Asymmetric Digital Subscriber Line (ADSL) borrows broadband technology from the local area network (LAN) world and uses it over ordinary copper phone lines. Unlike ISDN or domestic analogue modems no call is dialled specifically to send and receive data; the service is live continuously for data to be transmitted and received even when voice calls are in progress.

DSL's signal attenuation causes deterioration even over relatively short runs of 500m to 2000m, which could limit the distance customers can be from local exchanges unless some kind of signal regeneration is employed telecom providers' network cabinets. All DSL variants generate noise, which affects other copper pairs in the same bundle carrying DSL. ADSL is believed to be the worst offender.

A future answer may be VDSL - potentially offering up to 13Mbit/s outwards and up to 52Mbit/s inwards. It too, suffers from attenuation but will work better if telecom providers use an all-optical-fibre backbone network to join short run copper local loop in local neighbourhood cabinets.

ADSL/DSL Variants

ADSL 256kbit/s upload, 2Mbit/s download
ADSL MAX Up to 800kbit/s upload, 8Mbit/s download
ADSL 2 Plus Up to 1Mbit/s upload 24Mbit/s download
Annex M Up to 2.5Mbit/s upload 16Mbit/s download
HDSL 1.5Mbit/s to 2Mbit/s both ways (full duplex)
SDSL 2Mbit/s both ways (full duplex)
VDSL 13Mbit/s upload, 52Mbit/s download