ADSL (Asymmetric DSL) and variants

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 (Plain Old Telephone Service) 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).

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