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Conventional exchange functions; PCM switching advantages

Short answer

A conventional exchange performs line identification, digit reception, digit analysis, busy testing, path interconnection, ringing, supervision, signaling, and charging. PCM switching is economically better due to shared time-slot crosspoints, no per-exchange codecs, cheap VLSI, low maintenance; technically it gives no noise accumulation, constant loss, less crosstalk, integrated transmission and switching, ISDN/data, SS7, and SPC features.

What are the basic functions of a conventional exchange? Write economic and technical advantages of PCM switching compared to its analog switching.

Basic functions of a conventional exchange

  1. Attending (identification): continuously monitor all lines and detect a call request (off-hook) and the calling line.
  2. Information receiving: send dial tone and receive the dialled digits (pulse or DTMF).
  3. Information processing: analyse the digits to find the destination and route.
  4. Busy testing: check whether the called line or a trunk is free.
  5. Interconnection (switching): set up a speech path through the switching network between the calling and called line or trunk.
  6. Alerting: send ringing current to the called party and ring-back tone to the caller.
  7. Supervision: watch the call for answer and clear-down, and release the path when either party hangs up.
  8. Information sending: send address and line signals to other exchanges for outgoing calls.
  9. Charging and metering: record call duration and destination for billing.

Advantages of PCM (digital) switching over analog switching

Economic advantages

  • Shared crosspoints: one time-shared crosspoint or memory location serves many calls, so far fewer components are needed.
  • No codecs between exchanges in an integrated digital network: digital trunks are switched directly, removing costly A/D and D/A conversion at each exchange.
  • VLSI hardware: memories and logic chips are cheap, small and low power, so exchanges need less floor space, air conditioning and power.
  • Lower maintenance cost: no moving parts, self-diagnosis and remote maintenance by software.
  • Quick installation and easy expansion in modular units.

Technical advantages

  • No noise or loss accumulation: signals are regenerated, so quality does not depend on the number of switching stages or distance.
  • Constant transmission loss through the exchange, independent of path.
  • Less crosstalk and better speech quality.
  • Integration of transmission and switching on the same PCM format (2.048 Mbps E1).
  • Data and ISDN services carried directly, as everything is already digital.
  • Fast call set-up and easy use of common channel signaling (SS7).
  • SPC features: call waiting, forwarding, conference, itemised billing.
  • High reliability through duplicated processors and self-checking.

(Issues to note: need for network synchronisation, a codec and hybrid on every analogue line card, and echo control.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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