---
title: "Strowger switching mechanism"
subject: "Telecommunications"
chapter: 1
key: "semester_8/telecommunications:strowger-switching-principle"
source: https://www.bctengineer.com/q/semester_8/telecommunications/strowger-switching-principle
site: BCT Engineering
---

# Strowger switching mechanism

Telecommunications · Chapter 1: Telecommunication Networks

- Asked in: 2076 Asoj, 4 marks
- Video: [The Strowger Step By Step Telephone Exchange](https://www.youtube.com/watch?v=bvPH-tsD9ZM) — THIS MUSEUM IS (NOT) OBSOLETE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/strowger-switching-principle
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1

## Question

Explain the working principle of the Strowger switching mechanism and justify that it was the beginning of the automatic switching system.

## Short answer

Strowger selectors move directly with dial pulses: the first digit raises wipers to a level, group selectors hunt for free outlets, and the final selector uses the last two digits to reach the line. It began automatic switching because calls were connected without an operator using only the subscriber dial, introducing the rotary dial and multistage switching.

## Answer

#### Working principle
The **Strowger (step-by-step) system** uses **two-motion selectors** that move **directly in step with the dial pulses** sent by the subscriber (direct, progressive control).

```text
     Bank: 10 levels x 10 contacts
   level 10 o o o o o o o o o o
     ...
   level 2  o o o o o o o o o o
   level 1  o o o o o o o o o o
            ^
     wipers │ vertical step (one per pulse)
            └─> rotary step (hunt or per pulse)

 Caller ─> Uniselector/line finder ─> 1st group
 selector ─> 2nd group selector ─> final selector
 ─> called line
```
1. The subscriber lifts the handset; a **uniselector** or **line finder** connects him to a free **first group selector**, which sends dial tone.
2. Dialling the first digit sends $n$ pulses; the vertical magnet raises the wiper shaft $n$ steps to level $n$.
3. In a **group selector** the wipers then rotate automatically across that level, hunting for a free outlet (trunk to the next selector). Each digit thus selects a group of lines.
4. The **final selector (connector)** uses the last two digits: the tens digit gives vertical steps and the units digit gives rotary steps, reaching the exact called line.
5. The final selector tests the line; if free it rings it, otherwise returns busy tone. On clearing, the release magnet returns all switches to normal.

#### Why it was the beginning of automatic switching
- It was the first system (patented 1891; first exchange at La Porte, 1892) in which calls were connected **without a human operator**, using only the pulses from the subscriber's dial.
- It introduced the **rotary dial**, numbered subscriber lines and selection by digits – the basis of all later numbering schemes.
- It introduced selectors arranged in stages (grouping, hunting, final selection) – the idea of a multistage switching network still used today.
- It gave privacy, 24-hour service and reduced operating cost, which led to world-wide replacement of manual exchanges.

## Check yourself

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

1. Strowger control is described as
   A. Direct progressive control, with selectors moving in step with dial pulses
   B. Common control with markers
   C. Stored program control
   D. Time-division control
   Answer: A. Crossbar uses common control. (Key point: Control)

2. In the final selector the tens digit gives
   A. Rotary steps, and units give vertical
   B. Vertical steps, and the units digit gives rotary steps
   C. Only hunting
   D. Only release
   Answer: B. The two digits select the exact line. (Key point: Final selector)

3. After the vertical step in a group selector, the wipers
   A. Stay at level
   B. Return to normal
   C. Rotate automatically across the level hunting for a free outlet
   D. Are moved by the units digit
   Answer: C. Each digit selects a group. (Key point: Group selector)

4. Why is it the beginning of automatic switching?
   A. It was computer controlled
   B. It used digital time slots
   C. It needed a manual board
   D. Calls were connected without an operator using only dial pulses
   Answer: D. First exchange: La Porte 1892. (Key point: Beginning of automation)

## Related in Telecommunications

- [Strowger subscriber access design approaches](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Astrowger-subscriber-access) — Telecommunications, chapter 1
- [CCS and SS7 working principle](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs-ss7-working-principle) — Telecommunications, chapter 5, asked 2×
- [Working principle of FDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-principle) — Telecommunications, chapter 3
- [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4×
- [Digital switching system and its functions](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-switching-functions) — Telecommunications, chapter 4
- [Switching hierarchy and Marconi's transmitter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-marconi) — Telecommunications, chapter 1

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