DSL is a way to connect to the internet using digital capacity. There are two types: asymmetric and symmetrical. HDSL is a type of symmetrical DSL and has several types including HDSL2 and HDSL4. SHDSL is a speed-adaptable enterprise-class DSL technology based on HDSL2.
One of the ways a home or office computer user can connect to the Internet is by using DSL (Digital Subscriber Line) or one of its variants. DSL has more digital capacity than telephone lines, and speeds depend on the distance between your computer and the central office. There are two types of DSL: asymmetric DSL, which has fast downstream, but can have slow upstream; and symmetrical 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 symmetrical DSL.
There are several types of symmetric DSL, including HDSL, SDSL, and IDSL. HDSL is high-speed DSL, and it too has several types, all of which support T1, the North American dedicated digital broadband in the United States, and E-1, its European counterpart. Forms of HDSL include HDSL2–7 — of which two of the most discussed are HDSL2 and HDSL4 — and SHDSL.
Regional Bell Operating Companies (RBOC) were the driving force behind the development of HDSL – an early form 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 rates up to 1,544 Mbps over T1 with two pairs of wires at 12,000 feet (3,658 m) and up to 2,048 Mbps over E1 with two to three pairs of cables at 12000 feet (3658m) The T1 and E-1 implementations are slightly different.
HDSL2, also spelled HDSL-2, was implemented to reduce costs and achieved this by reducing cable pairs to one. At the same time, the speed was maintained over a longer distance. T1 speeds of 1544 Mbps and E-1 speeds of 2048 Mbps are achieved above 18000 feet (5486 m).
HDSL4 was developed to serve lines that had to cover a greater distance than HDSL2. By using four cables, HDSL4 also avoids the higher frequencies used by HDSL2. The extra pair of cables reduces the need for repeaters, and by using HDSL2 for shorter distances and HDSL4 for longer distances, HDSL becomes redundant. SHDSL (Symmetric High-bit-rate Digital Subscriber Line) is a speed-adaptable enterprise-class DSL technology based on HDSL2. While HDSL2 is rated for 1,544 Mbps, the SHDSL speed range is 192 kbps for a distance of 20,000 feet (6096 m) to 2,312 Mbps for a distance of 10,000 feet (3048 m).
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