A.6 Digital Subscriber Line
A.6.1 DSL background

Digital subscriber line (DSL) technology is a modem technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers. The term xDSL covers numerous similar yet competing forms of DSL, including asymmetric DSL (ADSL), single-line DSL (SDSL), high-data-rate DSL (HDSL), RADSL (Rate-adaptive DSL), and very-high-data-rate DSL (VDSL). xDSL is drawing significant attention from implementers and service providers because it promises to deliver high-bandwidth data rates to dispersed locations with relatively small changes to the existing telco infrastructure. xDSL services constitute dedicated, point-to-point, public network access over twisted-pair copper wire on the local loop ("last mile") between a network service provider's (NSP's) central office and the customer site, or on local loops created either intra-building or intra-campus. Currently the primary focus in xDSL is the development and deployment of ADSL and VDSL technologies and architectures. This section covers the characteristics and operations of ADSL and VDSL.