinations
that will make it in the market.

For the average user, the basic way of thinking about it is to segment
the options into three categories:

� Low End Residential

Speed ranges from 384kbps-128kbps, Asymmetric

� High End Residential or Business End User

Speed ranges from 1.5Mbps-384kbps, Asymmetric

� High End Server

Speed ranges from +2.0Mbps-1.1kbps, Symmetric

4. How it works.

ADSL is made up of several parts (shown by Figures 1 and 2):

� ADSL Network Termination (ANT) and Network Interface Card (NIC)

� Splitter or Splitterless Design

� DSLAM and Telco Loop

� ISP connection

Figure 1: ADSL Block Diagram (POTS Splitter)

<-------Home/Office---------------> <-----Telco Central Office---->
NID
----- -----
2 wire X-----------Voice-=| S | | D |
phone | P | | S |=----- Voice Switch
line | L | 2 wire | L |
| I |=-------------=| A |
| T | Local Loop | M |=----- ISP Connection
10baseT ---------- Data | T | | |
Ethernet X--=| |=----=| E | -----
or ---------- | R |
ATMF ADSL -----
NIC ANT

4.1. CPE: ADSL ANT and NIC

The Customer Premises Equipment (CPE) for ADSL consists of the ANT
and/or NIC card. The ADSL Network Termination (ANT), shown in Figure
1, is located at your home or office, provides an IP connection. ANTs
come in several types:

� Router ANT with 10/100baseT Interface

� Bridge ANT with 10/100baseT Interface

� ANT with ATMF Interface

� ANT with USB Interface

� Integrated ANT/NIC Card

In each case, the ANT/NIC provides the a router address to an ISP.
Each Telco will specify the configurations that they will allow. The
most desirable configuration for the Linux user is the ANT with a
10baseT Interface, since the cost/setup is the easiest. The other
options require special drivers, which have, to date, not been made
available for Linux. The bad news is that some providers allow only
integrated ANT/NIC PCI cards that do not have Linux Drivers.

Warning! Make sure any third party ANT/NIC you may purchase are
compatible with your Telco provider. There are two major line
encodings for ADSL (CAP, DMT), and several options for IP
encapsulation. Your Telco should provide you a list of allowable
options.

The ANT is connected to your house's inside wire (2 wire phone line).
This inside wire is connected to the data side of a POTS splitter, or,
in the case of the splitterless version, directly connected to the
local loop. Figure 1 shows the POTS splitter wiring, and Figure 2
shows a splitterless type.

In my case, I was provided with an Alcatel ANT, which supports a
10baseT (wired as a Crossover) RJ45 jack. I understand that a NIC card
that fits directly in the PC will be available sometime in the future.

Figure 2: ADSL Block Diagram (Splitterless Design)

<-------Home/Office---------------> <-----Telco Central Office---->
SNI
- -----
2 wire X-[RJ11]---Voice----| | D |
phone Filter | | S |=----- Voice Switch
line | 2 wire | L |
|=-------------=| A |
| Local Loop | M |=----- ISP Connection
10baseT ---------- Data | | |
Ethernet X--=| |=-----| -----
or ----------
ATMF ADSL
NIC ANT

4.2. Splitter or Splitterless Design

Somehow, the digital and analog signals need to be separated for all
of this to work. Thus, a filter needs to be placed in the signal path
at some point. There are two methods for doing this: Using a POTS
Splitter or using RJ11 phone jack filters.

First, in the POTS splitter method, device is located on the "side of
the house" where the Telco line is connected. The splitter provides
two functions. First, it is the "demarcation point" that separates the
Telco wiring from the inside wiring. Second, it "splits" the DSL
signal from Telco into a separated data channel and a voice channel.
The voice channel is a normal analog phone line (2 wire), and the data
channel is sent to the ANT. The splitter is a passive, non-powered
device, which will allow the voice channel to operate even if the
power fails at the home location. The Telco signal is sent to the
splitter using an existing 2 wire line to the home. The Splitter is
housed in the Network Interface Device (NID) on the outside of your
house.

