DACS is the name used by British Telecom (BT Group plc) in the United Kingdom for a 0+2 Pair gain system.
Why was it used ?
Where there were not enough copper pairs (also called Telephone lines or local_loops) going from the Telephone exchange ( or central office) to a subscribers address DACS was used in the 1980s,1990s and today (2000s) for getting 2 analog phone lines (or Plain old telephone service) down one physical copper pair. It was especially useful in the 1980s and 1990s when lots of new Telephone lines were needed for FAX machines.
DACS could also solve other problems:-
1) Because it is digital DACS exhibits less symptoms to low levels of RF interference than the more usual analog line. A normal phone line might annoy a subscriber by picking up music or speech from a local radio transmitter or radio ham. If using a RF Filter (signal processing) failed, the line might be converted to DACS (DACSed ?) to combat this interference. If the Interference was very high, clicks can be heard on a DACS line. If the Interference was higher the call would be lost or if the phone was off hook it might ring when it shoudn't.
2) Because a DACS line is digital for most of its length it is more difficult to BUG see bugging, so it could be used for slightly higher security
3) Because DACS is digital it can be used as a diagnostic aid for intermittent line faults. The DACS exchange unit monitors the digital part of the phone line 24 hours a day, keeping a log of what time trunk was cut or interfered with (if it led to an increase in the bit error rate) . The telephone engineer could then look back at a trunks log's to try and diagnose the cause of the line problems. Faults as strange as interference from electric trains were diagnosed by comparing DACS logs with train timetables
DACS and modems
The 56Kbps speed of recent modems requires only a single D-A conversion in the route from the isp to the end user. This means that the maximum speed possible over a dacs line was 33.6Kbps. Furthermore many 56K modems had problems negotiating down and ended up connecting at even lower speeds. Since bt don't officially support use of fast modems over thier lines there was no way for customers to force BT to remove the dacs box and fit ordinary lines suitable for fast modems.
Technical
The DACS system consisted of 2 main parts
1) The exchange equipment (EU) - Which took the analog lines two at a time and converted them to a digital trunk. One Telspec EU rack took upto 80 analog lines and produced upto 40 digital trunks. It consisted of 1 SMAC card, upto 8 ALCs and upto 8 DLCs. Pulling out the smac card on a fully populated rack could make all 80 phones rings for a little while !
2) The remote equipment or remote unit (RU) which converted the lines back to analog. The RUs were usually to be found up poles within a few 100 yards of the subscribers homes or business, unless both lines belonged to the same subscriber when the RU could be in the subscribers premises. There were 3 basic types of RU, internal, external and underground.
The remote unit contained a mini test head which was capable of testing both of the lines between the RU and subscriber for faults, it would then communicate the results back to the EU digitally where mimics were presented to the normal exchange testing equipment.
DACS2 could also provide on and off hook Caller ID (CLI)
DACS uses ISDN2 (Basic Rate ISDN ( Integrated Services Digital Network)) technology to get two phone lines down one copper pair). A call on ISDN differs from a DACS call in number of ways :-
1)Although the call travels most of the way from your home to the exchange digitally it is converted back to analog for the telephone exchange line card.
2)ISDN and DACS use different D channel signalling
3)DACS has upto 140v DC on the digital telephone line compared with the usual ISDN voltages of 48v or 90v
What was used for Pair gain before DACS ?
Before DACS a 1 + 1 analog carrier system was used, the first subscriber's phone would work more or less as normal, the 2nd subscriber would have his phone calls modulated onto an RF carrier or Carrier wave(eg WB900?) on the same physical phone line.
Before that the GPO Telephones (as BT was then known) offered Party lines, but only one subscriber could use the phone at once with this system.
Who made it ?
BT bought DACS from two different companies Telspec [1] and ECI [2]. Each BT region either installed one or the other. e.g. in South Wales ECI DACS is fitted, while in Kent Telspec DACS is fitted
Other Places to read about DACS
ADSL customers and DACS[3]
Problems with DACS and V.92 Modems
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