HDSL stands for high bit rate digital subscriber line. It is similar technology to the now familiar ADSL used for broadband internet access (ADSL stands for assymetrical digital subscriber line, because it's faster in the download direction than the upload).
Essentially, HDSL is a very high performance analogue modem that can use copper twisted pairs of telephony grade to carry higher bandwidth digital data. If you think of a standard modem, it converts digital signals into analogue frequencies within the normal voiceband (typically 1800 - 3000 Hz). HDSL uses much higher frequencies, normally starting at 4kHz. HDSL uses some fancy signal processing to continuously monitor and adjust for changes in line quality arising from factors like temperature changes, interference etc. It's only by doing this that it can support such high speed signalling over cables that were not designed to those frequencies.
This means that HDSL modems are significantly more expensive than standard modems. For a decent quality, industrial strength device you may expect to spend well over $1000 each. However, that should get you a modem that could carry up to 2Mbps (bidirectional) over maybe 8km. This is way in excess of the capability of normal analogue modems.
