DSL, or Digital Subscriber Loop, is a high-speed Internet access technology that uses a standard copper telephone line (a.k.a. "loop" in telco parlance). DSL provides a direct, dedicated connection to an ISP via the existing telco network. DSL is designed to run on up to 80% of the telephone lines available in the United States. By using line-adaptive modulation, DSL is capable of providing data speeds of 8 Mbps or more.
DSL services are now being aggressively marketed for home and small business use. DSL is typically priced below ISDN, and well below T1 service, yet can provide potentially even greater speeds than T1 without the cost, complexity, and availability issues of T1. Since DSL is a dedicated, often "always on" service, it avoids the delays and use charges that are common with ISDN. Making this quite a nice technology for the bandwidth starved masses.
While all this sounds exciting, DSL does have some drawbacks. The quality of the DSL signal, and thus the connection, depends on distance (the length of the copper "loop") and various other factors. Also, there is no such thing as standard "DSL". There are various flavors of DSL, and many, many ways DSL providers are implementing their networks. In typical fashion, Linux users are often left to fend for themselves, since the DSL providers are often taking the easy way out, and catering only to "mainstream" Operating Systems.
The topics included in this HOWTO include qualification and pre-installation, installation, configuration, troubleshooting and securing a DSL connection. As well as other related topics. There are also appendices including a comprehensive DSL Overview, Frequently Asked Questions, a listing of related links, and a glossary.
Due to the fast pace of change in the telco and DSL industries, please make sure you have the latest version of this document. The current official version can always be found at http://www.tldp.org/HOWTO/DSL-HOWTO/. Pre-release versions can be found at http://feenix.burgiss.net/ldp/adsl/.
This document attempts to give a comprehensive discussion of DSL. All aspects are hopefully addressed to one degree or another with what can be a complex topic since it deals with networking, hardware, new fangled technologies, and various approaches taken by various vendors. The core components of this document are:
The Installation section covers installation of DSL hardware and related components, including wiring considerations, splitter or microfilter installation, modem and Network card installation.
The Configuring Linux section covers mostly client and software aspects of getting the connection up and running. The Network card configuration is actually covered mostly in the above Installation section.
The Securing Your Connection section covers Security implications that are even more important with a full-time connection. Linux users seem especially targeted by crackers, because quite frankly, some don't understand how important security is, or don't understand the finer points of this. And who wants to "own" a Windows box?
The Tuning and Troubleshooting section covers post-installation topics like how well is our connection performing, and how to track down any show-stoppers or intermittent problems.
There is also a lengthy Appendix that covers various topics relating to Linux and DSL. None of these are directly related to simply getting that connection up and running, but may be of interest nonetheless.
To simplify the navigation of this document, below is a suggested reading guideline. Everyone should read the Introduction. Please pay special attention to the Conventions and Terminology section, as some of this terminology may be used somewhat differently in other contexts. Also, there is a Glossary if you get lost in the world of TA (telco acronyms) ;-).
If you don't know anything about DSL, you should probably read the entire document. You may want to start with the DSL Overview section in the Appendix, and then the FAQ. The DSL Overview explains how the various pieces of the puzzle fit together. DSL network implementations are more complex than traditional dialup networks.
If you have already done some homework, but have not ordered service from anyone yet, read the Choosing Providers section. Also, you might get a head start by reading the Configuring Linux section so you know what lies ahead.
If you have ordered service already, and are awaiting delivery, you can skip the sections on choosing a Provider. If you will be doing a self-install, you should read the pertinent parts of the Installation section, the Configuring Linux section, and the Securing Your Connection section.
If the installation is complete, and you can't get a working connection, skip right to the Troubleshooting Section. If you are not clear on what protocols are required, or what software you need to have installed, also read the Configuring Linux section. If not sure what terms like "sync" mean in this context, then be sure to read the DSL Overview section first so you know how it all fits together.
If you have never had a full-time Internet connection, or are not absolutely sure you fully understand how to secure you connection, be sure to read The Securing Your Connection section. If you don't understand some aspect of this, re-read it, or start looking for other references.
There is a comprehensive Links section that has references to some topics not touched on in the main body of the Document itself.
