- 2070 Chaitra · 7 marks
Digital electronics in upgrading switching
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.
Explain the role of Logic or digital electronics in upgrading the electromechanical switching system into digital switching system.
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
- 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).
- 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.
- 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.
- Digit reception: pulse counters and DTMF receivers built with digital filters replaced rotary selectors.
- Electronic crosspoints: reed relays first, then semiconductor gates replaced mechanical crossbars, giving faster and wear-free switching.
- 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.
- Digital transmission integration: exchanges connect directly to PCM E1 trunks, removing analogue/digital conversion between exchanges.
- Signaling: logic-based signaling units allowed common channel signaling (SS7) with fast message-based call set-up.
- Charging and administration: counters and memory replaced electromechanical meters; detailed call records are produced automatically.
- 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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