Digital Access Carrier System
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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.
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[edit] 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 which perform this task are known in Britain by the generic name "DACS".
DACS works by digitizing 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.
[edit] 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 subscriber's homes or business.
[edit] Advantages
- Because it uses a digital signal along most of the distance between the subscriber and the exchange, DACS is less prone to electrical interference than the more usual analogue line.
- The DACS system has in-built 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.
[edit] DACS and modems
The 56 kbit/s speed of recent 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 56K modems are unable to successfully negotiate even this speed over a DACS line.
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 56K Internet access.
[edit] Technical
This section contains more technical detail on the three main subsystems that make DACS.
1)The exchange equipment (EU) - Which took the analogue lines two at a time and converted them to a digital trunk.
One Telspec EU rack took up to 80 analogue lines, 10 per ALC (Analogue Line Card) and produced up to 40 digital trunks, 5 per DLC card. It consisted of 1 SMAC (System Maintenance and Clocks) card, up to 8 ALCs and up to 8 DLCs.
The SMAC card contained amongst other things:-
- The main 48v to 5v converter to supply the digital circuitry in the rack
- Fault mimics to present to the exchange’s test equipment
- An analogue modem to receive data calls for remote diagnostics
- A battery backed real time clock and memory to store the time and type of fault events like bit errors
- A 25 pin RS232 connector for local access to the SMAC card’s diagnostic logs
- A 2 digit 7 segment display and buttons, that formed a basic MMI, for an engineer who had no terminal
- Circuitry to generate the various clocks and pulses needed to keep the ISDN chipsets and codecs working together.
Pulling out the SMAC card on a live fully populated rack could make all 80 subscribers lines rings for a little while !
One ECI EU rack again took up to 80 analogue lines, but had just one type of card which supported 4 analogue lines, and 2 digital trunks and RUs
2)The copper pair between the EU and RU - Carries the 2B1Q signalling and the 140v DC for powering the RU and subscribers telephones.
The 140 v DC was not applied to the line until an RU was detected, so that lines men would not be shocked ! the 140 v was also removed as soon as the RU was disconnected, again for safety. The RU was distinguished from a phone or line fault, by the 8 milliamp it sinked when powered from a 48 v source. 8 milliamps were chosen because a working phone never draws a continuous 8mA under normal line conditions.
Although DACS(1+2) use the same 2B1Q Signalling as (Basic Rate ISDN ( Integrated Services Digital Network)). There are some significant differences:-
A DACS call travels most of the way from the subscriber to the exchange digitally, it is converted back to analogue to interface to the telephone exchange line card. ie ISDN has a digital interface at the exchange end and the subscriber end, DACS has an analogue interface at both the exchange end and the subscriber end.
ISDN and DACS use different D channel signalling
DACS has up to 140 V DC on the digital telephone line compared with the usual ISDN voltages of 48v or 90v
3)The RU which converted the digital trunk back to analogue.
The RUs were usually to be found within a few 100 metres of the subscribers homes or business (either up a pole or in a manhole), unless both lines belonged to the same subscriber when the RU (internal) could be in the subscribers premises.
There were 3 basic types of Telspec RU, internal (skirting board mountable) , external (pole mount) and underground (for manhole). The remote unit contains 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 provides on and off hook Caller ID (CLI) which means that an audio path is maintained between the exchange and subscriber even if the subs is on hook. Line reversals are also communicated between exchange and subscriber.
Telspec and ECI RUs, have been know to work from each others EU, but different Gain plans as well as subtle signalling and training differences means a less than perfect telephone service is provided.
[edit] Who makes it ?
BT sourced 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 used.
[edit] Some Definitions
WB900 - analogue radio frequency-based system which did not support even low speed data communications. Installed from the early 1980s. Now obsolete and rarely encountered.
DACS1 - first generation digital system which 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 installs.
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". In fact most DACS installations in the UK are now DACS2.
[edit] 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 onto 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.
[edit] See also
[edit] External links
Problems with DACS and V.92 Modems
