--- title: "Digital electronics in upgrading switching" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:electromechanical-to-digital" source: https://www.bctengineer.com/q/semester_8/telecommunications/electromechanical-to-digital site: BCT Engineering --- # Digital electronics in upgrading switching Telecommunications · Chapter 4: Digital Switching - Asked in: 2070 Chaitra, 7 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/electromechanical-to-digital - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question Explain the role of Logic or digital electronics in upgrading the electromechanical switching system into digital switching system. ## Short answer Digital electronics replaced electromechanical control with stored program control, registers/markers for digit translation, electronic scanners, and logic-based DTMF receivers. PCM codecs and time-space switches substituted metallic crosspoints, and digital trunks integrated directly. Common channel signaling, automatic charging, and self-test logic improved reliability and service flexibility. ## Answer Electromechanical exchanges (Strowger step-by-step and crossbar) used relays, selectors and moving contacts both to make the speech path and to control the call. **Logic and digital electronics** replaced these parts step by step and turned them into digital SPC exchanges. #### Role of logic / digital electronics 1. **Common control instead of direct control:** in Strowger exchanges each selector was stepped directly by dial pulses. Electronic **registers and markers** made of logic circuits store the digits first and then decide the route, so any free path can be chosen (alternate routing, digit translation). 2. **Stored program control:** hard-wired relay logic was replaced by a **processor and memory**. Call handling, routing tables and features became software, so new services (call waiting, forwarding, abbreviated dialling) need only a program change. 3. **Electronic scanning:** line relays were replaced by **scanners** (multiplexers and gates) that test every line every few milliseconds for off-hook, digits and on-hook. 4. **Digit reception:** pulse counters and **DTMF receivers** built with digital filters replaced rotary selectors. 5. **Electronic crosspoints:** reed relays first, then semiconductor gates replaced mechanical crossbars, giving faster and wear-free switching. 6. **Time division switching:** with PCM codecs, speech becomes 8-bit words. **Memories (TSIs) and logic gate matrices (space switches)** replace metallic crosspoints; one gate serves many calls in different slots. 7. **Digital transmission integration:** exchanges connect directly to PCM E1 trunks, removing analogue/digital conversion between exchanges. 8. **Signaling:** logic-based signaling units allowed **common channel signaling (SS7)** with fast message-based call set-up. 9. **Charging and administration:** counters and memory replaced electromechanical meters; detailed call records are produced automatically. 10. **Maintenance:** self-test logic, alarms and remote diagnostics reduce fault-finding time. #### Results of the upgrade | Point | Electromechanical | Digital (logic-based) | |---|---|---| | Switching element | Relays, selectors | Gates, memories (TSI) | | Control | Hard-wired | Stored program | | Speed of set-up | Seconds | Milliseconds | | Size and power | Large, high | Small, low | | Maintenance | Frequent, manual | Low, automatic | | New services | Difficult | Software update | Thus digital electronics is the basis of modern exchanges, giving lower cost, higher reliability and many more services. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the advantage of registers and markers over Strowger direct control? A. Digits are stored first, so any free path can be chosen B. They step selectors faster C. They remove dialling D. They use moving contacts Answer: A. Storing digits enables alternate routing and digit translation. (Key point: Common control) 2. What does SPC bring when replacing hard-wired relay logic? A. Faster contacts B. New services by program change alone C. Higher power use D. More moving parts Answer: B. Call handling and features become software. (Key point: Stored program control) 3. How do digital electronics replace metallic crosspoints? A. Memories (TSIs) and gate matrices, each serving many calls in different slots B. One gate per call C. Reed relays only D. Larger crossbars Answer: A. Time sharing lets one gate or memory location serve many calls. (Key point: Time division with PCM) 4. What did logic-based signaling units make possible? A. Rotary dialling B. Common channel signaling (SS7) with message-based set-up C. Manual switching D. Pulse counting Answer: B. SS7 allows fast message-based call set-up. 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