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DSL TechnologyThis page discusses some of the basic details of DSL technology.
What makes DSL technology appealing, first of all, is its speed. In its very fastest incarnations, DSL offers more than 100 times the network performance of a traditional analog modem. While the precise speed of a connection depends on the variety of xDSL deployed, even a basic ADSL setup should outperform those modems by a factor of 20 or more.
Because DSL uses the same telephone line wiring as traditional modems, it may not be immediately obvious how it achieves such high speeds. In a nutshell, DSL works on the unused (high) frequencies of the line. DSL modems contain an internal signal splitter that carries voice signals on the usual low frequencies (from 0 up to 4kHz) and data signals above that. This splitter, consequently, allows simultaneous access to the line by the telephone and the computer. Customers who might ordinarily have required a second phone line won't need it for DSL service.
Ideally, DSL service remains "on" all of the time. With an always-on connection customers no longer need to physically dial up to the Internet Service Provider (ISP) to "log in to the Internet." A few access providers (such as Pacific Bell) now offer DSL service implemented with PPPoE (Point-To-Point Protocol over Ethernet) that does not support always-on, but even in this case a DSL router can automate the connection process. People should be aware that long-lived connections like DSL can have security issues. Many DSL customers have installed personal firewall products on their home networks to guard against external attacks.
Needless to say, today's analog modems don't compete very well with DSL in terms of performance. Modem bonding, where two V.90 modems are close-coupled to one computer, may theoretically double the performance of a single modem. The increased network performance bonding provides (a factor of two theoretically) is good, but many users of the Internet need order-of-magnitude (factor of 10 or more) improvements in their connectivity speeds to support features like digital video in the home. For these purposes, DSL simply leaves analog in the dust.
Cable modems enable high-speed, always-on Internet access using the cable television lines that connect to some U.S. households. Cable can support network speeds comparable to those of DSL. Like some DSL services, the network speed upstream to the Internet will be slower than downstream to the home or office. But unlike DSL services (that all offer locally dedicated bandwidth), cable modem service involves locally shared bandwidth. This means the realized performance of a customer's cable will depend on how many other customers in that local area subscribe to the same service.
Satellite data service affords another option to those out of the reach of DSL (or cable) service. Satellite works at no more than one-third the speed that DSL does today -- and often less. But like DSL, satellite bandwidth is dedicated and speeds won't drop when others use it at the same time. Satellite "mini-dish" systems weren't designed for two-way communications (they're the same ones that deliver TV), so satellite access to the Internet generally requires an analog phone line and modem for outgoing traffic.
Integrated Services Digital Network (ISDN) technology has been available for a number of years from the public telephone companies. Under ideal conditions, ISDN allows customers to receive data at rates just twice that of ordinary dial-up -- nowhere close to the data rates of cable modems and DSL. ISDN has generally been more widely available than DSL service, but the rapid expansion of DSL networks suggests this advantage is fading.