High data rate digital subscriber line
High Data Rate Digital Subscriber Line ( HDSL ) was the first DSL technology that used a higher frequency spectrum of the copper wire pair . It was first developed in the USA . In the USA there are lines for 1,544 kbit / s (so-called T1 lines ) which are used to connect larger telephone systems , to connect routers to one another and to connect other communication systems. The transmission system used for this has so far mainly used the AMI code as the line code . This does not have a long range, which requires the use of repeaters for long lines and consumes a relatively large amount of electricity.
HDSL transmits symmetrical bit rates: in both directions 1544 kbit / s for T1 lines in the USA and 2048 kbit / s for E1 lines in Europe .
2B1Q technology
The first HDSL technology that was introduced in the USA was the line code 2B1Q , which was operated at 784 kbit / s on a twin wire. The full 1544 kbit / s was then achieved with two twin wires. Three wire pairs were used for greater ranges. In Europe, HDSL technology was preferably used for leased lines with 2048 kbit / s (E1). The range that can be achieved with HDSL increases both with the cross-section of the cable cores and with the number of twisted pairs: In multi-pair systems, the data stream is distributed over the twisted pairs. In a three-pair system, the transmission rate is 784 kbit / s per line pair, which enables 2048 kbit / s. By reducing the transmission rate of the data stream, the frequency spectrum required for transmission is shifted downwards. This avoids having to use high frequency components. At high frequencies, the line attenuation of the pair rises sharply (roughly with the square root of the frequency).
Range and repeater
Under typical crosstalk conditions , such as those in cable bundles in Germany, the range of a two-pair system of the first HDSL generation, which is based on 2B1Q technology, is around 2.6 km with a conductor diameter of 0.4 mm. If only one copper pair is available for the transmission, it is reduced to 2 km.
If, under comparable conditions to the user, the 2048 kbit / s with the precursor technology for E1 leased lines, the HDB3 , (for example for a code ISDN - Primary Rate Interface ), available requires so that except for two pairs (one for each of the reciprocating and reverse direction in simplex mode ) with a range of 500 m, four repeaters that regenerate the electrical signal. Because HDSL still works without a repeater, this technology has considerable cost advantages.
The increased range of HDSL technology at E1 is mainly due to the improved line coding with 2B1Q instead of HDB3. The distances that can be bridged by HDSL can also be increased by using repeaters, which, however, cause additional costs. HDSL repeaters can usually be monitored remotely so that the defective section can be identified in a route in which several repeaters are switched on. This technology was installed by Deutsche Bahn along some tracks.
Further developments
Since the first experiences with HDSL, a new line code, the so-called CAP ( Carrier-less Amplitude and Phase Modulation ) has been made ready for use. Newer HDSL transmission systems use this code.
The further development of HDSL technology is SDSL technology.