DSL

in full Digital Subscriber Line

Broadband digital communications connection that operates over standard copper telephone wires. It requires a DSL modem, which splits transmissions into two frequency bands: the lower frequencies for voice (ordinary telephone calls) and the upper band for digital data, especially for connection to the Internet. Data can be transferred via DSL at much higher rates than with ordinary dial-up modem service; the range of DSL signals, however, is very small. Connections can be made only within a few miles of the nearest transmitting station. DSL and “xDSL” are umbrella terms under which a variety of protocols and technologies fall. ADSL (Asymmetric DSL) is a popular type of DSL in which most of the bandwidth of the connection is devoted to downloading data from the network to the user, leaving only a small-scale connection for uploading data. In HDSL (High bit-rate DSL) and SDSL (Symmetric DSL) the data stream is symmetric; that is, the upstream and downstream rates are the same. UDSL (Unidirectional DSL), VDSL (Very high data rate DSL), and others still under development are intended to offer even greater rates of data transmission.

DSL

(1) See domain-specific language.

(2) (Digital Subscriber Line) A technology that significantly increases the digital capacity of ordinary telephone lines (the local loops) into the home or office. DSL speeds are based on the distance between the customer and telco central office. There are two main categories. Asymmetric DSL (ADSL) is used for Internet access, where fast downstream is required, but slow upstream is acceptable. Symmetric DSL (SDSL, HDSL, etc.) is designed for connections that require high speed in both directions.

DSL provides "always-on" operation. At the telco central office, DSL traffic is aggregated in a unit called the DSL Access Multiplexor (DSLAM) and forwarded to the appropriate ISP or data network. DSL arrived in the late 1990s with more versions and alphabet soup than most any other new transmission technology. See PPPoA and PPPoE.

Asymmetric: Fast Down - Slow Up

ADSL - (Asymmetric DSL)
ADSL shares ordinary telephone lines by using frequencies above the voice band, but the higher frequencies interfere with regular telephone usage. The first versions required a visit from the phone company to install a POTS splitter that divides the line into separate lines for DSL and telephone. Subsequent splitterless versions (also known as G.Lite, Universal ADSL and ADSL Lite) eliminate the phone company visit, but require that the user plug DSL low-pass filters into every telephone outlet that serves ordinary telephones, answering machines and faxes.

ADSL is available in two modulation schemes: Discrete Multitone (DMT) or Carrierless Amplitude Phase (CAP). See CDSL, G.shdsl, ATU-C and ATU-R.

ADSL Transmission

The higher frequencies of DSL have to be filtered out for regular telephones, answering and fax machines.

The Filter

Low-pass DSL filters split the line between phone and DSL modem and must be used wherever a telephone is plugged into the wall.

RADSL (Rate Adaptive DSL)
RADSL is a version of ADSL that adjusts speeds based on signal quality. Many ADSL technologies are actually RADSL.

VDSL/VHDSL (Very High Bit Rate DSL)
VDSL is used as the final drop from a fiber optic junction point to nearby customers. VDSL lets an apartment or office complex obtain high-bandwidth services using existing copper wires without having to replace the infrastructure with optical fiber. Like ADSL, VDSL can share the line with the telephone.

Symmetric - Same Speed Both Ways

HDSL (High Bit Rate DSL)
The most mature DSL, HDSL provides T1 transmission over existing twisted pair without the additional provisioning typically required for setting up T1 circuits, such as bridged tap removal and repeater installation. HDSL requires two cable pairs up to 12,000 feet, while HDSL-2 requires only one cable pair and spans 18,000 feet. HDSL does not allow line sharing with analog phones.

SDSL (Symmetric DSL)
SDSL is an HDSL variation that is rate adaptive, uses one cable pair and is offered in speeds from 144 Kbps to 1.5 Mbps. Like HDSL, SDSL does not share lines with analog phones.

IDSL (ISDN DSL)
IDSL is a slightly faster basic BRI ISDN service. It uses the 16 Kbps "D" channel for data rather than call setup to achieve 144 Kbps instead of 128 Kbps. It also offers the longest distance of 26,000 feet. Unlike standard ISDN, IDSL does not support analog phones, and signals are not switched through the telephone network. Since IDSL uses the same 2B1Q line coding as ISDN, ISDN customers can use existing BRI terminal adapters and routers. See ISDN.

1.	(communications)	DSL - Digital Subscriber Line.
2.	(language)	DSL - Digital Simulation Language.
3.	(language)	DSL - Denotational Semantics Language.