---
title: "Optical fibre system vs wireless"
subject: "Propagation and Antenna"
chapter: 6
key: "semester_6/propagation_and_antenna:ofc-working-vs-wireless"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ofc-working-vs-wireless
site: BCT Engineering
---

# Optical fibre system vs wireless

Propagation and Antenna · Chapter 6: Optical Fibres

- Asked in: 2081 Chaitra, 3+5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ofc-working-vs-wireless
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6

## Question

Explain the working principle of optical fiber communication system. What are the advantages and disadvantages of optical fiber over wireless communication system?

## Short answer

In an optical fibre system, electrical signals modulate an optical source (LED/laser); light propagates by total internal reflection in the core and is detected by a photodiode. Fibre offers higher bandwidth (Tb/s), immunity to interference, security, and no spectrum licence; disadvantages include lack of mobility, higher installation cost, and vulnerability to cuts.

## Answer

#### Working principle

An **optical fibre communication system** transmits information as modulated light through a glass fibre. The electrical message signal is converted to light by a source, guided along the fibre by total internal reflection, and converted back to an electrical signal at the receiver.

```text
 Message -> Driver/   -> LED or
 (input)    modulator    laser
                           |
           ====== fibre (TIR, repeaters) ======
                           |
 Output  <- Amplifier, <- PIN/APD
            decision      detector
```

1. The information signal (usually digital) drives the **optical source** (LED or laser diode), which varies its light intensity with the data.
2. The light is coupled into the **fibre** within its acceptance angle and travels by TIR. Splices, connectors and repeaters/optical amplifiers (EDFA) along the route maintain the signal.
3. At the far end a **photodetector** (PIN or APD) converts the light into current.
4. The **receiver** amplifies, equalises and regenerates the signal and passes it to the user.

#### Advantages of optical fibre over wireless

| Point | Optical fibre | Wireless |
|---|---|---|
| Bandwidth | Tb/s per fibre | Limited by allocated spectrum |
| Interference | Immune to EMI and RF interference | Suffers interference, noise |
| Fading/weather | No fading, rain or multipath effects | Multipath fading, rain attenuation |
| Security | Very hard to tap | Signal open to interception |
| Spectrum licence | Not required | Required, and costly |
| Reliability | Stable, predictable loss | Varies with path and weather |
| Latency | Low, constant | Can vary |

#### Disadvantages of optical fibre compared to wireless

- **No mobility:** users must be at a fixed point; wireless supports mobile and roaming users.
- **High installation cost and time:** trenching, ducts, poles and right-of-way, especially over mountains and rivers (a real problem in hilly Nepal); wireless can be set up quickly.
- **Vulnerable to cuts:** road construction and landslides often break cables; wireless has no cable to cut along the path.
- **Hard to reach remote or scattered users:** satellite or microwave is cheaper for sparse rural areas.
- **Repair** needs splicing equipment and trained staff.
- **Broadcasting** to many receivers is easy over radio but needs a separate fibre path to each user.

## Check yourself

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

1. Which sequence describes fibre communication?
   A. Antenna, air, antenna
   B. Detector, fibre, source
   C. Modulator, mirror
   D. Source, fibre (TIR), photodetector, receiver
   Answer: D. Electrical to light and back. (Key point: Working)

2. Over wireless, fibre has:
   A. No spectrum licence needed and immunity to fading
   B. Mobility
   C. Easy broadcasting
   D. Lower installation cost
   Answer: A. Fibre is not affected by weather or multipath. (Key point: Advantage)

3. A key disadvantage versus wireless is:
   A. Limited bandwidth
   B. No mobility and high installation cost
   C. Fading
   D. Interception
   Answer: B. Needs trenching and fixed points. (Key point: Disadvantage)

## Related in Propagation and Antenna

- [Elements of optical communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-communication-elements) — Propagation and Antenna, chapter 6, asked 3×
- [Properties for selecting optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-selection-properties) — Propagation and Antenna, chapter 6
- [Dispersion and attenuation in optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Adispersion-attenuation) — Propagation and Antenna, chapter 6
- [Advantages of fibre over metalled wire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-advantages-metalled-wire) — Propagation and Antenna, chapter 6
- [Types of optical fibre: construction and propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-fiber-types-construction) — Propagation and Antenna, chapter 6, asked 2×
- [Optical sources and fibre losses](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-sources-fiber-losses) — Propagation and Antenna, chapter 6, asked 2×

## Related in other subjects

- [Communication system; optical fiber merits](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-optical-merits) — Basic Electronics Engineering, chapter 5
- [Optical fiber advantages and wireless media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aoptical-fiber-wireless-media) — Telecommunications, chapter 2
- [Advantages of optical fiber over copper](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aoptical-fiber-advantages) — Instrumentation II, chapter 5, asked 3×
- [Optical fiber advantages over coax and twisted pair](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afiber-advantages) — Data Communication, chapter 4, asked 3×