Working principle of a digital telephone exchange

A digital telephone exchange is a stored-program-controlled (SPC) exchange in which speech is switched in digital (PCM) form, using time and space switching of 64 kbps channels, under the control of computer processors. Examples: Alcatel E10, Siemens EWSD, Ericsson AXE, and the exchanges installed by Nepal Telecom.

Block diagram

 analogue      +-------------+  +------+
 subscribers --| Subscriber  |--|      |
 (2-wire)      | line units  |  | conc-|   +-----------+
 (BORSCHT)     |  + codec    |  |entr. |===|           |
               +-------------+  +------+   |  Digital  |
 digital trunks (E1)--[trunk interface]====| switching |
 analogue trunks--[ATI + codec]===========| network   |
                                           |  (TST)    |
 tones/announcements --[tone generator]====|           |
 signalling --[CAS/CCS (SS7) units]=======|           |
                                           +-----+-----+
                                                 |
                 +-------------------------------+---+
                 |  Control subsystem (processors,   |
                 |  memory, stored programs)         |
                 +----+-------------------------+----+
                      |                         |
              [O&M terminal, billing]   [network sync clock]

Main blocks

1. Subscriber line interface (line card). Each analogue line needs the BORSCHT functions:
   • Battery feed (−48 V DC for the telephone),
   • Overvoltage protection (lightning, power-line contact),
   • Ringing (75 V, 25 Hz AC applied to the line),
   • Supervision (detecting off-hook, on-hook, dial pulses),
   • Coding (codec: A-law PCM at 64 kbps, with filters),
   • Hybrid (2-wire to 4-wire conversion),
   • Testing (access for line tests).
2. Concentrator / remote line unit. Since only a fraction of subscribers are busy at once, many lines (e.g. 128–1000) are concentrated onto fewer PCM channels (E1 links) to the switching network. Remote units can be placed near subscriber clusters.
3. Trunk interfaces. Digital trunks (E1, 2.048 Mbps, HDB3) are connected directly after frame alignment and clock adaptation; analogue trunks need codecs and signalling conversion.
4. Digital switching network. Usually a TST (time–space–time) or multi-stage network. Time switches (time slot interchangers) move a sample from one time slot to another using speech and control memories; space switches connect a time slot on one PCM highway to the same slot on another. This connects any 64 kbps channel to any other. The network is duplicated for reliability.
5. Signalling equipment. Receivers for dial pulses and DTMF, CAS units (E1 TS16) and common channel signalling (SS7) terminals for inter-exchange signalling.
6. Service circuits. Tone generators (dial, busy, ring-back), announcement machines, conference bridges.
7. Control subsystem. Duplicated processors with stored programs carry out call processing, routing translation, charging and resource management. Control may be centralised or distributed.
8. Operation and maintenance (O&M) and billing. Terminals for subscriber data changes, traffic measurement, fault alarms, diagnostics and call detail records.
9. Synchronisation. A clock locked to the national network clock keeps all PCM links in step to avoid slips.

Working principle (a local call)

1. Off-hook detection: subscriber A lifts the handset; loop current flows; the line card's supervision scans and reports to the processor.
2. Dial tone: the processor checks A's class of service, assigns a free path and a digit receiver, and connects dial tone.
3. Digit reception: dialled digits (pulses or DTMF) are received and stored; dial tone is removed after the first digit.
4. Analysis and routing: the processor translates the number. For a local number it checks B's line; for a distant number it selects an outgoing trunk and sends signalling (e.g. SS7 IAM).
5. Path set-up: a free time slot path through the TST network is found and the speech and control memories are written.
6. Ringing: ringing current is sent to B and ring-back tone to A.
7. Answer: when B lifts the handset, ringing is tripped, the speech path is through-connected and charging starts.
8. Conversation: speech from each phone is coded to 64 kbps, switched digitally in its time slots and decoded at the other line card.
9. Clear-down: on on-hook, the processor releases the path, stops charging and records the call details.

Advantages

• No noise build-up; uniform digital quality and easy integration with digital transmission (no codec at each trunk).
• Small size, low power, fast set-up, high reliability (duplicated control).
• Many subscriber services (call waiting, forwarding, conference, abbreviated dialling), easy changes by software, centralised O&M and accurate billing.
• Supports ISDN, data and common channel signalling.