High-bit-rate digital subscriber line
High Data Rate Digital Subscriber Line ( HDSL ) was the first DSL technology, which used a higher frequency spectrum of copper wire pairs. It was initially developed in the USA. In the U.S. there are lines for 1,544 kbit / s (called T1 lines ), which are used for the connection of larger PBXs, the connection of routers with each other and other communication systems. The transmission system used for this purpose previously used as a line code mainly the AMI code. This has no long range, then what with long lines using repeaters requires and consumes relatively much energy.
With HDSL symmetrical bit rates are transferred: In both directions 1544 kbit / s for T1 lines in the U.S. or 2048 kbit / s in the E1 lines in Europe.
2B1Q technology
The first HDSL technology which was introduced in the United States was the 2B1Q line code, operating at 784 kbit / s in a twisted pair. With two double wires were then the full 1544 kbit / s achieved. For longer ranges three pairs were used. In Europe, the HDSL technology was preferably used for s leased lines with 2048 kbit / (E1). The achievable with HDSL range increases with both the cross-section of the cable strands as well as the number of double cores: For multi- pair systems, the data flow is distributed to each of the pairs. In a three- pair system, the transmission rate is 784 kbit / s per line pair, and thereby the 2048kbit / s are possible. This reduction of the transmission rate of the data stream, the spectrum required for transmission shifts down. This is to avoid that high frequency components must be used. At high frequencies, the line loss of the twin cores increases sharply ( approximately with the square root of the frequency).
Reach and Repeater
Under typical crosstalk conditions as they exist in Germany in cable bundles, the range is a two - pair HDSL system of the first generation, which is based on 2B1Q technology, a wire diameter of 0.4 mm, about 2.6 km. If there is only one copper pair available for transmission, it is reduced to two kilometers.
If, under comparable conditions the user to 2048 kbit / s with the precursor technology for E1 leased lines, the HDB3 code so requires ( for example, for an ISDN Primary Rate Interface ) available that except for two pairs ( one for each way and reverse direction in the simplex mode ) at a range of 500 meters four repeaters that regenerate the electrical signal. Because HDSL still manages here without repeater, this technology has significant cost advantages.
The gain in range of HDSL technology at E1 is mainly produced by the improved line coding with 2B1Q instead of HDB3. The bridged by HDSL distances can also be larger, but which cause additional costs through the use of repeaters. HDSL repeater can usually be monitored remotely so that at a distance, in the more repeaters are turned on, the damaged section can be identified. This technique has been installed from the DB along some railway tracks.
Developments
Since the first experiences with HDSL, a new line code that brought so-called CAP (Carrier -less Amplitude and Phase Modulation) to deployment. Newer HDSL transmission systems use this code.
The development of the technique is the SDSL HDSL technology.