---
title: "Media types, four-wire system and hybrid"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:media-types-four-wire-hybrid"
source: https://www.bctengineer.com/q/semester_8/telecommunications/media-types-four-wire-hybrid
site: BCT Engineering
---

# Media types, four-wire system and hybrid

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2072 Chaitra, 2+6 marks
- Video: [Part 1.17 - Transmission Medium Wired & Guided Twisted Pair Cable Coaxial Cable Optical Fiber](https://www.youtube.com/watch?v=jxcW3Wfj8s8) — KnowledgeGATE by Sanchit Sir
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/media-types-four-wire-hybrid
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

List the types of transmission media. Briefly explain four-wire transmission system and operation of hybrid.

## Short answer

Guided media include twisted pair, coax, fibre, waveguide; unguided include ground/sky/space waves, microwave, satellite, infrared. A four-wire system uses separate GO and RETURN paths, with a hybrid at each end. The hybrid’s balancing network matches line impedance, coupling signals between two-wire and four-wire while isolating receive from send to prevent echo and singing.

## Answer

#### Types of transmission media
1. **Guided (wired) media**
   - Open-wire lines
   - Twisted pair cable (UTP/STP)
   - Coaxial cable
   - Optical fibre cable (single-mode, multimode)
2. **Unguided (wireless) media**
   - Radio waves (ground wave, sky wave, broadcast)
   - Terrestrial microwave (line of sight)
   - Satellite links
   - Infrared and light-wave (free-space optics)

#### Four-wire transmission system
In a **four-wire system** each direction of transmission has its own pair: one pair (GO) carries speech from A to B and the other pair (RETURN) carries speech from B to A. Subscriber lines are two-wire (both directions on one pair), but long trunk circuits need **one-way amplifiers** and multiplexing equipment, which work in one direction only, so trunks are four-wire.

```text
 Sub A                                         Sub B
 2-wire  +-----+   GO pair  ->[amp]->  +-----+ 2-wire
 o-------| HYB |=======================| HYB |-------o
         |  A  |=======================|  B  |
         +-----+  <-[amp]<-  RET pair  +-----+
              4-wire trunk / carrier
```
Features:
- Gain can be inserted separately in each direction, so long distances are possible.
- Required for FDM/TDM carrier and radio links, which are inherently one-way.
- Needs a **hybrid** (2W/4W termination) at each end to connect the two-wire subscriber loops.
- Risk of echo and singing if the hybrids are not balanced.

#### Operation of hybrid
A hybrid is a three-winding bridge transformer with a **balancing network** $Z_B$ that matches the two-wire line impedance $Z_L$.
1. **Transmit:** speech from the 2-wire line is coupled to the 4-wire GO (send) pair.
2. **Receive:** speech from the 4-wire RETURN pair is fed to the centre tap and splits equally between $Z_L$ and $Z_B$; the subscriber hears it.
3. **Isolation:** the two receive currents produce opposite fluxes in the send winding, so when $Z_B = Z_L$ they cancel and nothing from the RETURN pair reaches the GO pair. This high **transhybrid loss** prevents echo and singing.
4. Each through path loses about 3 dB: received power splits between the line and the balancing network, and power from the line splits between the GO and RETURN ports (the part reaching the RETURN amplifier output is absorbed).

## Check yourself

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

1. Which is a guided medium?
   A. Satellite link
   B. Infrared link
   C. Coaxial cable
   D. Sky wave radio
   Answer: C. Open wire, twisted pair, coax and fibre are guided; radio, microwave, satellite and infrared are unguided. (Key point: Media classes)

2. In a four-wire system:
   A. One pair carries both directions
   B. Each direction has its own pair, allowing one-way amplifiers
   C. Only digital signals are used
   D. Two pairs are used for one direction
   Answer: B. GO and RETURN pairs allow one-way amplification over long trunks. (Key point: Four-wire system)

3. Why does a signal from the 4-wire receive pair not appear on the transmit pair of a balanced hybrid?
   A. The amplifier on the receive pair blocks it
   B. Its two halves flow in opposite senses through the windings coupled to the transmit pair, so their fluxes cancel when Z_B = Z_L
   C. It is filtered out because it uses a different frequency band
   D. The transmit pair is shorted by the balance network
   Answer: B. The signal splits equally into Z_L and Z_B; with Z_B = Z_L the opposing magnetic effects cancel in the transmit winding. (Key point: Why no leakage)

4. In a hybrid, a signal arriving from the 4-wire receive pair reaches the 2-wire line with what loss, and why?
   A. 0 dB, because the whole signal goes to the line
   B. 6 dB, because it is attenuated twice
   C. About 3 dB, because half the power goes into the balance network
   D. Infinite, because the hybrid isolates the line
   Answer: C. The signal divides equally between the 2-wire line and the balance network, so half the power (3 dB loss) reaches the line. (Key point: Hybrid 3 dB loss)

## Related in Telecommunications

- [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×
- [Guided media comparison and hybrid operation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-media-hybrid-operation) — 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
- [Guided and unguided transmission media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atransmission-media-explain) — Telecommunications, chapter 2, asked 2×
- [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

## 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
- [Guided and other transmission media](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Atransmission-media-explain) — Computer Networks and Security, chapter 2, asked 3×
- [Define transmission media](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Adefine-transmission-media) — Computer Networks and Security, chapter 2, 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