1.7: Added comments on ISDN line filters being different than POTS, and other additions related to ADSL over ISDN in various places. Add another supported modem: Eci Hi Focus ADSL Modem (and various related chipsets). Removed the 'Linux Friendly ISP' section. The landscape has changed much since this section was started. Back then there were few options for DSL in many places, and all too often a non-compatible modem was the only thing available. Also, the advent of microfilters and self-installation has helped with the "do-it-yourselfer" approach, giving everybody more freedom. Then, maintaining this number of links was a PITA too. I still encourage new subscribers to shop their local markets if there are options. Many large ISPs and telcos have very poor ideas of what an Internet connection is and restrict severely what you can do with Linux. Or at least try to ;-) Updated LDP links to tldp.org (from linuxdoc.org).
1.5: New Tuning sub-section using iproute. Hot stuff! Other additions to the Tuning section. A few new ISPs. Alcatel SpeedTouch USB section updates. Thanks to Alex Bennee for clarifying things. Other minor updates to FAQ, Glossary and Tuning.
Version 1.3: Updates to the SpeedTouch USB HOWTO in the appendix. Minor update to PPPoE section, and two new Linux Friendly ISPs. A feeble attempt to make the document a little less U.S.-centric. Various minor updates.
Version 1.2 adds PPTP configuration section for Alcatel ethernet modems. Also, added are two additional sections in the "Tuning" section for the TCP Receive window, and ADSL/DMT interleaving. And the big news is the release of open source drivers for the Alcatel USB modem as of March 2001. There is an Alcatel SpeedTouch USB mini HOWTO in the appendix by Chris Jones. A number of miscellaneous updates as well.
Version 1.1 included quite a few minor corrections, updates, and additions. Not much that is substantially new. There are finally two Linux compatible DSL PCI modems from Xpeed. The drivers are now in the kernel 2.2.18 source (not ported to 2.4 as of this writing 05/23/02).
Version .99 addresses some of the many changes that have occurred since the original ADSL mini HOWTO was published. Originally, ADSL was the primary DSL technology being deployed, but more and more some of the other DSL flavors are entering the picture -- IDSL, SDSL, G.Lite, and RADSL. Thus the renaming from "ADSL mini HOWTO" to the "DSL HOWTO". There have been many other changes in DSL technology as well. PPPoE/A encapsulation has become more and more common as many ISPs are jumping on this bandwagon.
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 GNU GPL.
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
N Silberstein ( Xnick@tpdinc.com) Info on Netrunner and his experience with US Worst.
Many and various posters from comp.dcom.xdsl and bellsouth.net.support.adsl, too numerous to mention individually. (HB)
Juha Saarinen for suggestions and explanations on the TCP Receive Window, and related tuning topics.
Chris Jones <email@example.com> for his Alcatel SpeedTouch USB mini HOWTO which was previously incorporated into the Appendix. Also, Alex Bennee for clarifying the driver situation for this modem.
The authors accept no liability for the contents of this document. Use the concepts, examples and other content at your own risk. As this is a new edition, there may be errors and inaccuracies. Hopefully these are few and far between. The author(s) do not accept any responsibility for incorrect or misleading information, and would certainly appreciate any corrections. Also, this type of technology dates itself very quickly. What may be true today, is not guaranteed to be true tomorrow.
All copyrights are held by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark.
For the sake of simplicity and sanity, let's clarify some of the terminology that we will be using in this document, so that we are all on the same page. While many of the definitions below are not always 100% technically correct, they are close enough for our purposes here. In fast moving technologies like DSL, there are so many "ifs, ands, and buts" that it is difficult to say anything with any degree of certainty and have it stick. And there are exceptions to almost every rule. And sometimes exceptions to the exceptions. We will be dealing with generalities to a large degree here, please keep that in mind.
"DSL" will be used to refer to the entire family of DSL technologies now available -- ADSL, SDSL, IDSL, RADSL, etc. ADSL still seems to be the most prevalent at this time, but the others are being deployed as well. Where it is important to differentiate one type of DSL from another, the full proper name will be used: e.g. RADSL. xDSL is also commonly used to refer to the various DSL technologies as a group, but we will be using just "DSL" here.
