Understanding DSL Technology

Conventional telephone service (often referred to as plain old telephone service, or POTS)

transmits the frequencies most often used in human speech—roughly 300-3,300Hz. 18

Conventional telephone modems extend this range by only about 200Hz. DSL, on the other hand, uses an entirely different range of frequencies—typically about 4,000-1,100,000Hz. It's therefore possible to use DSL on the same wires that provide POTS to your home or office. In practice, though, many DSL installations use separate lines, either because existing lines are of poor quality or because local phone companies don't like sharing the line with a third party that provides DSL service. (The courts have ruled that phone companies must share lines, but implementing this ruling might take some time.)

Types of DSL Technology

There are several variant DSL technologies available:

• ADSL Asymmetric DSL (ADSL) is the most popular type in the home marketplace. This form of DSL takes its name from the fact that upstream speed is much slower than downstream speed. For instance, an ADSL line might provide 640Kbps downstream but only 90Kbps upstream (or 640/90 service, as it's often called). This discrepancy works well for most home users, who mostly browse the Web and download data; but it's not so good for businesses, which often run high-volume servers. There are two modulation techniques in common use for ADSL service, Discrete Multi-Tone (DMT) and Carrierless Amplitude and Phase Modulation (CAP). You needn't be too concerned about which method your DSL provider uses. There's a general trend toward DMT, though, so if you get CAP service you might be required to upgrade your modem at some point. ADSL speeds range from 384/90 to 7,100/1,500 (the upper end of this scale

Part IV

is highly variable depending upon your provider). Prices range from about $40/month to more than $100/month, depending upon the speed, ISP, level of service, and so on. ADSL service can generally be obtained only within about 18,000 feet of your telephone company's CO.

• G.Lite G.Lite, which is also known as ADSL Lite, is a variant of ADSL. The key selling point of G.Lite is that it obviates the need for a telephone company technician to visit your home. In most DSL installations, a technician installs a splitter on your line at the point where the phone line enters your building. This splitter breaks up the DSL and voice signals and puts them on separate lines in your building, so that you don't hear a high-pitched whine on your phone and so that your use of the telephone doesn't interfere with the DSL service. Alternatively, some providers run entirely separate lines to your building to provide DSL. In G.Lite, by contrast, the DSL provider sends you DSL microfilters to be used at every phone, so in theory a technician need not visit your home, which saves cost. (This arrangement is illustrated in Figure 18.5.) Unfortunately, many homes have substandard phone wiring, so a service call is required in many cases even with G.Lite. G.Lite speeds are also capped at about 1,500Kbps downstream.

Conventional ADSL G. Lite


Figure 18.5

G.Lite replaces an external splitter with a series of internal microfilters.

• SDSL Symmetric DSL (SDSL) provides the same speed in both directions. SDSL is generally marketed to business users, and it usually costs more than does ADSL service—usually $100-$300 per month. SDSL speed typically ranges from about 144Kbps to 1500Kbps. SDSL uses different modulation techniques than does ADSL, and works out to 11,000 feet or so from the CO. Like ADSL, it's usually installed with a single splitter at the point of entry to a building.

• HDSL High-bit-rate DSL (HDSL) is much like SDSL, but uses two pairs of wires to increase the overall data capacity. HDSL is fairly rare in today's marketplace.

Figure 18.5

G.Lite replaces an external splitter with a series of internal microfilters.

Chapter 18

• IDSL ISDN-based DSL (IDSL) uses ISDN technology to deliver symmetric DSL service at speeds of 128-144Kbps. This speed is low compared to other DSL technologies, but it can be used at distances of up to 38,000 feet. IDSL service typically sells for $100-$300/month.

• xDSL This isn't a separate type of DSL; it's simply a term that refers to any variety of DSL service.

The preceding summary of DSL technologies should be taken as a rough guideline. For various reasons, many providers limit DSL speed or won't install at distances near the limits I've outlined here. There are also more exotic forms of DSL that might be available in your area, although ADSL (and its G.Lite variant), SDSL, and IDSL dominate the marketplace.

DSL Authentication

DSL providers, like other ISPs, need some way to authenticate their users. There are three methods in common use:

• Static IP addresses Some providers assign each user one or more static IP addresses. You then enter this information into your Linux system as if you were connecting to an

Ethernet network. Static IP addresses are great if you want to run a server, but they're the 18 least secure form of authentication. It's often possible for another DSL user to "hijack" your DSL IP address. If this happens, you might find yourself unable to use your DSL service. o

• DHCP The dynamic host configuration protocol (DHCP) is a means for ISPs to assign g IP addresses dynamically. A server that the ISP maintains keeps a record of IP addresses available, and assigns them to computers as they come online. DHCP can be configured to use your Ethernet card's media access control (MAC) number as authentication, so it's somewhat more secure than is a static IP address. It's also possible for an ISP to assign a given customer the same IP address every time the customer boots the computer. Most DSL ISPs don't work in this way, though, so you might get a new IP address every time you boot. Configuring Linux to use DHCP is simple, as I described in Chapter 17, "Network Hardware."

• PPPoE PPP over Ethernet is the latest craze in the DSL world. This protocol uses PPP to authenticate computers, but over a DSL modem rather than a conventional telephone modem. The 2.2.x and 2.3.x kernels don't include support for PPPoE, but there is work underway to integrate PPPoE into the kernel. In the meantime, several alternative PPPoE implementations for Linux exist. Links to many of these are at http://www.rodsbooks. com/network/network-dsl.html. PPPoE is the most secure of the authentication methods currently in use, but also the least convenient for Linux users. Most PPPoE implementations assign a new IP address every time you boot the computer, so PPPoE is a poor choice if you want to run a server. Some ISPs provide a fixed IP address via PPPoE, typically for a higher price than a random IP address.

Part IV

For Linux use, I recommend you try to locate an ISP that uses static IP addresses or DHCP, at least until Linux's PPPoE support makes its way into the kernel. PPPoE schemes can be made to work, but they're more trouble, and they often require that you initialize the account using Windows. If you don't dual-boot your computer between Windows and Linux, using PPPoE might therefore be a major problem. (After the account is configured, there's no need to reboot Windows, so you can eliminate it from your computer and use only Linux if you like.)

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