Second, in the splitterless design, shown in Figure 2, the outside
local loop is connected directly to the inside wire at the Subscriber
Network Interface (SNI), the same box that is used today at your
house. At each extension jack where you wish to plug in an analog
phone, you place a special jack that contains a filter that removes
the digital signal. This is called an RJ11 filter (RJ11 is the
official Telco term used for your 4/6 pin phone jack). The extension
used for your ANT does NOT use a filter (otherwise it won't work).
That's all there is to it! It should also be noted that some low
speed ADSLs will not require RJ-11 filters.

The splitterless design is very desirable from the Telco point of
view, as they won't have to roll any trucks to do the install work,
and allows them to offer ADSL at a lower price. For most users, it
doesn't really matter, in fact, the analog phones will still work
without the RJ11 filter in place. The only thing is that you will hear
a bit of a high pitched whine when you use the phone. However, this
is not recommended, as later version may damage the phone or have some
other nasty effect.

4.3. DSLAM

The DSLAM is the equipment at the Telco Central Office that that
splits the signal and connects the user to the Voice Switch and ISP.
That's all you need to know from a user perspective.

4.4. ISP connection

An ISP connects to the DSLAM via a high-speed data connection, usually
ATM over a T3 (45Mbps) or OC-3 (155Mbps). The important thing here is
that an ISP must "subscribe" with your Telco to provide this
connection.

5. Ordering Service

The basic procedure is as follows:

� Check to see if you meet the following requirements for ADSL.

� Select a Telco and ISP, and see what configurations they offer.

� Call the Telco and order the service.

Once you have ordered the service, the Telco should provide you with a
Due Date. I was able to do the entire order in one phone call through
my Telco, and they coordinated with the order with my ISP. I was
given a due date that was one week later, and everything was completed
on time, with no problems.

5.1. Home Requirements

� An appropriate NIC card

The system (PC, workstation, router or hub) that will be directly
connected to the ADSL ANT .

� Inside Wiring

You need inside wire (2 or 4 wire) from the Telco Demarcation Point
(the side of your house) to your PC location. Your existing phone
line can be used. However, this inside wire can only be used for
the ANT, and no other phones or extensions can be connected to this
line. The Telcos or other companies can be contracted to install a
new wire for you as well.

� Installation

Who will wire it up in your house? You can do the splitter wiring
yourself (My local Telco provided a wiring kit and instructions for
performing the inside wiring ) or you can contract them do it for
an additional fee. You can save around $ 150-250 dollars by doing
it yourself, which I did, or you can save yourself the hassle and
have them do it. You can decide what you want to do after reading
the procedure that I have included in this document. The bottom
line is if you are used to dealing with phone and/or network LAN
wiring procedures, and feel comfortable with doing this, then you
should consider going for it. If you haven't, and your company is
going to pay for it, then have them do it.

5.2. Telco Options

� Qualified Copper Loop

The Telco will normally test your line before you order service to
see if it is can pass the ADSL signal. You must be within 2-3
miles (11-16 kilofeet) of the Central Office, and have a loop that
has no loading coils, bridge taps, DAMLs, or other impediments.
Most of the Loops that support ISDN can also support ADSL, but
since ADSL is more restrictive, this is no guarantee of success.
Your Telco will help you here.

� ADSL Available from your Telco

Is the ADSL/DSLAM equipment is available at your Central Office?
Again, the Telco will check to see if it available in your area.
Every major Telco in the US, and many new dedicated ADSL providers
have announced plans for ADSL. I expect wide scale deployment by
1Q99.

� Bit Rate

ADSL is priced according to the Bit Rate - Check with your Telco.
You can usually order some combination of 128k,256k,384k, or 1.5M
speeds. Your loop may restrict the bit rate you can be offered.
My Telco also prices the service differently if you're a Residence
or Business Customer.

� POTS Service

Some providers (usually the big Telcos) offer POTS with ASDL. If
you choose this option, you will need to specify which phone number
(new or existing) you want assigned to it. You will need to
designation whether you want the ADSL to go over an existing POTS
service, or whether you want a new phone number for the analog
phone line. If you have ISDN service, and want to use the same
line, the ISDN service must first be disconnected. Note that some
Telcos (usually dedicated DSL providers) do not offer this option.

5.3. ISP Options

� ISP Presubscription

Your ISP of choice must be presubscribed with the ADSL Telco
provider. Check with your ISP to see if they are connected. If
they are not connected then you must either move to another ISP,
convince them to connect up, or decide not to use ADSL.

� IP Subnet and Address.

