An HDSL variant that extends the T1 and E-1 spans to a maximum of 16,500 feet over two unshielded twisted pair (UTP) local loops. HDSL4 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 HDSL2 and SDSL,TCM features an inherent forward error correction (FEC) mechanism to overcome issues of attenuation and interference. See also attenuation, baud, bit, E-1, FEC, HDSL, HDSL2, interference, line coding, local loop, T1, TCM, and UTP.