---
title: "Exchange roles and demerits of manual switching"
subject: "Telecommunications"
chapter: 1
key: "semester_8/telecommunications:exchange-roles-manual-demerits"
source: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-roles-manual-demerits
site: BCT Engineering
---

# Exchange roles and demerits of manual switching

Telecommunications · Chapter 1: Telecommunication Networks

- Asked in: 2072 Kartik, 7+3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-roles-manual-demerits
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1

## Question

Describe the roles and operations of an exchange. Write down the demerits of manual switching in telecommunication.

## Short answer

An exchange connects subscribers on demand, detects calls, receives digits, processes, tests busy, interconnects, alerts, supervises, charges, and disconnects; it also routes traffic and bills. Manual switching is slow, lacks privacy, error-prone, labour-intensive, limited capacity, and depends on operator efficiency.

## Answer

#### Roles and operations of an exchange
An **exchange** connects a calling subscriber to any called subscriber (local or distant) on demand, using shared equipment, and releases the connection after the call. It avoids $n(n-1)/2$ direct links among $n$ users and allows costly trunks to be shared.

```text
 Subscribers                 Other exchanges
  ──┐   ┌─────────────────────────┐   ┌──
  ──┼MDF┤ line units ─ switching ─┤ trunks
  ──┘   │   network ─ trunk units │   └──
        │        │                │
        │  control + signalling   │
        └─────────────────────────┘
```

**Basic operations in setting up a call (switching functions):**
1. **Attending (call detection):** the exchange continuously scans lines; when the caller lifts the handset (loop closes), a call request is detected.
2. **Information receiving:** a free register/digit receiver is connected and **dial tone** is sent; the dialled digits (pulse or DTMF) are received and stored.
3. **Information processing:** the digits are analysed to identify the called line or route (local, trunk, international) and the charging rate.
4. **Busy testing:** the called line (or outgoing trunk) is tested to see if it is free.
5. **Interconnection:** a path is set up through the switching network between the caller and the called line/trunk.
6. **Alerting:** ringing current is sent to the called phone and **ringback tone** to the caller; if busy, **busy tone** is sent.
7. **Supervision:** when the called party answers, ringing stops, the speech path is completed and **charging (metering)** starts; the exchange watches for clear-down.
8. **Information sending:** for calls to other exchanges, the number is signalled to the next exchange (CAS or SS7).
9. **Disconnection:** when either party hangs up, the path is released, charging stops and equipment returns to idle.

**Other roles:** charging and billing, routing traffic over alternative routes when busy, maintenance and fault detection, traffic recording, providing supplementary services (call forwarding, waiting, conference), emergency and operator services, and power feeding (−48 V) to subscriber telephones.

#### Demerits of manual switching
- Slow call set-up and long waiting for the operator.
- No privacy; operator can listen to calls.
- Human errors, misunderstanding of numbers/language.
- High labour cost; operators needed 24 hours.
- Limited capacity; unsuitable for large cities.
- Service depends on operator efficiency, fatigue and behaviour.

## Check yourself

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

1. In call set-up, what happens immediately after the exchange detects the caller lifting the handset?
   A. Ringing current is sent to the called phone
   B. A free register is connected and dial tone is sent to receive digits
   C. Charging starts
   D. The path is released
   Answer: B. After attending, the exchange performs information receiving: a free register is connected and dial tone is sent. (Key point: Order of call set-up)

2. When does charging (metering) start?
   A. When the caller lifts the handset
   B. When the digits are dialled
   C. When the called party answers and the speech path is completed
   D. When ringback is first sent
   Answer: C. On answer, ringing stops, the speech path is completed and charging begins. (Key point: Supervision and charging)

3. Which is a demerit of manual switching?
   A. Complete privacy of calls
   B. Fast call set-up
   C. Slow set-up, human errors and no privacy because the operator can listen
   D. Low labour cost
   Answer: C. Manual switching is slow, error-prone, lacks privacy and needs costly 24-hour operators. (Key point: Demerits of manual switching)

4. How many direct links would n users need without an exchange?
   A. n
   B. n(n-1)/2
   C. n^2/2 + n
   D. 2n
   Answer: B. Fully connecting n users needs n(n-1)/2 links; an exchange avoids this by sharing equipment. (Key point: Why exchanges are needed)

## Related in Telecommunications

- [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
- [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×
- [Evolution of telecommunication and switching systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aevolution-telecom-switching) — Telecommunications, chapter 1, asked 3×
- [Conventional exchange functions; PCM switching advantages](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aconventional-exchange-pcm-advantages) — Telecommunications, chapter 4
- [Roles of NTA and ITU](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anta-roles-itu-roles) — Telecommunications, chapter 7
- [Exchange types and essential features](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-types-essential-features) — Telecommunications, chapter 1

## Related in other subjects

- [Linear and circular queue operations](https://www.bctengineer.com/question.md?key=semester_4%2Fdata_structure_and_algorithm%3Alinear-circular-queue-algorithms) — Data Structure and Algorithm, chapter 2
- [Basic forging operations](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Aforging-operations-explained) — Workshop Technology, chapter 9