The term "telco" here refers to any potential DSL provider. This includes the ILECs (Incumbent Local Exchange Carriers), a.k.a. the old guard phone companies or state run phone companies, and where the monopolies now have competition, the CLECs (Competitive Local Exchange Carriers), or independent providers such as Covad in the U.S.
"CO" is the telco acronym for "Central Office". Traditionally this is a building where one end of your phone line physically terminates. The other end terminates at your home, office, or wherever. It will be used here to refer to the telco end termination point, regardless of whether it is a traditional Central Office building or another, smaller, remote structure or device.
"Loop" is telco speak for "phone line". Essentially, you should think of your loop as one dedicated pair of copper wires that run uninterrupted from your residence or office directly to the CO. This is perhaps an oversimplification, but will serve our purposes. DSL availability, and signal quality, is tied directly to the characteristics of your physical line -- or "loop" as they say.
"POTS" is the acronym for Plain Old Telephone Service. In other words, traditional, non-digital devices like analog phones, faxes and answering machines. ISDN is used for DSL in some areas, so POTS is not the only way to piggy-back DSL. But certainly the most common in many places.
"NID", or Network Interface Device, is the small telco housing that is often typically attached to the outside wall of your house, and is the service entrance for telco services, though may be placed elsewhere depending on the phone company. This may variously also be referred to as "ONI", "SNI", "NIU", "TNI" or other creative telco acronyms. It represents the "demarcation" point that divides the customer's realm of responsibility from the telco's. Commercial structures, and multi-family housing will likely have something more sophisticated, and probably located inside somewhere.
"DSLAM" is the sophisticated hardware device in the telco's CO where your phone line physically terminates, and thus makes DSL happen. Increasingly, telcos are making use of smaller devices like the "mini-RAM" in remote locations. We'll use "DSLAM" here as a catch-all for any device that enables DSL service from a telco. These are now being manufactured by a number of companies.
"Modem" will be used to refer to the end user device that enables a DSL connection. Your "modem" is connected to the telco's DSLAM in the CO via your copper loop. When they are "talking" DSL to each other, they are in "sync". Without "sync", no connection to your ISP is possible.
"Modem" is indeed the correct terminology since there is MOdulation and DEModulation of the signal, even though it doesn't resemble an analog 56K modem like many of us have had before. These modems incorporate other features too -- so they are more than just a "modem". Some ISPs and manufacturers may be marketing simply "routers", "bridges", or even "brouters" for this purpose. These are essentially DSL modems with enhancements. A compatible "modem" of some kind is the minimum hardware requirement at the customer's end of the connection. The most commonly supplied modem is actually a combination bridge and modem.
One distinction here may be where ADSL is provided over ISDN lines. In this case, the term "modem" is not appropriate and the only physical difference is that the ISDN Network Terminator (NT), is equipped to handle DSL, but is still an NT. In any case, for brevity, we will take license here to refer to all such devices as "modems".
Unless stated otherwise, we will also be assuming the "modem" has an ethernet interface, and will connect to a standard ethernet Network Card (NIC). This is far and away the most prevalent configuration for Linux users, at least until more Linux drivers are available for PCI and USB modems. USB modem are quite popular otherwise, because they are "plug 'n play", and arguably less expensive.
It is worth noting that "routers" as supplied by DSL providers are typically modem/router combination devices. In our context, "router" will refer to these devices as such. There are also SOHO broadband routers available that are only dedicated routers and lack the modem functionality.
Previous versions of this document referred to the modem as an "ANT" (ADSL Network Termination). While this may be technically correct terminology, it is not used by ISPs, manufacturers, telcos, or most users to any extent. The "modem" will be just called a modem, regardless of whatever other features it may incorporate (i.e. router, bridge, etc.).
PPPoX will be used to refer to PPPoE (PPP over Ethernet) and PPPoA (PPPoATM, or PPP over ATM) collectively. These protocols are being used by many DSL providers now.
The information provided in this document is based mostly on the current state of DSL in the U.S. I will assume there are enough similarities with DSL services outside of the US that this document would still have some merit for everyone. Correct me if I am wrong by emailing <firstname.lastname@example.org>.
A "#" will be used to denote a command that typically is run by the root user. Otherwise, a "$" will be used as the prompt for non-root users.