--- title: "OFC block diagram vs cable" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:ofc-block-diagram-vs-cable" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ofc-block-diagram-vs-cable site: BCT Engineering --- # OFC block diagram vs cable Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2071 Magh, 3+5 marks - Video: [Fiber Optic Communication System (Block Diagram, Basics, Details & working) Explained](https://www.youtube.com/watch?v=q6_q2IBm93o) — Engineering Funda - 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-block-diagram-vs-cable - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question Explain the block diagram of optical communication and list out the advantages and disadvantages of it over cable communication. ## Short answer An OFC system encodes a message into an electrical signal, drives an LED/laser to launch light into a fibre, and a photodetector plus amplifier/decoder recovers it; repeaters or EDFAs extend long routes. Advantages over cable: huge bandwidth, low loss, EMI immunity, light weight, security; disadvantages: costly splicing, fragile, needs electrical-optical conversion and cannot power remote repeaters. ## