--- title: "OFC system; pros and cons over metal wire" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:ofc-system-vs-metal-wire" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ofc-system-vs-metal-wire site: BCT Engineering --- # OFC system; pros and cons over metal wire Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2074 Bhadra, 3+6 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-system-vs-metal-wire - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question Draw the optical fiber communication system. What are the advantages and disadvantages of optical fibers over metal wire communication? ## Short answer A fibre system comprises a transmitter (LED/laser), fibre, amplifiers/repeaters and a receiver (PIN/APD). Advantages over metal wire: enormous bandwidth, very low loss (~0.2 dB/km), immunity to EMI, small size and electrical isolation. Disadvantages: higher installation and splicing skill required, fragility, inability to carry power, and need for O/E conversion. ## Answer #### Optical fibre communication system ```text +--------+ +----------+ +---------+ | Info |-->| Electr. |-->| LED / | | source | | Tx/driver| | laser | +--------+ +----------+ +----+----+ | ===== fibre cable (splices, connectors, ===== repeaters / optical amplifiers) | +--------+ +----------+ +----v----+ | Dest. |<--| Amplifier|<--| PIN/APD | | | | /decision| | detector| +--------+ +----------+ +---------+ ``` The message is converted to an electrical drive signal, which modulates the intensity of an LED or laser. The light travels through the fibre by total internal reflection, is boosted by repeaters or optical amplifiers on long routes, and is converted back to current by a photodiode. The receiver amplifies and regenerates the signal for the user. #### Advantages over metal wire communication | Advantage over copper wire | Reason | |---|---| | Huge bandwidth | Optical carrier ~200 THz; Tb/s on one fibre | | Low loss | ~0.2 dB/km, repeaters every 80–100 km (copper: ~2 km) | | Immune to EMI and crosstalk | Light is not affected by electrical noise or lightning | | Electrical isolation | Glass is an insulator; no ground loops or sparks | | Small size, light weight | Fibre is ~125 µm; a cable is much lighter than copper | | Signal security | No radiation; tapping is hard to do undetected | | Raw material | Silica (sand) is plentiful; copper is costly | | Long life, corrosion-free | Glass does not corrode | #### Disadvantages compared to metal wire | Disadvantage | Reason | |---|---| | Costly installation | Fusion splicers, OTDR and skilled staff needed | | Fragile | Glass breaks if bent tightly or crushed | | Difficult joining | Micron-level alignment for splices/connectors | | No power feeding | Cannot carry DC power to repeaters/terminals like copper | | Optical–electrical conversion | Extra transmitter and receiver circuits needed | | Repair is harder | Locating and fixing a cut needs special tools | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does fibre offer huge bandwidth? A. It uses low-frequency currents B. It is thicker than copper C. It has high resistance D. Its optical carrier is around 200 THz Answer: D. The very high carrier frequency allows Tb/s on one fibre. (Key point: Bandwidth) 2. Why is fibre immune to EMI and crosstalk? A. Light is not affected by electrical noise or lightning and glass is an insulator B. It is shielded with metal C. It has a lower bit rate D. It uses amplifiers Answer: A. No electrical currents flow in the glass. (Key point: EMI immunity) 3. Which is a disadvantage of fibre against metal wire? A. Higher attenuation B. It cannot carry DC power to repeaters or terminals C. Susceptibility to lightning D. Heavier cable Answer: B. Copper can feed power; fibre cannot. (Key point: Disadvantage) 4. What is the repeater spacing for fibre versus copper? A. Fibre 2 km; copper 80-100 km B. Both 2 km C. Fibre 80-100 km; copper about 2 km D. Both 100 km Answer: C. Low loss of ~0.2 dB/km allows long spacing. (Key point: Repeater spacing) ## Related in Propagation and Antenna - [Fibre uses; pros and cons over metal wire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-uses-advantages-metal-wire) — Propagation and Antenna, chapter 6, asked 2× - [OFC pros and cons over metallic wire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-advantages-disadvantages-metallic) — Propagation and Antenna, chapter 6 - [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× - [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 - [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 ## Related in other subjects - [Role of antenna; optical fiber pros and cons](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aantenna-role-optical) — Basic Electronics Engineering, chapter 5 - [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 - [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×