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Telephone exchange structure; TST vs STS

Short answer

A digital exchange has subscriber line units (BORSCHT), switching network, trunk interfaces, signaling, SPC control, O&M, power and sync. TST uses Time-Space-Time with time expansion, lower blocking, non-blocking if l>=2c-1, for large exchanges; STS uses Space-Time-Space with space expansion, higher blocking, non-blocking if k>=2N-1, for small switches.

Describe technical structure of a telephone exchange. Compare TST and STS switch used in digital telephone exchange system.

Technical structure of a telephone exchange

A telephone exchange connects any subscriber line or trunk to any other on demand. It has the following main subsystems:

 Subscribers                         Other exchanges
   |                                       ^
 +------------+  +-------------+  +-------------+
 | Subscriber |->|  Switching  |->|  Trunk /    |
 | line units |<-|  network    |<-|  junction   |
 | (BORSCHT)  |  |  (T, S, TST)|  |  interface  |
 +------------+  +-------------+  +-------------+
      ^   ^             ^             ^
      |   +--------+    |    +--------+
      |   | Signal-|    |    |
      |   | ling   |<-->+<-->|  Control (SPC
      |   +--------+         |  processors)
      +--------------------> +---------------+
               O&M: charging, testing, admin
  1. Subscriber line interface: each line card does BORSCHT (battery feed, over-voltage protection, ringing, supervision, codec, hybrid, testing). Line concentrators combine many lightly used lines onto fewer switch ports.
  2. Switching network: makes the speech path. In digital exchanges it is a TST (or TSST) network of time switches and space switches.
  3. Trunk/junction interface: connects to other exchanges on E1 (2.048 Mbps) PCM links, with frame alignment and clock synchronisation.
  4. Signaling equipment: subscriber signaling (dial tone, DTMF receivers, ringing, busy tone) and inter-exchange signaling (CAS or CCS/SS7).
  5. Control unit: SPC processors that detect calls, analyse digits, find paths, set the switch, supervise calls and release them.
  6. Operation and maintenance: charging (billing) records, traffic measurement, alarms, testing and subscriber administration.
  7. Power and synchronisation: 48 V DC battery plant and a master clock.

Comparison of TST and STS switches

PointTST switchSTS switch
Stage orderTime – Space – TimeSpace – Time – Space
Centre stageSpace switch (TMS)TSIs
Alternative pathsll internal time slotskk centre TSIs
Blocking[1−(1−p/α)2]l[1-(1-p/\alpha)^2]^l, lower[1−(1−p/β)2]k[1-(1-p/\beta)^2]^k, higher
Expansion methodTime expansion, cheapSpace expansion, costly
Non-blocking conditionl≥2c−1l \ge 2c - 1k≥2N−1k \ge 2N - 1
Cost for large sizeLowerHigher
UseLarge digital exchanges (e.g. Ericsson AXE)Small switches, light load

TST is preferred in practice because, for the same blocking, it needs less hardware and its paths are easy to find by searching time slots.

✓ Answer checked by Claude Opus 5.5 (high effort)

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