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Classification of switching systems

Short answer

Switching systems are manual, electromechanical automatic (Strowger, crossbar), and electronic SPC with space or time division; transfer modes are circuit, message and packet. Digital exchange functions include supervision, digit reception, processing, busy test, interconnection, alerting, call supervision, signalling, release, charging and maintenance.

Classify various switching systems in telecommunication. Mention functions of digital switching system.

Classification of switching systems

            Switching systems
           /                 \
      Manual              Automatic
                       /            \
          Electromechanical      Electronic (SPC)
            /        \             /         \
      Strowger    Crossbar    Space       Time division
   (step-by-step)            division      (digital)
                         (centralised or distributed
                                control)
  • Manual: operator connects calls (local battery, central battery boards).
  • Electromechanical automatic:
    • Strowger (step-by-step) – direct progressive control by dial pulses.
    • Crossbar – common control with markers and registers.
  • Electronic (Stored Program Control): a computer controls switching.
    • Space-division – separate physical path per call (reed relays, crosspoints).
    • Time-division – shared paths with time slots; analogue PAM or digital PCM (time and T-S-T switches).
    • Control can be centralised or distributed.
  • By information transfer: circuit switching, message switching and packet switching.

Functions of a digital switching system

  1. Attending/supervision: continuously scan lines and trunks to detect off-hook (call request) and on-hook.
  2. Information receiving: give dial tone and receive dialled digits (pulse or DTMF).
  3. Information processing: analyse digits, translate the number and decide routing and charging.
  4. Busy testing: check whether the called line or trunk is free.
  5. Interconnection: set up a path through the time/space switch network (assign time slots).
  6. Alerting: ring the called party and send ringback tone to caller.
  7. Supervision during the call: watch for answer and clear-down; start/stop charging.
  8. Information sending: signal to other exchanges (CAS/SS7) for outgoing calls.
  9. Disconnection/release of the path at the end of call.
  10. Charging, maintenance and administration: billing records, fault detection, traffic statistics, subscriber data changes, and supplementary services (call forwarding, waiting, conference).

✓ Answer checked by Claude Opus 5.5 (high effort)

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