Symmetric digital subscriber line (SDSL) is a Digital Subscriber Line (DSL) variant with E1-like data rates (72Kbps to 2320Kbps). It runs over one pair of copper wires, with a maximum range of about 3 kilometers.
SDSL was never properly standardised until G.SHDSL was standardised by ITU.
SDSL is often confused with G.SHDSL.
Equipment touting SDSL support is usually proprietary equipment which only speaks to SDSL equipment from the same vendor, or to SDSL equipment from other vendors that use the same DSL chipset. Most new installations use G.SHDSL equipment instead of SDSL.
