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As of 2004, DSL provides the principal competition to cable modems for providing high speed Internet access to home consumers in Europe and North America.
When the telephone system was designed, it was intended for voice calls and, for reasons of economy, it was designed to transmit over a frequency range from around 300 Hz to 3 to 4 kHz (although human speech can range up to around 15 kHz, it can easily be understood if limited to this range).
However, the local loop of copper telephone connections to individual subscribers can usually communicate using a much wider range of frequencies, ranging from these low frequencies up to 200 - 800 kHz, depending on the quality of the circuit and the sophistication of the equipment.
DSL services were devised as a means of overcoming the restriction on the amount of data that could be transmitted over the "voiceband" (see Shannon capacity), by using these additional frequency ranges.
While DSL services often reserve the 0.3 - 4 kHz band for voice calls over the " plain old telephone service" (POTS), the frequencies above and below this range can be used to transmit data.
A DSL connection takes place between equipment of the subscriber and the telephone exchange, with some other protocol used between the exchange and whoever the subscriber really wants to connect to, typically an Internet service providerAn Internet Service Provider (an ISP is a provider of Internet services. Most telecommunications operators are ISPs. They provide services like internet transit, domain name registration and hosting, dial-up access, leased line access and colocation.. This differs from a normal telephone connection, where the public telephone network links subscribers to each other. The quality of service however falls when the distance between subscriber and telephone exchange exceed 5.5 kilometers.
The subscriber end of the connection consists of a DSL modemA modem (a portmanteau word constructed from mod ulator and dem odulator) is a device that modulates an analog carrier signal ( sound), to encode digital information, and that also demodulates such a carrier signal to decode the transmitted information.. This converts data from the digitalA digital system is one that uses discrete values rather than a continuous spectrum of values: compare analog. The word comes from the same source as the word digit: the Latin word for finger (counting on the fingers) as these are used for discrete counti electronic pulses used by computers into a digital audio stream of a suitable frequency range for the particular DSL variant in use.
In addition the subscriber may need to install a passive electronic filterAn electronic filter eliminates unwanted frequencies from an electronic signal. There are several kinds of electronic filters: A low-pass filter passes low frequencies. A high-pass filter passes high frequencies. A band-pass filter passes a limited range (known variously as a "filter" or "micro-filter") if using the POTS service on the same line (and possibly also to improve the DSL termination and prevent echoEcho is: Echo character in Greek mythology echo natural phenomenon Echo sounding Forward echo Echo, Minnesota Echo Township, Minnesota Echo Township, Michigan Echo Lake, Washington The rock band Echo & the Bunnymen The 1999 album Echo by Tom Petty the letes). This ensures that the DSL modem and the telephone only receive the frequencies they are designed to handle. Subscribers can plug a filter into an existing telephone socket when using a "wires-only" service; or alternatively the DSL provider may install it.
At the exchange a digital subscriber line access multiplexerA digital subscriber line access multiplexer (DSLAM) is a multiplexer located in the telephone company exchange that provides consumers access to DSL services over twisted pair copper cabling. The device separates the voice and data components from the su (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. It also separates out the voice component.