Short for symmetric digital subscriber line, a technology that allows more data to be sent over existing copper telephone lines (POTS). SDSL supports data rates up to 3 Mbps.
SDSL works by sending digitalpulses in the high-frequency area of telephone wires and can not operate simultaneously with voice connections over the same wires.
SDSL requires a special SDSL modem. SDSL is called symmetric because it supports the same data rates for upstream and downstream traffic. A similar technology that supports different data rates for upstream and downstream data is called asymmetric digital subscriber line (ADSL). ADSL is more popular in North America, whereas SDSL is being developed primarily in Europe.
Also see Cable vs. DSL in Webopedia’s “Did You Know…?” section.