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High Data Rate Digital Subscriber Line (HDSL) was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables. It was initially developed in the USA.
In the USA, there are circuits for 1.544 Mbit/s (so-called T1-lines), intented to be used for the interface to larger local exchange carrier systems, the connections from routers to other routers, and other communication systems. Before then, the transmission system used as the line code was the predominant AMI-Code (Alternate Mark Inversion). This did not have sufficient range, requires the application of repeaters over long circuits and consumes a great deal of power.
The first HDSL technique, as was established in the USA, was the line code 2B1Q, of which 784 kbit/s of a twisted pair cable's bandwidth was utilized. With two twisted pair cables, the full 1.544 Mbit/s was achieved. Since then, a new line code was implemented, the so-called CAP (Carrierless Amplitude Phase Modulation), that reached the maximum bandwidth. New HDSL transmission systems utilize this code.
With HDSL, the symmetrical bitrate will be overtaking: in both regions, respectively, 1.544 Mbit/s for T1-lines in the USA and 2.0 Mbit/s (2048 kbit/s) by the European E1-line, for example by Primary Rate Interface (PRI) from .
See also
"High data rate Digital Subscriber Line"