--- title: "Switching hierarchy and evolution until the telephone" subject: "Telecommunications" chapter: 1 key: "semester_8/telecommunications:switching-hierarchy-telephone-evolution" source: https://www.bctengineer.com/q/semester_8/telecommunications/switching-hierarchy-telephone-evolution site: BCT Engineering --- # Switching hierarchy and evolution until the telephone Telecommunications · Chapter 1: Telecommunication Networks - Asked in: 2081 Chaitra, 5+3 marks - 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/switching-hierarchy-telephone-evolution - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1 ## Question Draw the switching hierarchy used in the telecommunication switching system. Briefly explain the evolution of telecommunication until the telephone. ## Short answer Exchanges are arranged in levels (Class 5 end office up to Class 1 regional centre) with vertical final routes and direct high-usage trunks. Telecommunication evolved from ancient signalling and optical telegraph to electric telegraph (1837) and Alexander Graham Bell's telephone (1876), which led to telephone exchanges. ## Answer #### Switching hierarchy Exchanges are arranged in levels so that a call between any two subscribers passes through as few links as possible and traffic can overflow to higher levels when direct routes are busy. ```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 5 – End office (local exchange):** subscribers are connected through local loops. - **Class 4 – Toll centre / tandem:** joins end offices and gives access to long-distance network. - **Class 3 – Primary centre**, **Class 2 – Sectional centre**, **Class 1 – Regional centre:** higher levels of the long-distance network; regional centres are fully meshed. - **Routing rule:** first try a direct high-usage trunk; if busy, overflow upward; the last choice is the final route along the hierarchy. #### Evolution of telecommunication up to the telephone 1. **Ancient signalling:** drums, smoke and fire signals, messengers and carrier pigeons – line of sight or physical transport. 2. **Optical (semaphore) telegraph, 1790s:** Claude Chappe's towers with movable arms in France sent coded messages from tower to tower. 3. **Electric telegraph, 1837–1844:** Cooke and Wheatstone (UK) and Samuel Morse (USA) sent messages as electric pulses over wires using Morse code; the first Washington–Baltimore line (1844). Telegraph networks and undersea cables (1866 transatlantic) followed. 4. **Telephone, 1876:** Alexander Graham Bell patented the telephone, converting voice to a varying electric current with a transmitter and back to sound in a receiver. Edison's carbon microphone improved it. This led directly to telephone exchanges (1878) and the switching systems above. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the hierarchy, what is the last-choice route when direct trunks are busy? A. Another subscriber's line B. The final route along the hierarchy C. A manual operator D. A satellite link Answer: B. Traffic overflows upward and finally uses the final route. (Key point: Final route) 2. What is the function of a class 4 toll centre/tandem? A. Connects subscribers directly through local loops B. Is the top fully meshed level C. Joins end offices and gives access to the long-distance network D. Handles only charging Answer: C. Class 5 serves subscribers; class 4 joins end offices and provides toll access. (Key point: Class 4) 3. What was the 1790s Chappe system? A. An electric telegraph on wires B. A telephone using carbon microphones C. A radio transmitter using Morse code D. Towers with movable arms passing coded messages from tower to tower Answer: D. Chappe's semaphore was an optical telegraph in France, before electric systems. (Key point: Optical telegraph) 4. What did Bell's telephone do? A. Converted voice to a varying electric current and back to sound B. Sent on/off pulses coded in Morse C. Switched calls automatically D. Transmitted by radio waves Answer: A. It used a transmitter and receiver to carry voice, which led to exchanges in 1878. (Key point: Telephone) ## Close variants set in other exams - [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3× ## Related in Telecommunications - [Evolution of telecommunication and switching systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aevolution-telecom-switching) — Telecommunications, chapter 1, asked 3× - [Milestones in the evolution of telecommunication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aevolution-of-telecommunication) — Telecommunications, chapter 1, asked 2× - [Digital vs manual switching; evolution in Nepal](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-vs-manual-nepal-evolution) — Telecommunications, chapter 1 - [Switching hierarchy and Marconi's transmitter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-marconi) — Telecommunications, chapter 1 - [Digital switching system and its functions](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-switching-functions) — Telecommunications, chapter 4 - [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2× ## 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×