Exchange roles and demerits of manual switching

Roles and operations of an exchange

An exchange connects a calling subscriber to any called subscriber (local or distant) on demand, using shared equipment, and releases the connection after the call. It avoids $n(n-1)/2$ direct links among $n$ users and allows costly trunks to be shared.

 Subscribers                 Other exchanges
  ──┐   ┌─────────────────────────┐   ┌──
  ──┼MDF┤ line units ─ switching ─┤ trunks
  ──┘   │   network ─ trunk units │   └──
        │        │                │
        │  control + signalling   │
        └─────────────────────────┘

Basic operations in setting up a call (switching functions):

1. Attending (call detection): the exchange continuously scans lines; when the caller lifts the handset (loop closes), a call request is detected.
2. Information receiving: a free register/digit receiver is connected and dial tone is sent; the dialled digits (pulse or DTMF) are received and stored.
3. Information processing: the digits are analysed to identify the called line or route (local, trunk, international) and the charging rate.
4. Busy testing: the called line (or outgoing trunk) is tested to see if it is free.
5. Interconnection: a path is set up through the switching network between the caller and the called line/trunk.
6. Alerting: ringing current is sent to the called phone and ringback tone to the caller; if busy, busy tone is sent.
7. Supervision: when the called party answers, ringing stops, the speech path is completed and charging (metering) starts; the exchange watches for clear-down.
8. Information sending: for calls to other exchanges, the number is signalled to the next exchange (CAS or SS7).
9. Disconnection: when either party hangs up, the path is released, charging stops and equipment returns to idle.

Other roles: charging and billing, routing traffic over alternative routes when busy, maintenance and fault detection, traffic recording, providing supplementary services (call forwarding, waiting, conference), emergency and operator services, and power feeding (−48 V) to subscriber telephones.

Demerits of manual switching

• Slow call set-up and long waiting for the operator.
• No privacy; operator can listen to calls.
• Human errors, misunderstanding of numbers/language.
• High labour cost; operators needed 24 hours.
• Limited capacity; unsuitable for large cities.
• Service depends on operator efficiency, fatigue and behaviour.