(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. When DSL is installed, every telephone and every other analog device in the home, such as a fax machine and dial-up modem, must connect to the filter and then to the telephone line.
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.
