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Digital subscriber line

Digital Subscriber Line, or DSL, refers to a family of technologies that provide a digital connection over the copper wires of the telephone network. Analogue telephone connections are usually limited to a low frequency band of 0-4 kHz, but in practice it is usually possible to communicate using a much wider range of frequencies, depending on the quality of the circuit and the sophistication of the equipment. DSL services often reserve the 0-4 kHz band for the plain old Telephone service (POTS) over the same line.

A DSL connection is made 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 provider. This is unlike a normal telephone connection, where the public telephone network links subscribers to each other.

The subscriber end of the connection consists of a DSL modem, which has the job of converting the digital stream into analogue signals with various frequency bands, as required for the particular DSL variant in use. In addition an electronic filter known variously as a "splitter" or "filter" or "micro-filter" may be required if the POTS service is used on the same line (and possibly also to improve the DSL termination and prevent echoes). The filter can be plugged in to an existing telephone socket when a "wires-only" service is used, or alternatively it may be installed by the DSL provider.

At the exchange a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them where they are handed off onto other networking transports. It also separates out the voice component.

As of 2002, DSL is the principal competitor to cable modems for providing high speed Internet access to the home consumer.

The reach restraints (line length from Central Office to Subscriber) reduce as the data rates increase, with technologies like VDSL being used for short range links (typically "fibre to the curb" network scenarios).

Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers are effectively connected into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over ethernet or ATM (PPPoE[?] or PPPoA[?], also known as PPPoATM[?]), with authentication with a userid and password and using PPP mechanisms to provide network details.

Example DSL technologies (sometimes called xDSL) include:

  • ISDN (pre-dates the use of the term DSL)
  • IDSL[?] (ISDN Digital Subscribe Line, a data-only ISDN variant)
  • ADSL (Asymmetric Digital Subscriber Line)
  • HDSL[?] (High Bit Rate Digital Subscriber Line)
  • RADSL[?] (Rate Adaptive Digital Subscriber Line)
  • SDSL (Symmetric Digital Subscriber Line, a standardised version of HDSL)
  • VDSL (Very high speed Digitial Subscriber Line)
  • G.SHDSL[?] (ITU-T Standardised replacement for early proprietary SDSL)

See also: modem, COFDM

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