Wikipedia: Digital Subscriber Line

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Digital Subscriber Line (DSL) is a popular Internet access technology, currently competing with Cable TV Internet for supremacy. DSL allows for high-speed data connections over standard copper phone lines (identical to those used for traditional local telephone service.)

For a DSL connection to function, the customer must be located less than 1800 feet (by cable length) from a Central Office. It is important that the line be in good condition, and that no filters be present, as DSL utilizes non-voice-band frequencies.

Various types of DSL exist, including ADSL (Asymmetric Digital Subscriber Line) and SDSL? (single-pair Symmetric Digital Subscriber Line), and HDSL (High Bit Rate Digital Subscriber Line). ADSL, which provides a higher downlink bandwidth than uplink bandwidth, is the most common type of DSL. Many ADSL subscribers are actually receiving HDSL, because many of the larger phone service providers had this type of technology already installed in their Central Offices. The end user never really knows that it's HDSL, because the multiplexers and other network equipment is set to mimic a "normal" ADSL signal.

Recently, DSL has been the most common type of [broadband connection]? available, due to the relative ease with which it can be rolled out to individual customers using existing infrastructure. However, the rapid consumption of free copper pairs is limiting its future potential in some areas. Moreover, DSL can only be provided within certain distances: ADSL has a limit of about 12,000 feet; SDSL to 16-17,000 feet, and IDSL (DSL supplied over an ISDN pair) a bit further.

Some of the common "DSL Killers" are:

[Bridged taps]?
[Load coils]?
"SLIC" boxes, or splitters, where copper capacity is increased by extending with multiple fibres. DSL can't travel over fibre, and most telephone companies choose not to offer fibre connections to residential customers.

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.