---
title: "Role of exchange and rotary dial operation"
subject: "Telecommunications"
chapter: 1
key: "semester_8/telecommunications:exchange-role-rotary-dial"
source: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-role-rotary-dial
site: BCT Engineering
---

# Role of exchange and rotary dial operation

Telecommunications · Chapter 1: Telecommunication Networks

- Asked in: 2080 Baisakh, 4+4 marks
- Video: ["How Rotary Phones Work"](https://www.youtube.com/watch?v=cLGd6OB8D5k) — Telecom Training
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-role-rotary-dial
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1

## Question

Define the role and need of exchange or branch office used in telecommunication. Explain the operation of a rotary dial with associated drawbacks.

## Short answer

An exchange connects all subscribers via a central point, eliminating dedicated lines, and performs call detection, dial tone, digit reception, switching, supervision, and release. Rotary dial operation: winding the wheel winds a spring; release returns it at constant speed via governor, breaking loop current n times for digit n (0 gives 10 breaks) at 10 pps, with inter-digit pause. Drawbacks: slow, mechanical wear, limited to 10 signals, no * or #, pulses cannot pass end-to-end, distortion on long lines.

## Answer

#### Role and need of an exchange
Without an exchange, $n$ telephones would need $n(n-1)/2$ separate lines (1000 phones → 499,500 lines). An **exchange (central office or branch office)** connects all subscriber lines to one point and joins any two of them only when needed.

**Roles/functions:**
- Detect a call request (off-hook) and give dial tone.
- Receive and store the dialled digits.
- Find the called line or route to another exchange through trunks.
- Test whether the called line is busy; ring the called party and give ringback/busy tone.
- Set up the speech path, supervise the call, charge for it, and release it at the end.
- Provide shared use of costly trunks and services (operator, emergency, supplementary services).
- A **branch office/PBX** does the same for an organisation's internal phones, sharing a few external lines.

#### Rotary dial operation
```text
  finger wheel with 10 holes (1..9, 0)
  finger stop ─┐
               v        Off-normal contacts
  Wind clockwise  ─────> spring is wound
  Release ─────> wheel returns at fixed speed
                (governor) and cam opens the
                pulsing contact n times
  Loop current:
  ──┐  ┌──┐  ┌──┐  ┌──────   (digit 3)
    └──┘  └──┘  └──┘
    break make
```
1. The subscriber puts a finger in the hole of a digit and turns the wheel clockwise to the finger stop, winding a spring.
2. On release, the spring returns the wheel; a **centrifugal governor** keeps the return speed constant (10 pulses per second).
3. A cam opens and closes the **impulsing contacts** in the subscriber loop, giving one **break** (loop open) per digit unit: digit 1 = 1 break, digit 9 = 9 breaks, digit 0 = 10 breaks. Break:make ratio is about 2:1 (66.7 ms : 33.3 ms).
4. **Off-normal contacts** short the receiver during dialling to avoid clicks and keep the loop steady.
5. The time taken to rotate the dial for the next digit gives the **inter-digit pause** (at least ~200–300 ms) that the exchange uses to separate digits.

#### Drawbacks
- Slow: digit 0 takes about 1 s plus rotation time.
- Mechanical wear of governor and contacts changes pulse speed/ratio, causing wrong numbers.
- Only 10 signals; no * and # for services.
- Pulses cannot pass end-to-end over a call (no IVR use).
- Pulse distortion on long lines; needs DC loop signalling, not suitable for electronic systems.
- Uncomfortable for users; more misdialling.

## Check yourself

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

1. How many lines would 1000 telephones need without an exchange?
   A. 1,000
   B. 500
   C. 499,500
   D. 999,000
   Answer: C. n(n-1)/2 = 1000 x 999/2 = 499,500. (Key point: Need for exchange)

2. How is the speed of the returning rotary dial held constant?
   A. The off-normal contacts regulate it
   B. The subscriber's finger controls it
   C. The exchange regulates it with a clock
   D. A centrifugal governor keeps it at 10 pulses per second
   Answer: D. A governor fixes the return speed. (Key point: Pulse speed)

3. How many breaks does digit 0 give?
   A. 10
   B. 0
   C. 1
   D. 9
   Answer: A. Digit n gives n breaks, 0 gives 10. (Key point: Digit zero)

4. Which is a drawback of the rotary dial?
   A. It has too many keys
   B. Wear changes pulse speed/ratio causing wrong numbers, and pulses cannot pass end-to-end
   C. It is too fast for exchanges
   D. It can send * and #
   Answer: B. The dial is slow, wears, has only 10 signals and suits only DC loop signalling. (Key point: Drawbacks)

## Related in Telecommunications

- [Rotary dial dialing time calculation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arotary-dial-time-calculation) — Telecommunications, chapter 1
- [Time and space switch drawbacks](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-space-switch-drawbacks) — Telecommunications, chapter 4
- [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×
- [Exchange types and essential features](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-types-essential-features) — Telecommunications, chapter 1
- [Exchange roles and demerits of manual switching](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-roles-manual-demerits) — Telecommunications, chapter 1
- [Digital exchange and three-stage crosspoints](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-exchange-crosspoints) — Telecommunications, chapter 4

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