Section 13.4. ADSL

13.4. ADSL

The 64-Kbps rate that ISDN supports is nice, but if you want to access multimedia files via the Internet or simply are using the Internet a lot, you may want even more bandwidth. Without drawing additional cables to your house or office, ADSL (Asynchronous Digital Subscriber Line), a variant of DSL (Digital Subscriber Line), is a convenient alternative that gives you up to 128 times (depending on your provider and your type of subscription) the bandwidth of standard dial-up access and is run via your ordinary telephone line. A drawback with ADSL is that it only works within a distance of about 5 to 8 kilometers (3 to 5 miles), depending on cable quality around the next switching station, which makes this service unavailable in rural areas. Typical bandwidths are 0.5 to 8 Mbps (megabits per second) downstream (to your computerdownload operations, including viewing web pages and retrieving email) and 0.125 to 1 Mbps upstream (from your computerupload operations, including sending email). Note that there are other technologies with similar-sounding names, such as SDSL. Although these are fundamentally different on the wire level, setting them up on your Linux box should be no different from ADSL.

ADSL is not dial-up access; once you have logged into your account, you are always connected. Some providers cut your connection after a while (often after 24 hours), upon which you have to log in again in order to regain access.^[*]

^[*] The reason why the providers do this is that they want to prevent you from running a server, coercing you into upgrading to a more expensive "business" subscription if you wish to do that.

As we have already mentioned, there are no such things as ADSL cards or ADSL drivers. As far as hardware is concerned, an ADSL connection is just a normal Ethernet connection, using the same cables.

How you connect your Linux box to your ADSL line depends a lot on your ISP. With some ISPs, you rent the necessary equipment, such as an ADSL modem and an ADSL router, as part of your subscription. With others, you have to purchase the necessary hardware yourself, either on the free market or from the ISP. Your ISP can give you all the information you need.

There are two ways to use ADSL: either connecting directly to an ADSL modem or with an intervening ADSL router. If you have an ADSL router (either with a built-in ADSL modem, or in addition to one), you plug your Ethernet cable in there and are ready for action. If you want to connect your Linux box directly to your ADSL modem, you still connect the computer and the modem with an Ethernet cable, but you need to run a special protocol, called PPPoE (PPP over Ethernet), on it. This protocol is handled by a special daemon called pppoed. How this is set up depends on your distribution and should be documented there. Some distributions also let you set up PPPoE from their respective configuration programs.

Finally, we should mention that a small number of weirdo ADSL modems are not connected with an Ethernet cable , but rather with a USB cable. This is technically a bad idea, so you should avoid these modems if possible, but if you are stuck with one, you can find more information, including drivers for some devices that run PPPoE on a USB connection (that would be PPP over Ethernet over USB, then!), at http://eciadsl.flashtux.org and http://speedtouch.sourceforge.net.

Whichever way you use ADSL (with or without an ADSL router), you need to set up the correct IP address. This can either be static (in which case you should find it in the information you have received from your ISP) or dynamic and assigned via DHCP (Dynamic Host Communication Protocol), in which case you can request a dynamic IP address with the following command:

     dhclient eth0

Of course, if the Ethernet card you use has another name, you need to replace the eth0 with the correct name. Instead of dhclient, you can also use the utility pump. There is also a DHCP daemon called dhcpcd that runs in the background and assigns a dynamic IP address whenever necessary.

Finally, many ISPs require that you activate your line from time to time. How you do this depends on your ISP, but often the activation requires nothing more than browsing to a certain web site and entering the credentials there that your ISP has assigned to you. As mentioned before, you may have to repeat this step from time to time.