An HDSL variant that supports T1 and E-1 spans over a single unshielded twisted pair (UTP) local loop of 24 AWG up to 13.2 kft., 768 kbps up to 17.7 kft., and 384 kbps up to 22.5 kft. An HDSL2 line doubler, i.e., repeater, can double the distance for each speed rating. HDSL2 employs a line coding technique known as trellis-coded pulse amplitude modulation (TC-PAM), also known as trellis-coded modulation (TCM). This technique places three bits on a baud, which is an improvement over the two bits per baud realized through the 2B1Q technique used in HDSL. Also used in HDSL4 and SDSL, TCM features an inherent forward error correction (FEC) mechanism to overcome issues of attenuation and interference. See also 2B1Q, attenuation, AWG, bit, baud, E-1, FEC, interference, line coding, line doubler, local loop, PAM, repeater, SDSL, T1, TCM, and UTP.