Symmetric Digital Subscriber Line (SDSL) can have two meanings:
- In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream. It is considered the opposite of Asymmetric Digital Subscriber Line (ADSL) technologies where the upstream bandwidth is lower than the downstream bandwidth.
- In the narrow sense SDSL is a particular DSL variant that supports data only on a single line and does not support analog calls.
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.
S-DSL is a proprietary 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.
SDSL typically falls between ADSL and T1/E1 in price and was mainly targeted at small and medium businesses who don't need the service guarantees of Frame Relay or the higher performance of a leased line..
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