An access technology developed as a more costeffective means of providing T1 local loop circuits over existing unshielded twisted pair (UTP). HSDL was developed by Bellcore (now Telcordia Technologies) at the request of the Regional Bell Operating Companies (RBOCs) in the United States, and was later standardized in 1999 by the American National Standards Institute (ANSI) as T1E-1.4. HDSL eliminates repeaters in the T1 local loop for distances up to 12,000 feet, which can be extended another 12,000 feet through the use of a line doubler, which essentially is an HDSL repeater. In the North American implementation of HDSL, the upstream and downstream signals are split across both pairs, with each pair operating in full-duplex (FDX) mode at 784 kbps, which is half the T1 rate plus additional overhead. In the European implementation, each of two pairs operates at 1.168 Mbps, which is roughly half the E-1 rate, plus additional overhead. The HDSL line coding scheme is 2B1Q, also known as 4 PAM (4-level Pulse Amplitude Modulation). As 2B1Q impresses two bits on each symbol, each of which is in the form of one of four voltage levels. The symbol rate, therefore, is one-fourth the line rate, meaning that an HDSL T1 implementation at a line rate of 784 kbps across each of two pairs requires a carrier frequency of only 196 kHz, at least at the peak power level. At this relatively low frequency, issues of attenuation and crosstalk are mitigated. 0 +3V -3V 0 +3V -3V 1 1 0 0 0 0 0 0 1 1 1 1 0 0 0 V 0 0 1 1 Bit Pattern HDB3 with Bipolar Violation HDSL2 is an HDSL variant that supports T1 and E-1 over a single twisted pair, with a maximum transmission span of 13,200 feet. A variant known as HDSL4 can run over two twisted pair in order to extend the maximum transmission span to as much as 16,500 feet. See also attenuation, 2B1Q, crosstalk, E-1, FDX, HDSL2, HDSL4, line doubler, local loop, overhead, power, repeater, SDSL, symbol, T1, UTP, and voltage.