First, you'll need to know whether your IP addressing will be
static or dynamic. Dynamic addressing is set through the use of
DHCP.

Additionally, for static addressing, ISPs will allow anywhere from
1 to 254 IP addresses to be assigned to an ADSL line, with the
corresponding Variable Length Subnet Mask (VLSM).

My ISP charges more money as you get more hosts. I suspect that
most applications will use either 1 host (32 bit mask) or 6 hosts
(29 bit mask). If your confused, then read the IP-Subnetting
HOWTO. Also remember than multi-host subnets require 1 of the
hosts to be the ISP router address (meaning that the 6 host subnet
is really 5 hosts to you) and you can use IP Masquerading/ (see the
HOWTO) or Network Address Translation (NAT) to support a larger
number of addresses behind your LAN. If you request more than 30
hosts, be prepared to fill out a justification form for your ISP.

� Gateway Address

The default gateway address. Normally, this is the first host
address in your subnet. (e.g. if your subnet range and mask is
192.168.1.240/29, then your host range is .241 through .246, with
the default gateway assigned to the .241 address.

� Number of MAC addresses

The ANTs can support a set number of MAC addresses for bridging.
For example, the ADSL ANT that I use is limited to 16 MAC
addresses. This should be sufficient for most applications.
However, if you plan to support a large number of machines directly
off the hub of the ANT, you should check first to see what your
limit is.

� DNS Servers

The ISP should provide you with one or more DNS server addresses to
support Domain Name Lookups. Some ISPs also provide Domain Name
Server hosting (either primary or secondary server) for you as part
of the dedicated service packages. See your ISP for more details.

� User ID and Password.

The ISP will normally assign you a login/password for shell, mail,
etc. access. My ISP also provides web space, mail server, and some
other goodies.

6. Wiring it up

If you have ordered the installation option from the Telco, then you
can skip this section and move to " Configuring Linux" . The
following procedures are meant to illustrate the wiring process.
Please note that your procedures may be different at your location.
Make sure you follow any warnings or safety instructions provided, and
that you are familiar with Telco wiring procedures.

The first step will be to wire up the connections from your Telco.
Identify the line on which service will be installed, and the
locations of your splitter and DSL jacks.

6.1. Wire the Splitter/NID (at the SNI)

If you have the splitterless design, you can skip this part.

The splitter will consist of two parts, the splitter and a small
outdoor housing, called the Network Interface Device. Mount the
splitter and NID housing per the Telcos instructions at the Subscriber
Network Interface (SNI) point, usually the side of your house where
the phone line is located. The phone company will need to access the
splitter for maintenance, so its advisable to locate it on the outside
where they can get at it. Connect the incoming 2W Telco line to the "
LINE"side of the splitter. Then wire the inside pair for your
telephone to the "VOICE" , and your inside wire pair for the ANT to
"DATA".

Checkstep At this point, you should be able to pull dial tone off the
voice side of the splitter. If this doesn't work, then either you've
wired it wrong, or the ADSL service is not yet connected on the Telco
side.

6.2. Wire the DSL Jack (at the computer location)

Wire up the DSL jack (RJ11) at your computer location (connected to
the DATA side of the splitter). The specifics differ greatly for each
situation, but basically you will have a 2 wire pair that you will
connect to the DSL jack. Make sure you read the directions, as the
DSL-RJ11 wiring is different for phones and ADSL jacks. In my case,
you wire the phone jack on the red/green pair (the two inside prongs
of the RJ11 jack) and the ADSL jack is wired on the yellow/black pair
(the two outside prongs of the RJ11 jack).

6.3. Install the ANT (at the computer location)

Connect up the ANT's power cord, and connect the phone line (category
3 cable) between the DSL jack and the ANT. This cable is usually
provided.

Checkstep At this point, verify the ANT syncs up to the Telco signal.
Most ANTs have an green LED that lights up when the signal is good.
If it doesn't sync, then check your wiring, or make sure that the
Telco signal is being sent (do this by calling your Telco and
verifying they have activated the service. Note that having dial tone
on the line does NOT confirm the presence of the ADSL data signal.)

If you have completed the previous steps, you are now ready to move on
to connecting your Linux System.

