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.

The main difference between ADSL and SDSL is that SDSL has the same upstream data transfer
rate as downstream (symmetrical), whereas ADSL always has smaller upstream bandwidth (asymmetrical).

SDSL was never properly standardised until G.SHDSL was standardised by ITU. SDSL
is often confused with G.SHDSL and unfortunately, 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.

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.