This Recommendation specifies a High bit rate Digital Subscriber Line (HDSL) which is a bidirectional and symmetrical transmission system that allows the transport of signals with a bit rate of 1544 kbit/s or 2048 kbit/s on the copper twisted pairs of an access network. The basic work has been carried out in the ANSI T1 committee for 1544 kbit/s signals. The results of this work were taken by TM6 of ETSI and adopted for 2048 kbit/s signals in a technical specification, which built the basis for this Recommendation.
The HDSL system uses echo cancellation technique for the separation of the directions of transmission, so that one twisted pair can carry both directions. Two different options for the line code are recommended, the Pulse Amplitude Modulation 2B1Q and the Carrierless Amplitude/Phase Modulation CAP. CAP is applicable for 2048 kbit/s only, while for 2B1Q two different frames for 1544 kbit/s and 2048 kbit/s are defined. The 2B1Q for 2048 kbit/s caters for both duplex transmission on a single pair and parallel transmission on two or three-pairs. This allows for the distribution of the signal to several pairs and for reduction of the symbol rate and an increase of the line length. CAP is defined for one- or two-pairs only and the 1544 kbit/s 2B1Q for two-pairs only.