7. Configuring Linux

After you have wired up the ANT and you're getting the sync signal,
then you're ready to configure your Linux System and verify your
connection to your ISP. Although I will refer to a Linux System, you
can connect any type of 10baseT device to the ANT. This includes a
router, hub, PC, or any other system that you wish to use.

Caution! Before you connect to your ISP, make sure you understand all
security issues of having a direct connection to the Internet via
ADSL. Depending on your ISP, most outside uses can access your
systems, and you should setup any firewalls, deactivate
ports/services, and setup any passwords prior to connecting your
machine to the world. Read the Security-HOWTO if you need a good
overview on this subject.

7.1. Install and Connect the NIC card

Install your NIC card in your Linux machine, configure the kernel,
etc., etc. See the various Linux references for doing this. See the
Ethernet-HOWTO for more information.

Connect the RJ45 cable between the NIC and the ANT. Note A gotcha here
is that some ANTs are already wired as a 10baseT crossover, and
require a direct Category 5 cable for a direct connection to a NIC,
rather than a crossover cable. I lost around 12 hours figuring this
one out, so don't make the same mistake - make sure you read the
instructions first.

7.2. Configure the Ethernet Interface

Configure the IP address, Subnet Mask, Default Gateway, and DNS server
information. Each Linux Distribution (RH, debian, Slackware,
S.U.S.E.) has a different way of doing this, so check on your
particulars. You can also do this manually using the ifconfig and
route commands. See the NET3-HOWTO for information for more
information.

Once your system is configured, see if you can ping to the your
default gateway address provided by the ISP. If the ping is
successful, the you should see around 20 ms roundtrip delay for this
connection. Congratulations, you're connected to the Net!

7.3. Setting up a Router

Depending on your local setup, you should consider some other issues.
These include a firewall setup, and any associated configurations.
For my setup, shown in Figure 3, I use an old i486 machine configured
as a firewall/router between the ADSL connection and the rest of my
machines. I use private IP addresses on my Private LAN subnet, and
have configured my router to provide IP Masquerading and Firewalling
between the LAN and Internet connection. See the IP_Masquerading-
HOWTO, and Firewall-HOWTO for more information. My experience is that
Linux provides superior routing/firewalling performance , and is much
cheap than a commercial router, if you find an old 386/486 machine
that you may be using as a doorstop somewhere.

Figure 3: My SOHO Network Setup

<-Private Subnet--> <-Public Subnet-> <-ADSL Line--------->
|
X----|
|
X------| X----| |----|
| |--------| | |ADSL| Internet
| | Linux | |-----|ANT |----------> Service
X------|------| System |--------| | | Provider
| E1|(Router)|E0 | |----| Router
| |--------| |
X------| IP_Masq 10baseT
IP_Firewall Hub

What I did is setup a router (Linux RH 5.0 on a i486) with two
Ethernet interfaces. One interface routes to the ISP subnet/gateway,
and the other interface supports a class private network address
(i.e. 192.168.2.x). Using the private network address behind your
router allows some additional security because it is not directly
addressable from outside your ISP. You have to explicitly masquerade
your private addresses in order to connect to the Internet.

Caution Make sure your kernel is complied with IP forwarding and the
IP forwarding is turned on. You can check this by

cat /proc/sys/net/ipv4/ip_forward

The value is "1" for on, and "0" for off. You can change this value
by echoing the desired value into this file.

(e.g.) echo 1 > /proc/sys/net/ipv4/ip_forward

Will turn forwarding on.

7.4. Setting up a Firewall/Masquerading

If you have a direct connection to the Internet, then you want to also
turn on Firewall Administration and Masquerading. Figure 4 shows a
picture of this.

Caution! I also need to stress that this configuration is only part
of the things that need to be performed to create a secure
environment. Other considerations include turning off ftp, telnet, and
other services on the Router, and ensuring all password, login, etc.
configurations are correctly setup for your environment. Make sure
you read the Security-HOWTO.

