Telecommunication Networks and Switching
This domain covers how calls are routed and switched: circuit, message and packet switching, PSTN hierarchy, signalling, numbering and traffic engineering. Expect Erlang calculations and questions on SS7 and exchange components.
Core rules
- Circuit switching reserves a dedicated path for the call duration; packet switching shares links and suits data.
- The PSTN hierarchy runs from local exchange to tandem/secondary exchange to trunk exchange and international gateway.
- Traffic intensity in Erlangs: A = (number of calls × average holding time in seconds) / 3600, measured in the busy hour.
- Grade of service (GOS) is the probability of call blocking; Erlang B tables relate offered traffic, trunks and blocking.
- SS7 (CCS7) is an out-of-band common-channel signalling system using signalling links separate from voice circuits.
- NGN and VoIP separate call control from media, using protocols such as SIP and RTP.
Worked example
During the busy hour an exchange handles 60 calls with an average holding time of 3 minutes. Total holding time = 60 × 180 = 10,800 seconds. Traffic A = 10,800/3600 = 3 Erlangs. If one trunk carries 1 Erlang when fully occupied, at least 3 Erlangs of trunk capacity is needed before allowing for GOS and peakedness.
Traps that cost marks
- Using call count instead of call-seconds in the Erlang formula.
- Measuring traffic outside the busy hour.
- Confusing GOS with call completion rate.
- Treating SS7 as in-band signalling.
- Mixing exchange hierarchy terms used by different administrations.
Exam approach
For traffic sums, convert minutes to seconds and divide by 3600. For switching questions, state whether the service is connection-oriented or connectionless and justify it.