DSL (or xDSL, an umbrella term for the several types of DSL technology available) stands for Digital Subscriber Line (originally Digital Subscriber Loop). The term refers to a set of networking technologies that provide digital data transmission over local network telephone wires.

It comes in several variants, such as

• Asymmetric Digital Subscriber Line (ADSL),
• Symmetric DSL (SDSL),
• Rate-Adaptive DSL (RADSL),
• Powerline DSL (PDSL),
• High data rate DSL (HDSL), and
• Very high speed DSL (VDSL).

It can transmit and receive data at speeds ranging from 256 kbps to 24,000 kbps.

DSL typically works by using two primary bands, a high frequency band (25 kHz and above) which carries Internet Service Provider (ISP) data, and a low frequency band (4 kHz and below) which carries voice data, to divide the frequencies used in a single telephone line.

A DSL modem is required to convert data from digital signals used by computers into a voltage signal of an appropriate frequency rage that is applied to the telephone line. Also, each telephone should normally have a DSL filter installed on it to filter out the high frequencies.

It was initially implemented as part of the Integrated Services Digital Network (ISDN) specification, later reused as the ISDN Digital Subscriber Line (IDSL). The original DSL design, ADSL, was developed by Joe Lechleider at Bellcore/Telcordia Technologies in 1988.

He placed wideband digital signals above the existing baseband analog voice signal that was transmitted between telephone company central offices and users that had the conventional twisted pair cabling as their medium. He also discovered that an asymmetric arrangement (faster downloading rate than uploading rate) provided more than twice the bandwidth capacity of SDSL, so Internet Service Providers are able to provide more efficient service to consumers, enabling them to download large amounts of data without needing to upload as much.

There are two modes of transport supported by ADSL: fast channel, which is the better option for streaming multimedia; and interleaved channel, which is the better option for file transfers.

DSL may use a variety of transmission methods, depending on the market, region, carrier, and equipment. These transmission methods are

• Carrierless Amplitude Phase Modulation (CAP),
• Discrete Multitone Modulation (DMT), and
• Orthogonal Frequency-Division Multiplexing (OFDM).

Older ADSL standards could transmit data at a rate of 8 Mbps over an estimate of 2 km of unshielded twisted pair (UTP) copper wire, while the latest standard, ADSL2+, is able to transmit data at up to 24 Mbps. This depends, however, on the distance of the user from the DSL access multiplexer (DSLAM), since distances greater than 2 km negatively affect the amount of usable bandwidth on the wires, effectively lowering the data rate.