Figure 4: Firewall/Masquerading for ADSL

|-------| |-------| |-X
======X| ADSL |=------| Linux |-----|
ADSL | ANT | E0| |E1 |-X Private Network
Line |-------| |-------| | (e.g. 192.168.2.x)
<-------> |...
ISP Subnet or host
(Public Net Address)

The kernel for the Linux router is complied for IP
forwarding/masquerading, and has the "ipfwadm" (IP firewall software)
installed with the following options:

file: /etc/rc.d/rc.firewall (called by rc.sysinit in RH5.0)

echo "Setting up the firewall"
#
# From the "Firewall-HOWTO"
#
# flushes all setting
#
ipfwadm -F -f
#
# set the firewall
#
ipfwadm -F -p deny
#
# allow any machine with address 192.168.2.x to masquerade.
#
ipfwadm -F -a accept -m -S 192.168.2.0/24 -D 0.0.0.0/0
#
# allow the domain name server to work (udp 53)
#
ipfwadm -F -a accept -b -P udp -S 0.0.0.0/0 53 -D 192.168.2.0/24
ipfwadm -F -p masquerade
#
# the rest just list out the options for your enjoyment
#
ipfwadm -F -l
ipfwadm -O -l
ipfwadm -I -l

You need to be careful, as some application will still not work
without special modules (namely ftp, real audio, and some others).
Check the ipfwadm documentation for more information. I found this
pretty easy to set up.

Additionally, using the private network addressing scheme is cheap,
and gives an administrator complete flexibility in setting up their
local LAN. The drawback is that Masquerading has a limit on the
number of private hosts that it can reasonably support, and that some
IP applications that pass the host address in their data fields will
not work, but this tends to be a limited number.

8. Appendix

8.1. FAQs

This is the Frequently Asked Questions (FAQ) section for ADSL.

1. 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 <http://www.aware.com>. 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
<http://www.ansi.org>
Asymmetric Digital Subscriber Line (ADSL) Metallic Interface
ANSI TI.413-1995
Note: ANSI TI.413 Issue 2 was released September 26, 1997

2. 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.

3. 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.

4. 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.

5. 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
<http://www.adsl.com>. 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.

� Router ANT with 10/100baseT Interface

Examples: Flowpoint 2000 DSL (CAP), Netspeed Speedrunner 202 (CAP),
Speedrunner 204 (CAP), 3COM Viper-DSL (CAP), StarNet Ezlink 500/100
(DMT), Westell ATU-R-Flexcap (CAP), Aware x200

� Bridge ANT with 10/100baseT Interface

Examples: Alcatel A1000 (DMT), Westell ATU-R-Flexcap2 (CAP)

� ANT with ATMF Interface

Examples: Alcatel A1000 (DMT), Netspeed Speedrunner 203 (CAP),
Ariel Horizon II

� Bridge ANT with V.35 Serial Interface (T1, Serial Router)

Examples: Westell ATU-R

� ANT with USB Interface

Rumored to being pushed by Intel.

� Integrated ANT/NIC Card

Examples: Netspeed PCI Runner (CAP), Efficient Networks Speedstream
3020 (DMT)

These are NOT endorsements of the products listed, just provided
for illustration.;-).

8.2. Links

� ADSL Forum Home Page <http://www.adsl.com> A comprehensive web site
created by the adsl vendors. Fairly complete for reference
information on ADSL.

� Dan Kegels ADSL Page
<http://www.alumni.caltech.edu/~dank/isdn/adsl.html> 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
<http://www.pacbell.com/products/business/fastrak/adsl/index.html>
Pacific Bell is the local Telco and my provider of ADSL service.

�

� Jeremie's Unofficial Ameritech ADSL FAQ
<http://www.rc.lsa.umich.edu/~jeremie/adsl/>

� Telechoice xDSL News Page
<http://www.telechoice.com/xdslnewz/indexDSL.cgi>

� ADSL Deployment 'round the World
<http://www.geocities.com/Paris/Metro/5013/adsl.html> Claims to
have a complete list - looked accurate for my area - gives
providers, prices, speeds, etc.

� Bell Atlantic ADSL Home Page <http://www.bell-atl.com/adsl>

� comp.dcom.xdsl FAQ <http://homepage.interaccess.com/~jkristof/xdsl-
faq.txt>

8.3. Credits

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 ([email protected]) Great Description of loop impairment.

� C Wiesner ( [email protected]) List of many ADSL URLs.

� J Leeuw ( [email protected]) Many tips on ADSL, especially in Europe

� J Kass ( [email protected]) Unoffical Ameritech ASDL FAQ

� N Silberstein ( [email protected]) Info on Netrunner and his
experience with US Worst.

8.4. Glossary

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