Types of DSL Modems

DSL modems come in two basic varieties: internal and external. For the most part, internal DSL modems are useless under Linux, because they lack drivers. I know of only one internal modem for which DSL drivers currently exist, the Diamond 1MM. A link to the driver for this board exists on http://www.rodsbooks.com/network/network-dsl.html. Other internal DSL modems will no doubt be supported in the future, however.

External DSL modems come in a variety of shapes and sizes. In general, they're somewhat larger than conventional telephone modems, but they otherwise look similar. Figure 18.6 shows a typical external DSL modem. Like telephone modems, DSL modems usually include a number of status LEDs, an ordinary telephone jack, and a jack for an external power brick. Unlike telephone modems, DSL modems don't include RS-232 serial ports. Instead, these devices interface through Ethernet or USB ports.

Figure 18.6

DSL modems look much like conventional telephone modems.

Figure 18.6

DSL modems look much like conventional telephone modems.

Chapter 18

For use in Linux, I strongly recommend you use an external DSL modem that uses an Ethernet interface. These devices enable you to treat a DSL connection as if it were an ordinary Ethernet connection. In fact, it is an ordinary Ethernet connection. To use such a modem, you must have an Ethernet card. Because DSL speeds are usually well below 10Mbps, a 10Mbps Ethernet card is sufficient. A 10/100Mbps card is also usable, however. Given that the price difference is so small, you might as well use a 10/100Mbps NIC, unless your ISP provides you with a 10Mbps NIC as part of the installation package.

Some external DSL modems interface via the USB port. This is a suboptimal arrangement, particularly in Linux, because driver support is so weak and because high-speed USB devices tend to chew up a fair amount of CPU time. If you find yourself with a USB-interfaced DSL modem, check http://www.linux-usb.org to locate the latest information on drivers that might support it. You'll need to use a 2.3.x or later kernel, or apply a USB back-port to a 2.2.x kernel.

Most DSL modems function as network bridges. This means that they pack up any Ethernet frames they receive and send them out over the telephone wire using a DSL encoding method. When the data reaches your ISP, the DSL encoding is stripped away and the raw Ethernet frames sent by your system are decoded and sent on their way. More advanced DSL modems 18

function as routers. A router is more advanced than a bridge, and can often be programmed to not pass certain types of Ethernet frames, or to process the frames in one way or another. For instance, some DSL routers can handle the PPPoE encapsulation that some DSL ISPs use. If you use such a device, you can configure your computer just as you would if your ISP gave you a static IP address. The DSL router then converts your data into PPPoE form and sends it on its way. If your chosen ISP uses PPPoE, therefore, you might want to look into the possibility of using a DSL router rather than a DSL bridge. You'll pay more for a router, however.

Most DSL providers include a DSL modem in the installation cost for the service. From time to time, providers run installation specials in which you pay a very low cost for installation, including the hardware. It's therefore generally best to use whatever DSL modem your ISP provides. If you decide to buy another one, be sure to check that it's compatible with your ISP's service. Because there are so many forms of DSL—including two major modulation techniques for ADSL alone—it's easy to buy a modem that can't be used with your ISP's service.

With the exception of G.Lite devices, you won't find DSL modems in most computer stores. Nonetheless, there are cases where you might need to buy one. For instance, you might have an internal model and want to switch to Linux. You can often find external DSL modems on Internet auction sites such as eBay (http://www.ebay.com). Be very careful when buying, though; you don't want to wind up with a DSL modem that's incompatible with your ISP's DSL protocols.

Part IV

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