  ADSL HOWTO for Linux Systems
  David Fannin, dfannin@sushisoft.com
  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
  dfannin@dnai.com .

  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 (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

  ·  J Kass ( Xjeremie@umich.edu) Unoffical Ameritech ASDL FAQ

  ·  N Silberstein ( Xnick@tpdinc.com) 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







