---
title: "Digital switching system and its functions"
subject: "Telecommunications"
chapter: 4
key: "semester_8/telecommunications:digital-switching-functions"
source: https://www.bctengineer.com/q/semester_8/telecommunications/digital-switching-functions
site: BCT Engineering
---

# Digital switching system and its functions

Telecommunications · Chapter 4: Digital Switching

- Asked in: 2072 Chaitra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/digital-switching-functions
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4

## Question

Explain digital switching system. Mention functions of switching system in telecommunication.

## Short answer

A digital switching system exchanges PCM voice/data using time-division switching under stored program control, typically with a TST network. Its telecommunication functions include detecting off-hook, receiving digits, route analysis, busy testing, path interconnection, alerting, supervision, signaling, charging, and operation/maintenance.

## Answer

#### Digital switching system
A **digital switching system** is an exchange that switches voice and data in digital (PCM) form, using **time-division** switching under **stored program control**. Speech from analogue lines is sampled at 8 kHz, coded into 8-bit words and multiplexed onto 32-slot (2.048 Mbps) highways. The switching network then moves each word from its incoming slot and highway to the required outgoing slot and highway using time switches (T) and space switches (S), normally arranged as TST.

```text
 Analog   +---------+  +------------+  +--------+  Digital
 lines -->| Line    |->| Switching  |->| Trunk  |--> trunks
          | card +  |  | network    |  | inter- |   (E1)
          | codec   |<-| (T-S-T)    |<-| face   |
          +---------+  +------------+  +--------+
               ^              ^             ^
               +------ Control processor ---+
                     (SPC) + signaling
```

**Main parts:** line interface (BORSCHT), concentrator/multiplexer, digital switching network, trunk interface, signaling units (DTMF receivers, SS7), control processor, and O&M system.

**Features:** no moving parts, time-shared hardware, regeneration of signals (no noise build-up), integration with digital transmission, and easy new services through software.

#### Functions of a switching system
1. **Attending:** detect a call request (off-hook) on any line.
2. **Information receiving:** give dial tone and receive dialled digits.
3. **Information processing:** analyse digits to find the called party and route.
4. **Busy testing:** check if the called line or outgoing trunk is free.
5. **Interconnection:** set up a path through the switching network.
6. **Alerting:** ring the called party; send ring-back tone to the caller.
7. **Supervision:** watch for answer and clearing; release the path at the end.
8. **Information sending:** send signaling to other exchanges for outgoing calls.
9. **Charging:** record call details for billing.
10. **Operation and maintenance:** fault detection, testing, traffic records, subscriber data changes.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is speech prepared for a digital exchange?
   A. Sampled at 4 kHz, 4 bits
   B. Sampled at 8 kHz, coded into 8-bit words
   C. Sampled at 8 kHz, 16 bits
   D. Left analog but multiplexed
   Answer: B. 8 kHz sampling with 8-bit words gives 64 kbps per channel; 32 slots form a 2.048 Mbps highway. (Key point: PCM format)

2. Which elements form the digital switching network?
   A. Relays and crossbars
   B. Time and space switches, usually arranged as TST
   C. Only space switches
   D. Rotary selectors
   Answer: B. Time (T) and space (S) switches are combined, normally as TST. (Key point: Switching network)

3. What does busy testing do?
   A. Checks if the called line or trunk is free
   B. Detects off-hook
   C. Sends ringing
   D. Records call duration
   Answer: A. Busy testing checks the availability of the called line or outgoing trunk. (Key point: Busy testing)

4. What controls a digital switching system?
   A. Hard-wired relays
   B. Stored program control processor
   C. Human operator
   D. Rotary selectors
   Answer: B. A processor under stored program control handles call processing, signaling and maintenance. (Key point: Control)

## 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×
- [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 functions and three-stage crosspoints](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-functions-crosspoints-4x4) — Telecommunications, chapter 4, asked 2×
- [Digital electronics in upgrading switching](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aelectromechanical-to-digital) — Telecommunications, chapter 4
- [Classification of switching systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aclassify-switching-systems) — Telecommunications, chapter 1
- [Conventional exchange functions; PCM switching advantages](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aconventional-exchange-pcm-advantages) — Telecommunications, chapter 4

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