High bit rate Digital Subscriber Line

High-bit-rate digital subscriber line (HDSL) is a telecommunications protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum of copper, twisted pair cables. HDSL was developed to transport DS1 services at 1.544 Mbit/s and 2.048 Mbit/s over telephone local loops without a need for repeaters. Successor technology to HDSL includes HDSL2 and HDSL4, proprietary SDSL, and G.SHDSL.

HDSL was developed for T1 service at 1.544 Mbit/s by the American National Standards Institute (ANSI) Committee T1E1.4 and published in February 1994 as ANSI Technical Report TR-28. This American variant uses two wire pairs with at a rate of 784 kbit/s each, using the 2B1Q line code, which is also used in the American variant of the ISDN U interface. First products were developed in 1993. A European version of the standard for E1 service at 2.048 Mbit/s was published in February 1995 by the European Telecommunications Standards Institute (ETSI) as ETSI ETR 152. The first edition of ETR 152 specified the line code 2B1Q on either three pairs at 784 kbit/s each or two pairs at 1,168 kbit/s each. A second edition of ETR 152, published in June 1995, specified trellis coded carrierless amplitude/phase modulation (CAP) as an alternative modulation scheme, running on two pairs at 1,168 kbit/s each. A third version of ETR 152, published in December 1996, added the possibility of using a single CAP-modulated pair at 2,320 kbit/s. Later, an international HDSL standard was published by Study Group 15 of the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) on 26 August 1998 and adopted as recommendation ITU-T G.991.1 on 13 October 1998.

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