---
title: "Two-wire vs four-wire communication"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:two-vs-four-wire"
source: https://www.bctengineer.com/q/semester_8/telecommunications/two-vs-four-wire
site: BCT Engineering
---

# Two-wire vs four-wire communication

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2071 Chaitra, 4+6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/two-vs-four-wire
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

What is the difference between two and four wire communication? Describe the four wire communication.

## Short answer

Two-wire uses a single pair for both directions; four-wire uses separate GO and RETURN paths with one-way amplifiers. Four-wire requires hybrids at ends to interface with two-wire subscriber lines. The hybrids use a balancing network to isolate directions, preventing echo and singing, and enabling long-distance multiplexed trunks.

## Answer

**Two-wire communication** carries both directions of speech on a single pair of wires; **four-wire communication** uses two separate pairs (or two separate channels), one for each direction.

#### Difference between two-wire and four-wire communication
| Point | Two-wire | Four-wire |
|---|---|---|
| Conductors | One pair for both directions | Two pairs, one per direction |
| Direction on a pair | Both ways (full duplex on one pair) | One way only (GO or RETURN) |
| Amplification | Needs bidirectional (negative impedance) repeaters; hard | Simple one-way amplifiers in each path |
| Distance | Short (subscriber loop, a few km) | Long trunk, toll and international |
| Use | Subscriber line, local junctions | Trunks, carrier (FDM/TDM), radio, satellite |
| Cost | Cheaper, half the copper | More copper / equipment |
| Hybrid needed | No | Yes, 2W/4W hybrid at each end |
| Echo / singing | Little | Possible if hybrids unbalanced |
| Multiplexing | Difficult | Natural (one-way channels) |

#### Four-wire communication
In long-distance circuits the signal must be amplified many times. Amplifiers are one-way devices, so the two directions are separated: a **GO path** from A to B and a **RETURN path** from B to A. Subscriber lines remain two-wire, so a **hybrid** (four-wire termination set) at each end converts 2-wire to 4-wire.

```text
 Sub A                                         Sub B
         +-----+  GO  ->[A1]->[A2]->  +-----+
 o-------| HYB |=======================| HYB |-------o
 2-wire  |  A  |=======================|  B  | 2-wire
     Z_B-+-----+  <-[A3]<-[A4]<- RET  +-----+-Z_B
```

**Working:**
1. Speech from subscriber A enters hybrid A and is sent on the GO pair. Amplifiers along the route make up for line loss.
2. At B the hybrid passes the GO signal to subscriber B's two-wire line.
3. B's reply travels on the RETURN pair the same way to A.
4. In each hybrid the **balancing network** $Z_B$ copies the 2-wire line impedance, so the incoming RETURN signal is not passed back into the GO pair (high transhybrid loss).

**Equivalent four-wire:** on carrier and radio systems there are no physical pairs; the two directions use different frequency bands (FDM) or time slots/channels (TDM) — still treated as four-wire.

**Advantages:**
- Gain can be set separately for each direction and the circuit can be made very long with low net loss.
- Directly suits multiplexing, digital transmission and radio/satellite links.
- Four-wire switching in trunk exchanges avoids repeated 2W/4W conversions and the loss and echo they cause.

**Problems and remedies:**
- **Echo:** imperfect hybrid balance reflects some received speech back to the talker. Long-delay circuits use echo suppressors or cancellers.
- **Singing:** if total loop gain $G_1 + G_2 - (L_A + L_B) \geq 0$ dB the loop oscillates. Good balancing and a net-loss margin prevent it.
- **Cost:** twice the conductors and the hybrids add cost, justified only on long routes.

## Check yourself

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

1. What is the basic difference between two-wire and four-wire communication?
   A. Two-wire uses fibre
   B. Two-wire carries both directions on one pair; four-wire uses separate pairs per direction
   C. Four-wire is analogue only
   D. Two-wire has amplifiers in each direction
   Answer: B. Four-wire has a separate GO and RETURN path. (Key point: Basic difference)

2. Why are long trunks four-wire?
   A. Amplifiers are one-way, so gain is inserted separately per direction
   B. Copper is cheaper
   C. Hybrids are not needed
   D. Two-wire has more echo
   Answer: A. Separate one-way paths allow simple amplifiers and natural multiplexing. (Key point: Why four-wire for trunks)

3. When does a four-wire circuit with hybrids at both ends sing (oscillate)?
   A. When the line is longer than 100 km
   B. When the balance return loss is very large
   C. When Z_B is exactly equal to Z_L
   D. When the loop gain round the four-wire circuit reaches or exceeds unity (0 dB)
   Answer: D. Leakage lets signal circulate round the loop; if loop gain is at least unity the loop oscillates and sings. (Key point: Singing condition)

4. How is four-wire achieved on carrier and radio systems with no physical pairs?
   A. It is not possible
   B. By using extra wires
   C. Directions use different frequency bands (FDM) or time slots (TDM)
   D. By using hybrids in the middle only
   Answer: C. These are still treated as four-wire because the directions are separate channels. (Key point: Equivalent four-wire)

## Related in Telecommunications

- [Noise and two-wire to four-wire converter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anoise-two-four-wire-converter) — Telecommunications, chapter 2
- [Wired vs wireless media; four-wire and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awired-wireless-four-wire-hybrid) — Telecommunications, chapter 2, asked 2×
- [Talker and listener echo in four-wire circuit](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afour-wire-echo-numerical) — Telecommunications, chapter 2
- [Media types, four-wire system and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amedia-types-four-wire-hybrid) — Telecommunications, chapter 2
- [Guided vs unguided media; 2-wire to 4-wire](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-unguided-two-four-wire) — Telecommunications, chapter 2, asked 2×

## Related in other subjects

- [Star three impedances four-wire system](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Astar-three-impedances-4-wire-400v) — Basic Electrical Engineering, chapter 8
- [Two-ray model path and phase difference](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atwo-ray-path-phase-difference) — Wireless Communication, chapter 3, asked 2×
- [Two-wire 40 Ω line, input and output voltage](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Atwo-wire-40-ohm-line-voltages) — Electromagnetism, chapter 5
- [Inductance of a single-phase two-wire line](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Asingle-phase-inductance-derivation) — Power System Analysis I, chapter 4