---
title: "Wired vs wireless media; four-wire and hybrid"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:wired-wireless-four-wire-hybrid"
source: https://www.bctengineer.com/q/semester_8/telecommunications/wired-wireless-four-wire-hybrid
site: BCT Engineering
---

# Wired vs wireless media; four-wire and hybrid

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2081 Baisakh, 3+5 marks; 2074 Chaitra, 3+5 marks (asked 2 times)
- Video: [Wired and Wireless Communication Medium | Computer Networks | In Hindi](https://www.youtube.com/watch?v=wwsD3gT48Ag) — codeitup
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/wired-wireless-four-wire-hybrid
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

Differentiate between wireless and wired communication media with examples. Briefly explain four-wire transmission system and operation of hybrid.

## Short answer

Wired media (twisted pair, fibre) guide signals; wireless (radio, microwave) propagate through air. A four-wire system uses separate transmit and receive paths for one-way amplifiers. The hybrid transformer converts 2-wire to 4-wire: a centre-tapped winding splits the receive signal to line and balance network, cancelling in the transmit winding when impedances match, preventing echo.

## Answer

#### Wired vs wireless communication media
| Point | Wired (guided) | Wireless (unguided) |
|---|---|---|
| Path | Signal guided by a physical conductor | Signal propagates through air/space |
| Examples | Twisted pair, coaxial cable, optical fibre | Radio, microwave, satellite, infrared |
| Mobility | Fixed terminals | Supports mobile users |
| Installation | Costly digging/laying, slow | Quick, good for remote/hilly areas |
| Interference | Low (esp. fibre) | Affected by fading, rain, interference |
| Security | More secure | Easier to intercept; needs encryption |
| Bandwidth | Very high (fibre) | Limited by spectrum licence |

#### Four-wire transmission system
In a **four-wire circuit**, each direction of transmission uses a separate pair (or separate channel), so amplifiers, multiplexers and digital equipment, which are one-way devices, can be used. The subscriber loop, however, is two-wire (both directions on one pair). A **hybrid** converts between the two at each end.

```text
  West                                       East
 2-wire  ┌──────┐   Tx pair  ──>  (amp)  ┌──────┐ 2-wire
 ────────┤Hybrid├──────────────────────> ┤Hybrid├────────
 sub A   │  H1  │ <──────────────────────┤  H2  │ sub B
         └──┬───┘   Rx pair  <──  (amp)  └──┬───┘
           Z_B                             Z_B
```

- Speech from A enters hybrid H1 and goes only to the **transmit (go) pair**, is amplified and reaches hybrid H2, which sends it to subscriber B.
- Speech from B goes through H2 to the **return pair** and then through H1 to A.
- Long-distance and carrier/PCM systems are four-wire; the conversion happens at the exchange or line card.

#### Operation of the hybrid
```text
                    4-wire receive (from far end)
                         │  (amplifier)
                         v
              ┌──────────────────────┐
  2-wire  ────┤ T1             T2    │
  line   ─────┤  (hybrid transformer)│
  (Z_L)       │   centre-tapped      │
              │   windings           │
  Balance ────┤                      ├───> 4-wire transmit
  network Z_B └──────────────────────┘     (to far end)
```

**Operation of the hybrid (bridge principle):**
1. **Receive → 2-wire:** a signal arriving on the 4-wire receive pair is fed to the centre-tapped winding. It divides equally between the **2-wire line ($Z_L$)** and the **balance network ($Z_B$)**. Half the power reaches the line (3 dB loss).
2. **No leakage to transmit pair:** the two halves flow in opposite senses through the windings coupled to the transmit pair, so their magnetic effects cancel when $Z_B = Z_L$. Ideally no receive signal appears on the transmit side.
3. **2-wire → transmit:** speech from the 2-wire line divides between the transmit and receive ports; the transmit port gets half (3 dB loss), and the part going to the receive port is absorbed by the amplifier output and does no harm.
4. **Balance:** in practice $Z_B$ only approximates $Z_L$ over the band, so a small signal leaks across. This is measured by the **balance return loss**
$$
B = 20\log_{10}\left|\frac{Z_L + Z_B}{Z_L - Z_B}\right|\ \text{dB}
$$
Poor balance causes **echo**, and if the loop gain round the four-wire circuit exceeds unity, **singing** (oscillation).

## Check yourself

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

1. Which is true for wireless media compared with wired?
   A. More secure and low interference
   B. Supports mobility and is quick to install in remote areas but is affected by fading and interference
   C. Unlimited bandwidth
   D. Needs digging and laying
   Answer: B. Wireless supports mobility and quick deployment, but suffers fading, interference and easier interception. (Key point: Wired vs wireless)

2. Why is a four-wire circuit used for long distance?
   A. Copper is cheaper
   B. Amplifiers, multiplexers and digital equipment are one-way devices needing separate paths per direction
   C. Four wires have lower loss
   D. Subscriber loops are four-wire
   Answer: B. One-way devices need separate transmit and receive paths, while the subscriber loop is two-wire. (Key point: Need for four-wire)

3. When is there no leakage of the receive signal into the transmit pair?
   A. When Z_B = Z_L, so the magnetic effects cancel
   B. When Z_B is zero
   C. When the loop gain exceeds unity
   D. When the amplifier is removed
   Answer: A. With the balance network matching the line impedance, the two halves cancel; imperfect balance gives echo and possible singing. (Key point: Hybrid balance)

## Related in Telecommunications

- [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 media comparison and hybrid operation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-media-hybrid-operation) — Telecommunications, chapter 2
- [Two-wire vs four-wire communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atwo-vs-four-wire) — 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×
- [Media classes and hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amedia-classes-hybrid-transformer) — Telecommunications, chapter 2
- [Optical fiber advantages and wireless media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aoptical-fiber-wireless-media) — Telecommunications, chapter 2

## Related in other subjects

- [Differentiate wired and wireless systems](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Awired-vs-wireless-diff) — Basic Electronics Engineering, chapter 5, asked 2×
- [Wired and wireless communication systems](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Awired-wireless-systems) — Basic Electronics Engineering, chapter 5, asked 2×
- [Guided vs unguided transmission media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-vs-unguided-media) — Data Communication, chapter 4
- [Optical fibre system vs wireless](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-working-vs-wireless) — Propagation and Antenna, chapter 6