Specified by the ITU-T as G.991.2 (February 2001), SHDSL is a business-class DSL technology that supports symmetric rate-adaptive transmission rates ranging range from 192 kbps at 20,000 feet (6 kilometers) to 2.312 Mbps at 10,000 feet (3 kilometers) in increments of 8 kbps over a single unshielded twisted pair (UTP) local loop. An optional two-pair mode supports transmission rates ranging from 384 kbps to 4.624 Mbps in increments of 16 kbps. In either mode, line doublers can approximately double the distance. SHDSL uses the same trellis-coded pulse amplitude modulation (TC-PAM) line coding technique as HDSL2 and HDSL4. SHDSL also operates in a fixed-rate mode at 784 kbps and 1.544 Mbps. See also DSL, G Series, ITU-T, line coding, line doubler, local loop, modulation, rate adaption, symmetric, TC-PAM, transmission rates, UTP, and xDSL.