---
title: "Strowger subscriber access design approaches"
subject: "Telecommunications"
chapter: 1
key: "semester_8/telecommunications:strowger-subscriber-access"
source: https://www.bctengineer.com/q/semester_8/telecommunications/strowger-subscriber-access
site: BCT Engineering
---

# Strowger subscriber access design approaches

Telecommunications · Chapter 1: Telecommunication Networks

- Asked in: 2079 Bhadra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/strowger-subscriber-access
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1

## Question

In the strowger switching system, briefly describe the two design approaches used to provide subscriber access.

## Short answer

In Strowger access, each subscriber may have a dedicated uniselector that hunts for a free first group selector, or a group of 100-200 lines may share line finders that hunt for the calling line. Uniselectors give fast access but cost one switch per subscriber; line finders are cheaper for low-traffic lines.

## Answer

In a Strowger (step-by-step) exchange, a calling subscriber must first be connected to a free **first group selector** that will receive the first dialled digit. Two design approaches are used for this subscriber access:

#### 1. Uniselector (line switch / pre-selector) approach
```text
 Sub A ─> [Uniselector A] ─┐
 Sub B ─> [Uniselector B] ─┼─> free 1st group selector
 Sub C ─> [Uniselector C] ─┘    (outlets of uniselectors)
```
- Every subscriber has his own **uniselector** (a single-motion rotary switch) at the exchange.
- When the subscriber lifts the handset, his uniselector rotates and hunts over its outlets until it finds a free first selector, then stops and gives dial tone.
- Fast access (no searching for the caller), simple control.
- Costly: one switch per subscriber, though most subscribers are idle most of the time; poor equipment utilisation.

#### 2. Line finder approach
```text
 Sub A ─┐
 Sub B ─┼─ bank of ─> [Line finder 1] ─> 1st selector
 Sub C ─┘  contacts   [Line finder 2] ─> 1st selector
 (100-200 lines)      (few finders shared)
```
- A small number of **line finders** (two-motion switches, each permanently paired with a first selector) is shared by a group of 100–200 subscribers.
- When a subscriber goes off-hook, an allotter assigns a free line finder, which searches its bank (vertical then rotary) to **find the calling line** and connects it to the first selector.
- Fewer switches are needed because the number of finders is set by traffic, not by the number of subscribers – cheaper for low-traffic lines.
- Slightly slower (search time) and needs an allotter/control circuit.

| Point | Uniselector | Line finder |
|---|---|---|
| Switch per | Subscriber | Group (shared) |
| Hunts for | Free selector | Calling line |
| Cost | High | Lower |
| Best for | High-calling-rate lines | Low-calling-rate lines |

## Check yourself

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

1. In the uniselector approach
   A. A few shared switches search for the calling line
   B. Every subscriber has his own switch that hunts for a free first selector
   C. An allotter assigns a free finder
   D. The first selector is chosen by the caller
   Answer: B. The uniselector is one per subscriber. (Key point: Uniselector)

2. What does a line finder do?
   A. Hunts for a free selector from a single line
   B. Rings the called party
   C. Searches its bank to find the calling line and connects it to the first selector
   D. Counts dial pulses
   Answer: C. Line finders are shared by 100-200 subscribers and find the caller. (Key point: Line finder)

3. Why is the line finder approach cheaper?
   A. It uses no switches
   B. It needs no control circuit
   C. Each subscriber has a switch
   D. The number of finders depends on traffic, not on subscribers
   Answer: D. Shared finders suit low-traffic lines. (Key point: Cost)

4. What is a drawback of line finders compared with uniselectors?
   A. Slightly slower because of search time, and needs an allotter
   B. Higher cost per subscriber
   C. No dial tone
   D. Needs more switches
   Answer: A. They have search time and need an allotter. (Key point: Disadvantage)

## Related in Telecommunications

- [Strowger switching mechanism](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Astrowger-switching-principle) — Telecommunications, chapter 1
- [Digital switching system and its functions](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-switching-functions) — Telecommunications, chapter 4
- [Subscriber loop and cable hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asubscriber-loop-cable-hierarchy) — Telecommunications, chapter 1
- [Space switch and why TST is used](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-why-tst) — Telecommunications, chapter 4
- [IP and soft switching for PC-to-PSTN calls](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aip-soft-switching-skype-pstn) — Telecommunications, chapter 8
- [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×

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