Telecom Networks and Switching (PSTN, exchanges, numbering, routing)
This section covers Telecom Networks and Switching: PSTN structure, exchanges, numbering, and routing. The exam tests whether you can trace a call from a subscriber line through the network and explain how a number becomes a route.
Core rules and concepts
- The PSTN is a circuit-switched network. A call reserves a path for its duration; resources are released at disconnect.
- Hierarchy: local exchange (LE) serves subscribers; tandem or trunk automatic exchanges (TAX) aggregate traffic; international gateways connect other countries. Subscriber lines run to the LE; trunks run between exchanges.
- Modern exchanges are stored-program-controlled digital switches using time-division switching, with space switching in larger fabrics.
- Numbering follows ITU-T E.164: maximum 15 digits, country code plus national significant number. India's country code is +91. Mobile numbers are 10 digits, commonly beginning 6-9; landlines use an STD code plus subscriber number.
- Number analysis: the exchange examines a prefix to choose an outgoing trunk group. Routing may be hierarchical, least-cost, or alternate. SS7/CCS7 signaling sets up and releases calls on a separate signaling path.
- Charging starts at answer, not at dialing, and depends on distance, time, and tariff.
Worked example
A Jaipur landline user dials 0141-2345678. The LE recognizes 0141 as a local STD code and routes locally. If the user dials 09876543210, the LE sees a 10-digit mobile beginning 98, selects a trunk group to the mobile network, and the gateway queries the HLR for the subscriber's serving MSC/VLR. The call is then paged and connected. The voice path may differ from the signaling path.
Traps that cost marks
Confusing STD code with country code; assuming every call goes through one central exchange; thinking SS7 carries the voice; treating a numbering plan as the routing table; and ignoring that mobile numbers are not 15 digits just because E.164 allows 15.