Digital switching system and its functions

Digital switching system

A digital switching system is an exchange that switches voice and data in digital (PCM) form, using time-division switching under stored program control. Speech from analogue lines is sampled at 8 kHz, coded into 8-bit words and multiplexed onto 32-slot (2.048 Mbps) highways. The switching network then moves each word from its incoming slot and highway to the required outgoing slot and highway using time switches (T) and space switches (S), normally arranged as TST.

 Analog   +---------+  +------------+  +--------+  Digital
 lines -->| Line    |->| Switching  |->| Trunk  |--> trunks
          | card +  |  | network    |  | inter- |   (E1)
          | codec   |<-| (T-S-T)    |<-| face   |
          +---------+  +------------+  +--------+
               ^              ^             ^
               +------ Control processor ---+
                     (SPC) + signaling

Main parts: line interface (BORSCHT), concentrator/multiplexer, digital switching network, trunk interface, signaling units (DTMF receivers, SS7), control processor, and O&M system.

Features: no moving parts, time-shared hardware, regeneration of signals (no noise build-up), integration with digital transmission, and easy new services through software.

Functions of a switching system

1. Attending: detect a call request (off-hook) on any line.
2. Information receiving: give dial tone and receive dialled digits.
3. Information processing: analyse digits to find the called party and route.
4. Busy testing: check if the called line or outgoing trunk is free.
5. Interconnection: set up a path through the switching network.
6. Alerting: ring the called party; send ring-back tone to the caller.
7. Supervision: watch for answer and clearing; release the path at the end.
8. Information sending: send signaling to other exchanges for outgoing calls.
9. Charging: record call details for billing.
10. Operation and maintenance: fault detection, testing, traffic records, subscriber data changes.