DSL (digital subscriber line)

DSL (digital subscriber line) is one of the two most preva-lent forms of high-speed wired access to the Internet (see broadband and cable modem). DSL can operate over regular phone lines (sometimes called POTS or “plain old telephone service”). DSL takes advantage of the fact that existing phone lines can carry frequencies far beyond the narrow band used for voice telephony. When install-ing DSL, the phone company must evaluate the quality of existing lines to determine how many frequency bands are usable, and thus how much data can be transmitted. Fur-ther, because the higher the frequency the shorter the dis-tance the signal can travel, the available bandwidth drops as one gets farther from the central office or a local DSL access Multiplexer (DSLAM).

Typical DSL services can range in speed from 128 kbps to 3 Mbps. Many providers offer higher speeds at additional cost. Speeds quoted are generally maximums; actual speed may be less due to poor line quality or greater distance from the central office.

The most common form of DSL is ADSL (asymmetric DSL), which has much higher download speeds than upload speeds. This is generally not a problem, since most users consume much more content than they generate. The lower frequencies are generally reserved for regular voice and fax service. A single DSL modem can serve multiple users in a local network by being connected to a router.

As more people move from land-line phone service to cellular, there has been greater demand for offering so-called naked DSL—DSL without traditional phone ser-vice. DSL can also be provided over optical fiber (see fiber optics).

Note that an older and lower-bandwidth version of the technology called ISDN (Integrated Services Digital Net-work) is still in use, but has largely been superseded by DSL/ADSL.

Alternatives to DSL

Cable is still more popular than DSL, though the latter has closed the gap somewhat. The fact that the two services can both provide fast Internet access (mostly) through existing infrastructure has created considerable competition. Thus a cable provider can now offer telephone service via the Internet (see voip) at the same time a phone provider using DSL can offer movies and television programming streamed over the network. The fact that in many locations DSL and cable providers are in competition can result in lower rates or more attractive “bundles” of services for consumers.

On average, cable modem speeds are somewhat faster than DSL; however, cable speeds can degrade as more users are added to a circuit. Although both services have had their share of glitches, they now both tend to be quite reliable.