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Symmetric digital subscriber line (SDSL) is a technology based on DSL, which enables data transfer on a single line and allows symmetric bandwidth on the upstream and downstream. The working mechanism of SDSL is considered opposite to that of asymmetric digital subscriber line (ADSL) technology, which offers much faster download than upload speeds.
SDSL supports data rates up to 3 Mbps through a single pair of copper wires running from a telephone company. It has a maximum range of 3000 meters and uses the entire bandwidth. SDSL cannot be combined with a conventional voice service on the same channel.
SDSL was developed as a proprietary technology, but standardization never took place. Therefore, it was restricted to only connect and communicate with devices from the same vendor.
SDSL is the predecessor of single-pair high-speed digital subscriber line (SDHSL), which is a data communication technology offering faster data transmission over copper telephone wires. SDHSL was standardized in February 2001 by ITU-T with recommendation G.991.2. SDSL is a DSL variant and provides T1/E1-like data rates.