ASYMMETRIC DIGITAL SUBSCRIBER LINE

Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide.

As compared to other forms of DSL, ADSL has the distinguishing characteristic that the data can flow faster in one direction than the other, i.e., asymmetrically. Providers usually market ADSL 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.

ADSL can use any of a variety of modulation techniques, but the ANSI and ETSI standards use DMT modulation schemes.

Downstream rates start at 256 kbit/s and typically reach 2 Mbit/s but can go as high as 8 Mbit/s over short ranges (so-called VDSL). Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 768 kbit/s. The name ADSL Lite is sometimes used for the slower versions.

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 runs widely over ADSL technology to ensure that this remains a possibility.

ADSL service providers may offer either static or dynamic IP addressing. Static addressing is preferable for people who may wish to connect to their office via a virtual private network, for some Internet gaming, and for those wishing to use ADSL to connect a Web server.

UK

In the United Kingdom, users had to live within 3.5 kilometers of the local telephone exchange to receive ADSL, but the range has grown to 5.5 kilometers 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 512 kbit/s downstream, and 256 kbit/s upstream (it may run slower if the user has RADSL), with a 50:1 contention ratio. Packages designed for offices or businesses have a 20:1 contention ratio and range from 512 kbit/s to 2 Mbit/s in downstream speed.

See Also

• See DSL for further details and other varieties.
• digital subscriber line access multiplexer

ADSL standards

• ITU G.992.3
• ITU G.992.4
• ITU G.992.5

External links

• DSL Forum - promotional trade organization for the ADSL industry
• adslguide.org.uk