Digital subscriber line
Digital subscriber line (DSL) is a high-speed method of transferring data over existing twisted-pair telephone lines. DSL is commonly available through your local telephone provider or a third-party provider, if the service is available in your area.

DSL encompasses various protocols, including asymmetric DSL (ADSL), rate adaptive DSL (RADSL), high bit-rate DSL (HDSL), and symmetric DSL (SDSL). It also includes different subvariations, such as CAP-encoding ADSL versus discrete multitone (DMT)- encoding ADSL. The acronym xDSL is used to refer to the set of DSL protocols, rather than to a specific protocol.

DSL is available in several different bandwidths and flavors. The main types of DSL are ADSL – a DSL for which upload and download speeds are different, SDSL – in which upload and download speeds are the same, and Internet DSL (IDSL), which uses ISDN technology to deliver DSL. The minimum bandwidth for DSL is 128 Kbps, but speeds up to 52 Mbps are available.

If a DSL user connects via an Internet service provider (ISP), the connection to the ISP occurs over a LAN or WAN connection that the ISP has arranged with the local telephone company, rather than via the ISP’s standard modem bank. This connection may use, for example, 10Base-T Ethernet, T1, T3, Asynchronous Transfer Mode (ATM), or frame relay. The ISP then routes the data onto the Internet. Alternatively, a user may connect directly to a company network. DSL is always “on” – the connection is always available, ready to pass bits up and down the pipe, so there is no need to re-establish a connection each time you need to use it

The main limitation on DSL is that it is distance-sensitive. The signaling required for a DSL connection cannot travel more than a certain number of feet over the phone wire from the DSL equipment at the local phone switch. The bandwidth rate available to you will depend on the distance you are from the DSL equipment – the closer you are, the higher the bandwidth you can expect.