Acronym for Single Line Digital Subscriber Line
Although HDSL is attractive for replacing existing T1 lines, it is not suitable for residential subscribers because it requires two twisted pair, whereas the typical residential subscriber has a single twisted pair. SDSL was developed to provide the same type of service as HDSL by over a single twisted-pair line. As with HDSL, 2B1Q encoding is used. Echo cancellation isused to acheive full-duplex transmission over a single pair.
-- Taken from "Data and Computer Communications" - William Stalling - 6th Ed, 
ISBN:0130863882
