Digital access carrier system (DACS) is the name used by British Telecom (BT Group plc) in the United Kingdom for a 0+2 pair gain system. Usage For almost as long as telephones have been a common feature in homes and offices, telecommunication companies have regularly been faced with a situation where demand in a particular street or area exceeds the number of physical copper pairs available from the pole to the exchange. Until the early 1980s, this situation was often dealt with by providing shared or 'party' lines, which were connected to multiple customers. This raised privacy problems since any subscriber connected to the line could listen to (or indeed, interrupt) another subscriber's call. With advances in the size, price, and reliability of electronic equipment, it eventually became possible to provide two normal subscriber lines over one copper pair, eliminating the need for party lines. The more modern ISDN technology based digital systems that perform this task are known in Britain by the generic name 'DACS'. DACS works by digitising the analogue signal and sending the combined digital information for both lines over the same copper pair between the exchange and the pole. The cost of the DACS equipment is significantly less than the cost of installing additional copper pairs. Overview The DACS system consists of three main parts: The exchange unit (EU), which connects multiple pairs of analogue lines to their corresponding single digital lines. One Telspec EU rack connects as many as 80 analogue lines over 40 digital copper pairs. The copper pair between the exchange and the remote unit, carrying the digital signal between the exchange unit and the remote unit. The remote unit (RU), which connects two analogue customer lines to one digital copper pair. The RUs are usually to be found on poles within a few hundred metres of the subscribers' homes or businesses. Advantages Because it uses a digital signal along most of the distance between subscriber and exchange, DACS is less prone to electrical interference than the more usual analogue line. The DACS system has built-in monitoring from the exchange. An alert is generated if the connection is lost or errors occur. This contrasts with a conventional analogue line, where the fault will usually not be known until a customer complains. DACS and modems The 56kbit/s speed of analogue modems can only be achieved if there is a single digital to analogue conversion