DSL or xDSL is a group of technologies that provide digital data transfer over the telephone network. DSL originally stood for digital subscriber loop, but is now commonly referred to as digital subscriber line. The typical transfer speeds of DSL services range from 128 kilobits per second to 24,000 kbit/s. DSL technology was originally begun as a part of the Integrated Services Digital Network (ISDN). ISDN is a series of protocals designed implemented to allow for digital transmission of voice and data over the telephone networks standard copper wires. This resulted in better speeds and quality than the old analog system.

Most Common Forms of DSL

Asymmetric Digital Subscriber Line (ADSL) is a form of DSL that enables faster transmission of data over these same copper wires than a standard dial up modem can. It accomplishes this by using the frequencies that are normally not used by a voice telephone call, in particular, frequencies higher than normal human hearing.

The drawback to ADSL is that it is ony effective over relatively short distances. The user must be within 5 km of the telephone companies Central Office. Once there, the ADSL signal is stripped off and immediately routed onto a conventional internet network, while any voice-frequency signal is switched into the conventional phone network. This is allows a single phone line to be used for both voice and data transfer.

The most distinguishing characteristic of ADSL is that data transmission speeds are greater in one direction than the other. This is often marketed to small businesses or residential users who are most interested in accessing the internet at an affordable price. It allows for higher download speeds, thus decreasing the users time to view websites and download files. This is not recommended if one has a need to upload large quantities of information or run servers that would require large amounts of bandwidth in the other direction.

Symmetric Digital Subscriber Line (SDSL) is a DSL variant that enables the user to have the same data transfer rate in both directions. This technology has a transfer speed range of 72 to 2320 kbit/s. The primary drawback is that you must be even closer to the telephone companies central office. A maximum distance of 3 km is all that is supported. It is also much more expensive.

Typical DSL technologies include:

High Data Rate Digital Subscriber Line (HDSL), the first DSL technology that uses the higher frequesncy spectrum to transmit data

Symmetric Digital Subscriber Line (SDSL), a standardised version of HDSL

Asymmetric Digital Subscriber Line (ADSL), a version of DSL with a slower upload speed

Rate-Adaptive Digital Subscriber Line (RADSL), A variation of ADSL that adjusts the upstream speed of the connection in an attempt to maintain a specific downstream speed.

Very-high-bit-rate Digital Subscriber Line (VDSL), An improvement on HDSL that Provides a faster rate of transmission speed. It uses up to 4 different frequency bands, two for upstream (from the client to the telco) and two for downstream.