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Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over analog telephone lines than can be obtained with a conventional modem. The distinguishing characteristic of ADSL compared to other forms of DSL is that the data can flow faster in one direction than the other, i.e., it is asymmetric. It is usually marketed as a service for people to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require bandwidth in the other direction.

Downstream rates start at 256 kbps, are typically 2 Mbps but can go as high as 8 Mbps over short ranges. Upstream rates start at 64 kbps, are typically 256 kbps but can go as high as 768 kbps. The slower versions are called ADSL Lite[?].

In the United Kingdom, users had to live within 3.5km of the local telephone exchange to receive ADSL, but the range has been extended to 5.5km thanks to RADSL[?] (Rate Adaptive Digital Subscriber Line), although users with RADSL will have a lower upstream rate. The typical home ADSL connection in the UK has a 512kbps downstream, and a 256kbps (may be lower if the user is on RADSL) downstream with a 50:1 contention ratio. Packages designed for offices, or businesses have a 20:1 contention ratio and range from 512kbps to 2mbps downstream speed.

Because of the relatively low data rate (compared to optical backbone networks) ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as Web traffic; ATM is widely employed over ADSL technology to ensure that this remains a possibility.

See DSL for all the different varieties.

External links

  • DSL Forum (http://www.dslforum.org) - promotional trade organization for the ADSL industry
  • adslguide.org.uk (http://www.adslguide.org.uk)

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