---
title: "Physical description of guided media"
subject: "Data Communication"
chapter: 4
key: "semester_5/data_communication:guided-media-description"
source: https://www.bctengineer.com/q/semester_5/data_communication/guided-media-description
site: BCT Engineering
---

# Physical description of guided media

Data Communication · Chapter 4: Transmission Media

- Asked in: 2069 Chaitra, 1 marks; 2069 Asar, 3 marks (asked 2 times)
- Video: [Guided Transmission Media | Wired Media 🔥](https://www.youtube.com/watch?v=CYXeiIakVXY) — Perfect Computer Engineer
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/guided-media-description
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-4

## Question

Briefly describe the physical description of each guided transmission medium.

## Short answer

Twisted pair consists of two insulated copper wires twisted together, bundled in a sheath; UTP has no shield, STP has a metal shield. Coaxial cable has a central conductor, insulating dielectric, braided shield, and outer jacket. Optical fibre has a glass or plastic core, cladding with lower refractive index, and a protective jacket.

## Answer

Guided media are physical cables that direct the signal along a fixed path. The three common types are:

#### 1. Twisted pair cable
Two insulated copper wires (typically 22-26 AWG, a conductor about 0.4 to 0.65 mm in diameter) twisted together in a regular spiral. Many pairs are bundled in one sheath. There are two types:
- **UTP**: no shield, only a plastic jacket (Cat 3, 5e, 6).
- **STP**: pairs covered by a metal foil or braid.

The twisting cancels interference and crosstalk.

#### 2. Coaxial cable
```text
 [ outer plastic jacket ]
   [ braided metal shield (outer conductor) ]
     [ insulating dielectric ]
       ( inner copper conductor )
```
A central solid or stranded copper conductor, surrounded by an insulator, then a braided metal shield, and an outer plastic jacket. The shield gives better noise protection and higher bandwidth than twisted pair. Types: 50 ohm (baseband) and 75 ohm (cable TV).

#### 3. Optical fibre
A thin strand of glass or plastic with three parts: the **core** (carries light), the **cladding** (glass of lower refractive index, which keeps light in the core by total internal reflection) and a protective **jacket**. Core diameters are about 8-10 µm (single mode) or 50-62.5 µm (multimode). Light pulses from an LED or laser carry the data.

## Check yourself

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

1. In optical fibre, what is the job of the cladding?
   A. It carries the light pulses
   B. It has a lower refractive index than the core, keeping light in by total internal reflection
   C. It shields the fibre from electromagnetic noise like a braid
   D. It is the LED or laser source
   Answer: B. Cladding has lower refractive index, so light is totally internally reflected in the core. (Key point: Cladding role)

2. What is the correct order of layers in a coaxial cable from the centre outward?
   A. Inner conductor, braided shield, dielectric, outer jacket
   B. Dielectric, inner conductor, outer jacket, shield
   C. Inner conductor, dielectric insulator, braided shield, outer jacket
   D. Inner conductor, jacket, shield, dielectric
   Answer: C. The centre conductor is insulated by a dielectric, then covered by the braided shield and a plastic jacket. (Key point: Coax construction)

3. What distinguishes STP from UTP?
   A. STP pairs are covered by a metal foil or braid; UTP has only a plastic jacket
   B. STP has more twists per metre than UTP
   C. UTP uses fibre while STP uses copper
   D. STP has a single pair, UTP has many pairs
   Answer: A. The metal foil or braid shield is what makes it shielded. (Key point: UTP vs STP)

## Related in Data Communication

- [Guided vs unguided transmission media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-vs-unguided-media) — Data Communication, chapter 4
- [Transmission characteristics of guided media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-media-transmission-characteristics) — Data Communication, chapter 4

## Related in other subjects

- [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×
- [Guided and unguided transmission media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atransmission-media-explain) — 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 and unguided media for radio signals](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aradio-guided-unguided-compare) — Telecommunications, chapter 2