Digital Access Carrier System - Technical

Technical

This section contains more technical detail on the 3 main subsystems that make DACS.

1. The exchange equipment (EU), which converts 2 analogue lines to a digital trunk. One Telspec EU rack takes up to 80 analogue lines, 10 per ALC (Analogue Line Card), and produces up to 40 digital trunks, 5 per DLC card. It consists of 1 SMAC (System Maintenance and Clocks) card, up to 8 ALCs and up to 8 DLCs. The SMAC card contains, 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, which forms a basic MMI, for an engineer without a 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 ring for a little while! Again, one ECI EU rack takes up to 80 analogue lines, but has just one type of card, which supports 4 analogue lines, and 2 digital trunks and RUs.
2. The copper pair between the EU and RU, which carries the 2B1Q signalling and the 140V DC for powering the RU and subscribers' telephones. The 140V DC is not applied to the line until an RU is detected so that engineers do not get a shock. It is also removed as soon as the RU is disconnected, again for safety. The RU is distinguished from a phone or line fault by the 8mA it draws when powered from a 48V source. 8mA was chosen because a working phone never draws a continuous 8mA under normal line conditions. Although DACS (1 + 2) uses the same 2B1Q signalling as Basic Rate ISDN, 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, i.e. 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 140V DC on the digital telephone line as opposed to the usual ISDN voltages of 48V or 90V.
3. The RU, which converts the digital trunk back to 2 analogue trunks. The RUs are usually to be found within a few hundred metres of the subscribers' homes or businesses (either up a pole or in a manhole), unless both lines belong to the same subscriber, where the RU (internal) could be on the subscriber's premises. There are 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 that is capable of testing both lines between the RU and subscriber for faults. It then communicates the results back to the EU digitally, where mimics are 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 known to work from each other's EU, but different Gain plans as well as subtle signalling and training differences mean a less than perfect telephone service is provided.