DSL
Digital Subscriber Line

A family of digital telecommunications protocols designed to allow voice and high speed data communication simultaneously over existing telephone lines.

When two modems are connected through the telephone system, it treats the communication in the same way as voice conversations. This has the advantage that there is no investment required from the telephone company, but the disadvantage is that the bandwidth available is only the same as that for voice conversations, usually 64 kb/s at most. The twisted-pair copper telephone cables can usually carry significantly more than 64 kb/s but the telephone company needs to handle the signal as digital rather than analogue.

There are many implementations of the basic scheme, differing in the communication protocol used and providing varying levels of service. Throughput can be anything from 128 kb/s to over 8 Mb/s and the communication can be either symmetric or asymmetric, i.e. the available bandwidth may ormay not be the same upstream and downstream.

The first technology based on DSL was ISDN, although ISDN is not often recognised as such nowadays. Since then other protocols have been developed, including HDSL, SDSL, ADSL, and VDSL.

See also:
[ADSL]  [Broadband]  [Internet Access]  [ISDN]