High bit rate Digital Subscriber Line

High bit rate Digital Subscriber Line (HDSL) was the first DSL technology to use a higher frequency spectrum of copper, twisted pair cables. HDSL was developed in the USA, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier systems, and also to carry high-speed corporate data links and voice channels, using T1 lines.

T-carrier circuits operate at 1.544 Mbit/s. These circuits were originally carried using a line code called Alternate Mark Inversion (AMI). Later the line code used was B8ZS. AMI did not have sufficient range, requiring the application of repeaters over long circuits. As with any wire circuit, they were subject to lightning and cable trouble such as inferior splices and backhoe fade. In troubleshooting these type of services, the *felt* frequency on each conductor is 772 kHz and the repeaters are usually spaced every mile to 1.2 miles depending on conductor gauge and the whim of the engineers.

As in classical T-carrier, HDSL has a positive and negative polarity to the side of the repeater. In splicing this type of service the telcos placed the low voltage side of the repeater cable together and then the High voltage side together in the splice. The telcos have a powering end to the circuit path and this gives the polarity and the repeaters are typically powered up to 130 volts dc. Usually if you see 130 volts there is trouble because the repeaters are running FULL power to try to compensate for the trouble. They require 60 milliamps and if they cannot get it they try to achieve it by raising the voltage.

The first attempts to use DSL technology to solve the problem were done in the USA, using the line code 2B1Q. This modulation allowed for a 784 kbit/s data rate over a single twisted pair cable. With two twisted pair cables, the full 1.544 Mbit/s was achieved. The new technology attracted the attention of the industry, but could not be directly used worldwide, due to the differences between the T1 and E1 standards. A new standard was then developed by the ITU for HDSL, using the CAP (Carrierless Amplitude Phase Modulation) line code, that reached the maximum bandwidth of 2.0 Mbit/s using two pairs of copper.

HDSL gave the telcos a greater distance reach when delivering a T-1 circuit. It was marketed originally as a Non Repeated T-1, with a distance of 12k feet over 24 gauge cable. The cable gauge affects the distance. To allow for longer distances, a repeater can be used. The repeater actually terminates the circuit and regenerates the signal. Up to four repeaters can be used for a reach of 60k feet (about 20 km). This reduced the cost of maintenance when compared with AMI-based repeaters that had to be used at every 35 db of attenuation (about 1 mile).

HDSL can be used either at the T1 rate (1.544 Mbit/s) or the E1 rate (2 Mbit/s). Slower speeds are obtained by using multiples of 64 kbit/s channels, inside the T1/E1 frame. This is usually known as channelized T1/E1, and it's used to provide slow-speed data links to customers. In this case, the line rate is still the full T1/E1 rate, but the customer only gets the limited (64 multiple) data rate over the local serial interface. Unlike later ADSL, HDSL did not allow POTS at baseband.

HDSL gave way to two new technologies, called HDSL2 and SDSL. HDSL2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192 kbit/s to 2.3 Mbit/s, using a single pair of copper. SDSL is used as a replacement (and in some cases, as a generic designation) for the entire HDSL family of protocols.

ee also

* Asymmetric Digital Subscriber Line
* Symmetric Digital Subscriber Line
* Digital Subscriber Line
* Very high data rate Digital Subscriber Line

External links

* [http://www.itu.int/rec/T-REC-G.991.1/en ITU-T Recommendation G.991.1: High bit rate Digital Subscriber Line (HDSL) transceivers]
* [http://www.commsdesign.com/main/1999/08/9908feat2.htm HDSL Primer]