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

# Subscriber loop and cable hierarchy

Telecommunications · Chapter 1: Telecommunication Networks

- Asked in: 2070 Asar, 1+5 marks
- Video: [Lec-24_Subscriber/Local loop system | Telecommunication Engineering | ICT Engineering](https://www.youtube.com/watch?v=r383HJsEALo) — Computer_IT_ICT Engineering Department : LJIET
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/subscriber-loop-cable-hierarchy
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1

## Question

Define subscriber loop. Explain the subscriber loop system showing cable hierarchy for subscriber loops.

## Short answer

The subscriber loop is the dedicated pair connecting subscriber to exchange. Cable hierarchy: feeder cable from MDF to cross-connect cabinet, distribution cables to distribution points, then drop wires to subscribers; loop resistance and loss are limited.

## Answer

#### Subscriber loop
The **subscriber loop** is the dedicated pair of wires (local loop) that connects a subscriber's telephone to the local exchange, forming a closed circuit through the telephone when it is off-hook.

#### Subscriber loop system and cable hierarchy
```text
 Exchange MDF
      │
      │ Feeder / main cable (e.g. 1200 pairs)
      v
 ┌─────────────┐
 │Cross-connect│ (cabinet / pillar)
 │ point (PCP) │
 └──┬───────┬──┘
    │       │  Distribution cables (e.g. 100 pairs)
    v       v
  [DP]     [DP]   Distribution points (10-20 pairs)
   │ │      │
   Drop wires (1-2 pairs)
   │ │      │
  Tel Tel  Tel   Subscribers
```
1. **Main (feeder) cable:** runs from the MDF of the exchange; large pair count; laid in ducts.
2. **Cross-connection point (cabinet/pillar):** jumpers connect any feeder pair to any distribution pair, giving flexibility and better use of pairs.
3. **Distribution (branch) cables:** smaller cables running along streets, underground or aerial.
4. **Distribution point (DP):** pole-mounted or wall-mounted terminal block.
5. **Drop/service wire:** individual pair from the DP to the subscriber's socket.

**Loop limits:** loop resistance is kept below about 1200–1800 Ω and loss below about 7–8 dB at 800 Hz, so that −48 V feeding, ringing and dial signalling work; thicker conductors or loading coils are used on long loops. The loop carries DC feed, ringing, dialling and two-way speech on the same two wires.

## Check yourself

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

1. What does the subscriber loop form when the telephone is off-hook?
   A. An open circuit
   B. A closed circuit through the telephone over a dedicated pair to the local exchange
   C. A shared radio path
   D. A four-wire circuit
   Answer: B. The local loop is a dedicated pair that forms a closed circuit through the telephone when off-hook. (Key point: Definition)

2. Which cable has the largest pair count?
   A. Drop wire
   B. Distribution cable
   C. DP wiring
   D. Feeder (main) cable from the MDF
   Answer: D. Feeder cables (e.g. 1200 pairs) are largest, reducing to distribution cables, DPs and drop wires. (Key point: Cable hierarchy)

3. What is carried on the two wires of the loop?
   A. Only speech
   B. DC feed, ringing, dialling and two-way speech
   C. Only signalling
   D. Only DC power
   Answer: B. The same two wires carry DC feed, ringing, dial signals and speech in both directions. (Key point: Loop limits)

## Related in Telecommunications

- [Subscriber loop or line plant](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asubscriber-loop-line-plant) — Telecommunications, chapter 1
- [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2×
- [Strowger subscriber access design approaches](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Astrowger-subscriber-access) — 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×
- [Switching hierarchy and evolution until the telephone](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-telephone-evolution) — Telecommunications, chapter 1, asked 3×
- [Switching hierarchy and Marconi's transmitter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-hierarchy-marconi) — Telecommunications, chapter 1

## Related in other subjects

- [Murray loop test for cable faults](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Amurray-loop-test) — Power System Analysis I, chapter 2, asked 5×
- [Cable fault location techniques and Murray loop](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-fault-techniques-murray) — Power System Analysis I, chapter 2
- [Varley loop test for cable faults](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Avarley-loop-test) — Power System Analysis I, chapter 2
- [FDM hierarchy in telephone system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-hierarchy-telephone) — Data Communication, chapter 6