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 56 kbit/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. Technical This section contains more technical detail on the 3 main subsystems that make DACS. Who makes it? BT sourced DACS from two different companies: Telspec and ECI. Each BT region installed either one or the other; e.g. in South Wales, ECI DACS is fitted, while in Kent, Telspec DACS is used. Definitions WB900 – an analogue radio frequency based system that did not support even low speed data communications. Installed from the early 1980s. Now rarely encountered. DACS1 – first generation digital system that did not support CLI but supported low-speed data communication devices such as fax machines. Installed from around 1990. DACS1 is no longer used in new installations. DACS2 – released in the mid 1990s, DACS2 was an upgrade to DACS1 with support for CLI and higher data speeds (but see below). DACS2 is fundamentally similar to DACS1 in operation. DACS – DACS1 and DACS2 are commonly known simply as 'DACS'. Most DACS installations in the UK are now DACS2. How did WB900 work? Before DACS, WB900 (a 1 + 1 analogue carrier system) was used. The first subscriber's phone (called the 'audio customer') would be connected as normal. The second subscriber (called the 'carrier customer') would have his phone calls modulated on to an RF carrier or Carrier wave on the same physical phone line at around 40 kHz – high enough not to be noticeable to the audio customer. See also Subscriber Loop Carrier Digital access and cross-connect system – an American system for which the initialism DACS is also used. Notes External links ADSL customers and DACS at virgin.net frank.gwc.org.uk Problems with DACS and V.92 Modems BT Group Local loop Telecommunications equipment