--- title: "Evolution of telecommunication and switching systems" subject: "Telecommunications" chapter: 1 key: "semester_8/telecommunications:evolution-telecom-switching" source: https://www.bctengineer.com/q/semester_8/telecommunications/evolution-telecom-switching site: BCT Engineering --- # Evolution of telecommunication and switching systems Telecommunications · Chapter 1: Telecommunication Networks - Asked in: 2079 Chaitra, 2+6 marks; 2075 Chaitra, 2+6 marks; 2069 Bhadra (old course), 2+10 marks (asked 3 times) - Video: [02 Evolution of Telecommunication BASICS OF SWITCHING SYSTEMS](https://www.youtube.com/watch?v=89iAwhvw_8k) — Suresh Kumar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/evolution-telecom-switching - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1 ## Question What is telecommunication? Explain the evolution of telecommunication with reference to the evolution of switching system. ## Short answer Telecommunication is communication over distance using electromagnetic systems. Switching evolved from manual exchanges to Strowger automatic, crossbar, electronic SPC, digital time-division, and now IP-based softswitches, progressively increasing capacity and services. ## Answer #### Telecommunication **Telecommunication** is communication at a distance: the transmission, emission or reception of signs, signals, writing, images, sounds or data of any kind by wire, radio, optical or other electromagnetic systems (ITU definition). A telecom system has terminals, transmission links and **switching** (exchanges) that connect any user to any other user on demand. #### Evolution of telecommunication with switching The growth of telecom is closely tied to the way calls were switched: 1. **Telegraph era (1837–1876)** – Morse's electric telegraph (1837/1844). Messages were carried by hand or by store-and-forward at telegraph offices (message switching by operators). 2. **Telephone and manual switching (1876–1890s)** – Bell's telephone (1876). With many telephones, point-to-point wiring needs $n(n-1)/2$ lines, so the **manual exchange** (first at New Haven, 1878) was introduced. Operators connected calls with cords and jacks on a switchboard; early boards used local-battery and later central-battery (CB) systems. 3. **Automatic electromechanical switching** - **Strowger step-by-step (1889–1892)** – Almon B. Strowger invented a two-motion selector driven directly by dial pulses; the first automatic exchange opened at La Porte, USA (1892). The rotary dial followed in 1896. - **Crossbar switching (1920s–1950s)** – crossbar switch with common control (markers, registers); faster, less wear, more flexible routing. 4. **Electronic switching – Stored Program Control (SPC), 1965** – Bell's No. 1 ESS: switching controlled by a computer program. Easy to add services (abbreviated dialling, call forwarding), centralised and later distributed control. The switching fabric was still space-division (reed relays). 5. **Digital time-division switching (1970s)** – with PCM, speech is digitised and switched using time slots (TSI, T-S-T networks): E10 (France, 1970), No. 4 ESS (1976), and later EWSD, 5ESS, AXE. Smaller, cheaper, works with digital transmission (integrated digital network) and common channel signalling (SS7). 6. **ISDN and packet switching (1980s)** – end-to-end digital voice and data; X.25 and later frame relay, ATM. 7. **Mobile and IP era (1990s–now)** – mobile switching centres (GSM), softswitches, IMS and all-IP networks (VoIP, 4G/5G core), where calls are carried as packets and switching is done by routers and software. | Era | Switching | Control | |---|---|---| | 1878 | Manual switchboard | Human operator | | 1892 | Strowger step-by-step | Direct (dial pulses) | | 1938 | Crossbar | Common control | | 1965 | Electronic SPC (space) | Computer program | | 1976 | Digital (time division) | SPC | | 2000s | Softswitch / IP | Software, packets | Each step reduced manual work, increased speed and capacity, and added new services. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why was the manual exchange introduced after the telephone was invented? A. Point-to-point wiring of n telephones needs n(n-1)/2 lines, which becomes impractical B. Telephones could not work without an operator's voice C. Morse code could not be sent over telephone wires D. Automatic dialling had already made wiring cheap Answer: A. With many telephones, wiring every pair needs n(n-1)/2 lines, so a central exchange was introduced. (Key point: Need for exchange) 2. What was the key feature of the Strowger step-by-step exchange? A. A computer program controlled each call B. A two-motion selector driven directly by dial pulses, with no operator C. Calls were switched using digitised time slots D. Common control using markers and registers Answer: B. Strowger's selector moved in step with the subscriber's dial pulses, making switching automatic. (Key point: Strowger) 3. What did the 1965 No. 1 ESS introduce? A. Digital time-division switching of PCM speech B. The crossbar switch with markers C. Stored Program Control, switching controlled by a computer program D. Packet switching using routers Answer: C. No. 1 ESS was the first stored program control exchange; its fabric was still space-division. (Key point: SPC) 4. Digital time-division switching of the 1970s works by A. Moving wipers vertically and then rotating them B. Connecting cords between jacks on a switchboard C. Using reed relays for a separate path per call D. Digitising speech with PCM and switching it in time slots (TSI, T-S-T) Answer: D. With PCM, speech is digitised and switched in time slots, matching digital transmission. 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