Symmetric digital subscriber line

Symmetric digital subscriber line (SDSL) refers to digital subscriber line (DSL) technologies, that is, technologies for transmission of digital data over the copper wires of the telephone network where the bandwidth in the downstream direction, from the network to the subscriber, is identical to the bandwidth in the upstream direction, from the subscriber to the network. This symmetric bandwidth can be considered to be the opposite of the asymmetric bandwidth offered by asymmetric digital subscriber line (ADSL) technologies, where the upstream bandwidth is lower than the downstream bandwidth. SDSL is generally marketed at business customers, while ADSL is marketed at private as well as business customers.

More specifically, SDSL can be understood as:

SDSL is a rate-adaptive digital subscriber line (DSL) variant with T1/E1-like data rates (T1: 1.544 Mbit/s, E1: 2.048 Mbit/s). It runs over one pair of copper wires, with a maximum range of 10,000 feet (3,000 m). It cannot co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth.

SDSL is a technology that was never standardized. As such it usually only interoperates with devices from the same vendor. It is the predecessor of G.SHDSL which was standardized in February 2001 by ITU-T with recommendation G.991.2. SDSL is often confused with G.SHDSL and HDSL; 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.

As there is a standardised successor available, SDSL installations today are considered legacy. Most new installations use G.SHDSL equipment instead of SDSL.

...
Wikipedia