---
title: "Subscriber loop or line plant"
subject: "Telecommunications"
chapter: 1
key: "semester_8/telecommunications:subscriber-loop-line-plant"
source: https://www.bctengineer.com/q/semester_8/telecommunications/subscriber-loop-line-plant
site: BCT Engineering
---

# Subscriber loop or line plant

Telecommunications · Chapter 1: Telecommunication Networks

- Asked in: 2071 Shrawan, 7 marks
- Video: [Lecture 25: Telephone System | Computer Networks](https://www.youtube.com/watch?v=bZEoHues200) — Gursimran Singh Dhillon
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/subscriber-loop-line-plant
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1

## Question

Explain the subscriber loop or line plant in case of structure of a telephone office or exchange.

## Short answer

The subscriber loop is the dedicated twisted pair from subscriber to exchange, forming the outside plant. It runs from MDF through feeder cable to cross-connect cabinet, then distribution cable to distribution point, then drop wire to premises; loop resistance and loss limits are maintained.

## Answer

The **subscriber loop (line plant or outside plant)** is the network of cables and wires that connects each subscriber's telephone to the local exchange. It is usually a twisted copper pair per subscriber (two wires forming a "loop"), and it is the most expensive part of a local network because it is not shared.

#### Structure
```text
 Exchange
 ┌──────────┐
 │ Switch ─ │ MDF
 └──────┬───┘
        │ Main (feeder) cable, 600-2400 pairs
        v
   ┌──────────┐
   │ Cabinet/ │  primary cross-connection
   │ pillar   │  point (flexibility)
   └──┬───┬───┘
      │   │ Distribution (branch) cables, 50-200 pairs
      v   v
     [DP] [DP]  distribution points (pole/wall box)
      │ │  │
     drop wires (1-2 pairs)
      │ │  │
     subscribers' premises
```
1. **Main Distribution Frame (MDF)** at the exchange – outside cables end here and are cross-connected to the exchange line equipment; protection devices fitted.
2. **Feeder (main/primary) cable** – large multi-pair cable, usually underground in ducts, from MDF to cross-connect points.
3. **Cross-connection cabinet (pillar)** – lets any feeder pair be jumpered to any distribution pair, giving flexibility when demand changes.
4. **Distribution (secondary) cable** – smaller cables, underground or aerial, running along streets.
5. **Distribution point (DP)** – a terminal box on a pole or wall serving a few buildings.
6. **Drop wire / service wire** – from DP to the subscriber, ending at the subscriber's terminal/socket.

#### Electrical features
- Loop resistance limited (about 1200–1800 Ω) so that DC signalling and −48 V feeding work; loading coils used on long analogue loops.
- Carries DC power, ringing (75 V, 25 Hz), dial signals and speech in both directions on two wires (hybrid at each end).
- Modern loops are being replaced by fibre (FTTC/FTTH) and wireless access.

## Check yourself

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

1. What is the subscriber loop?
   A. The shared trunk between exchanges
   B. The network of cables, usually a twisted copper pair per subscriber, connecting the telephone to the local exchange
   C. The power plant of the exchange
   D. The switching network
   Answer: B. The loop (line plant) connects each subscriber to the exchange and is expensive because it is not shared. (Key point: Definition of subscriber loop)

2. Which is the correct order from exchange to subscriber?
   A. MDF - distribution cable - cabinet - feeder - DP - drop
   B. MDF - feeder cable - cabinet - distribution cable - DP - drop wire
   C. DP - MDF - cabinet - feeder - drop
   D. MDF - drop - DP - cabinet - feeder
   Answer: B. Pairs go MDF, feeder cable, cross-connection cabinet, distribution cable, DP, then drop wire. (Key point: Order of plant elements)

3. Why is a cross-connection cabinet used?
   A. To amplify the signal
   B. To let any feeder pair be jumpered to any distribution pair for flexibility
   C. To supply ringing current
   D. To convert to fibre
   Answer: B. Jumpering gives flexibility when demand changes and better use of pairs. (Key point: Purpose of cabinet)

4. Why is loop resistance limited to about 1200-1800 ohm?
   A. To reduce cable cost
   B. So DC signalling and -48 V feeding work
   C. To allow loading coils
   D. To avoid fibre use
   Answer: B. Higher resistance would prevent DC signalling and battery feed from working properly. (Key point: Loop resistance limit)

## Related in Telecommunications

- [Telephone office structure and distribution plant](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atelephone-office-distribution-plant) — Telecommunications, chapter 1
- [Telephone exchange structure; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-structure-tst-sts) — Telecommunications, chapter 4
- [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×
- [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×
- [Subscriber loop and cable hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asubscriber-loop-cable-hierarchy) — Telecommunications, chapter 1
- [Short note: Digital Subscriber Line](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-dsl) — Telecommunications, chapter 8, asked 2×

## Related in other subjects

- [Distortionless line and telephone lines](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adistortionless-line-telephone) — Electromagnetism, chapter 5