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DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. more...

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DSL originally stood for digital subscriber loop, although in recent years, many have adopted digital subscriber line as a more marketing-friendly term for the most popular version of DSL, ADSL over UNE.

Typically, the download speed of DSL ranges from 640 kilobits per second (kbit/s) to 3,000, or exceptionally from 128 to 24,000 kbit/s depending on DSL technology and service level implemented. Typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for the rarer Symmetric Digital Subscriber Line (SDSL).

History

Digital subscriber line technology was originally implemented as part of the ISDN specification, thus can operate on a BRI ISDN line as well as an analog phone line.

Joe Lechleider at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone company central offices and customers on conventional twisted pair cabling.

US telephone companies promote DSL to compete with cable modems. DSL service was first provided over a dedicated "dry loop", but when the FCC required ILECs to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over UNE), this allows a single pair to carry data (via a DSLAM) and analog voice (via a circuit switched telephone switch) at the same time. Inline low-pass filter/splitters keep the high frequency DSL signals out of the user's telephones. Although DSL avoids the voice frequency band, the nonlinear elements in the phone would otherwise generate audible intermodulation products and impair the operation of the data modem.

Older ADSL standards can deliver 8 Mbit/s to the customer over about 2 km (1.25 miles) of unshielded twisted pair copper wire. The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM. Some customers, however, are located farther than 2 km (1.25 miles) from the central office, which significantly reduces the amount of bandwidth available (thereby reducing the data rate) on the wires.

Operation

The local loop of the Public Switched Telephone Network was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voiceband or commercial bandwidth.

Read more at Wikipedia.org