Switching and Exchanges
Switching and Exchanges
This domain tests how calls are connected through exchanges. Expect questions on circuit switching, space and time switching, exchange hierarchy, signalling, traffic measurement and blocking.
Core rules
- Circuit switching reserves a path for the entire call; it suits voice but wastes capacity when no data is sent.
- Space switching changes the physical path; time switching moves samples between time slots. Digital exchanges often combine both.
- Exchanges are hierarchical: local, tandem and trunk/transit levels. A call may use several exchanges.
- Signalling sets up, supervises and releases calls. Common-channel signalling carries control separately from voice.
- Traffic is measured in Erlangs: one Erlang is one circuit occupied continuously. Grade of service is the probability that a call is blocked.
Worked example
During a 30-minute observation, five calls each last 3 minutes. Total call time = 15 minutes, so offered traffic = 15/30 = 0.5 Erlang. This is not five Erlangs: Erlang measures occupancy, not the number of calls. If one circuit is available, 0.5 Erlang means it is busy half the time on average, but blocking can still occur at peak because arrivals are random.
Traps that cost marks
Confusing Erlang with call count; assuming a low average load prevents peak blocking; mixing signalling with switching; and forgetting that a tandem exchange does not serve subscribers directly. In numerical answers, keep minutes and hours consistent.