Digital Access Carrier System - DACS and Modems

DACS and Modems

The 56kbit/s speed of analogue modems can only be achieved if there is a single digital to analogue conversion in the route from the ISP to the end user. Since DACS involves an additional conversion to digital, and then back to analogue, this means that the maximum possible bitrate over a DACS line is 33.6 kbit/s. Furthermore, many 56kbit/s modems are unable to successfully negotiate even this speed over a DACS line. DSL broadband internet connections cannot work on a DACS line as they rely on a copper pair running all the way to the telephone exchange.

Since BT's traditional telephone line service is contractually only required to support voice and fax communication, BT are not obliged to remove a DACS because of problems with 56kbit/s modems.