--- title: "Telephone exchange and switching hierarchy" subject: "Telecommunications" chapter: 1 key: "semester_8/telecommunications:exchange-switching-hierarchy" source: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-switching-hierarchy site: BCT Engineering --- # Telephone exchange and switching hierarchy Telecommunications · Chapter 1: Telecommunication Networks - Asked in: 2081 Baisakh, 3+5 marks; 2078 Bhadra, 3+4 marks (asked 2 times) - Video: [PSTN - Public Switched Telephone Network](https://www.youtube.com/watch?v=fukXUnLVCuE) — Sunny Classroom - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-switching-hierarchy - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1 ## Question What is telephone exchange? Draw the switching hierarchy used in telecommunication switching system. ## Short answer A telephone exchange connects subscribers and trunks. The hierarchy has five levels: Class 5 end offices, Class 4 toll centres, Class 3 primary, Class 2 sectional, and Class 1 regional centres. Direct high-usage trunks handle heavy traffic; overflow uses the hierarchical final route. ## Answer #### Telephone exchange A **telephone exchange** (switching office or central office) is the system that connects the line of a calling subscriber to that of the called subscriber, or to a trunk leading to another exchange, for the duration of a call and releases it afterwards. It avoids wiring every telephone to every other ($n(n-1)/2$ links). Its functions include detecting a call, receiving the dialled number, finding a free path, ringing the called party, supervising the call, charging and clearing. #### Switching hierarchy Exchanges are arranged in a hierarchy so that any two subscribers can be connected with a small number of trunks. The classic (AT&T/North American) five-level hierarchy is: ```text Class 1: Regional centre / \ Class 2: Sectional Class 2: Sectional centre centre | | Class 3: Primary Class 3: Primary centre centre | | Class 4: Toll centre Class 4: Toll centre / \ | Class 5 Class 5 Class 5 end office end office end office | | | | subscribers subscribers subscribers Final (hierarchical) route: ─── (vertical links) High-usage trunks: direct links between busy offices at any level (shortcuts) ``` | Class | Name | Function | |---|---|---| | 5 | End office (local exchange) | Subscribers connected directly | | 4 | Toll centre / tandem | Connects local exchanges, long-distance entry | | 3 | Primary centre | Groups toll centres | | 2 | Sectional centre | Groups primary centres | | 1 | Regional centre | Top level, fully interconnected | **Routing:** a call first tries a **high-usage trunk** (direct route) between the two exchanges; if all are busy it overflows upward to the next level, and finally uses the **final route** through the hierarchy. ITU-T uses similar levels: local exchange, primary, secondary, tertiary and quaternary (international) centres. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a telephone exchange do? A. Connects a calling line to the called line or a trunk for the duration of a call and releases it afterwards B. Permanently wires every telephone to every other C. Only converts voice to digital form D. Only generates ringing current for the subscriber Answer: A. An exchange sets up, supervises and releases connections on demand, avoiding n(n-1)/2 links. (Key point: Exchange role) 2. In the five-level hierarchy, which class is the end office where subscribers connect? A. Class 1 B. Class 5 C. Class 3 D. Class 4 Answer: B. Class 5 is the local end office; class 1 (regional centre) is the top. (Key point: Hierarchy classes) 3. In the hierarchy, how is a call routed? A. Always go through the regional centre first B. Use only the final route and never direct trunks C. Try a direct high-usage trunk first, overflow upward if busy, and finally use the final route D. Try the final route first, then a high-usage trunk Answer: C. High-usage trunks are shortcuts; the final route along the hierarchy is the last choice. (Key point: Routing) 4. Which statement about the top class is true? A. Class 4 toll centres are fully meshed at the top B. Class 5 offices are fully meshed with each other C. Sectional centres connect subscribers directly D. Regional centres (class 1) are fully interconnected Answer: D. The regional centres at the top are fully meshed. (Key point: Top level) ## Close variants set in other exams - [Switching hierarchy and evolution until the telephone](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-telephone-evolution) — Telecommunications, chapter 1, asked 3× ## Related in Telecommunications - [Telephone exchange structure; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-structure-tst-sts) — Telecommunications, chapter 4 - [Switching hierarchy and Marconi's transmitter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-marconi) — Telecommunications, chapter 1 - [Evolution of telecommunication and switching systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aevolution-telecom-switching) — Telecommunications, chapter 1, asked 3× - [Exchange roles and demerits of manual switching](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-roles-manual-demerits) — Telecommunications, chapter 1 - [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4× - [Conventional exchange functions; PCM switching advantages](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aconventional-exchange-pcm-advantages) — Telecommunications, chapter 4 ## Related in other subjects - [FDM hierarchy in telephone system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-hierarchy-telephone) — Data Communication, chapter 6 - [E1 telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Ae1-hierarchy) — Computer Networks and Security, chapter 2 - [FDM vs TDM; FDM telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-vs-tdm-fdm-hierarchy) — Communication Systems, chapter 5 - [FDMA; FDM in telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-fdm-telephony-hierarchy) — Communication Systems, chapter 5, asked 2×