One of the ways a home or office computer user can connect to the Internet is using DSL (Digital Subscriber Line) or one of its variations. DSL has greater digital capacity than telephone lines, and the speeds depend on the distance between the computer and the central office. There are two types of DSL: asymmetric DSL, which has a fast downstream, but may have slow upstream; and symmetric DSL, which is high speed for both downstream and upstream transactions and cannot share a line with an analog telephone. HDSL is a type of symmetric DSL.
There are several different types of symmetric DSL, including HDSL, SDSL, and IDSL. HDSL is High-bit-rate DSL, and it, too, has several types, all of which support T1, the North American dedicated digital broadband in the US, and E-1, its European counterpart. Forms of HDSL include HDSL2–7 — of which two of the most often discussed are HDSL2 and HDSL4 — and SHDSL.
The Regional Bell Operating Companies (RBOCs) were the driving force behind the development of HDSL — one of the earliest forms of DSL — in the United States, with Bellcore — which later became Telcordia Technologies — doing the initial work. It was later standardized as T1E-1.4 by ANSI (American National Standards Institute in 1999. HDSL transmits with speeds up to 1.544 Mbps on T1 with two cable pairs at 12000 ft. (3658 m) and up to 2.048 Mbps on E1 with two to three cable pairs at 12000 ft. (3658 m). The T1 and E-1 implementations are slightly different.
HDSL2, also spelled HDSL-2, was implemented to reduce costs and accomplished this by reducing the cable pairs to one. At the same time, speed was maintained over a longer distance. T1 speeds of 1.544 Mbps and E-1 speeds of 2.048 Mbps are achieved over 18000 ft. (5486 m).
HDSL4 was developed to serve lines that had to cover a farther distance than HDSL2. By using four cables, HDSL4 also steers clear of the higher frequencies that HDSL2 uses. The extra pair of cables reduces the need for repeaters, and by using HDSL2 for shorter distances and HDSL4 for longer distances, HDSL becomes unnecessary. SHDSL (Symmetric High-bit-rate Digital Subscriber Line) is rate-adaptive business-class DSL technology that is based on HDSL2. While HDSL2 is made for 1.544 Mbps, the SHDSL rate range is 192 kbps for a distance of 20,000 ft (6096 m) to 2.312 Mbps for a distance of 10,000 ft (3048 m).