Symmetric Digital Subscriber Line

Symmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line (DSL) variant with E1-like data rates (72 to 2320 kbit/s). It runs over one pair of copper wires, with a maximum range of about 3 kilometers or 1.86 miles. The main difference between ADSL and SDSL is that SDSL has the same upstream data rate as downstream (symmetrical), whereas ADSL always has smaller upstream bandwidth (asymmetrical). However, unlike ADSL, it can't co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth. It typically falls between ADSL and T-1/E-1 in price, and it is mainly targeted at small and medium businesses who may host a server on site, (eg a Terminal Server or Virtual Private Network) and want to use DSL, but don't need the higher performance of a leased line.

SDSL was never properly standardised until Recommendation G.991.2 (ex-G.shdsl) was approved by ITU-T. SDSL is often confused with G.SHDSL; in Europe, G.SHDSL was standardized by ETSI using the name 'SDSL'. This ETSI variant is compatible with the ITU-T G.SHDSL standardized regional variant for Europe.

SDSL equipment usually only interoperates with devices from the same vendor, though devices from other vendors using the same DSL chipset may be compatible. Most new installations use G.SHDSL equipment instead of SDSL.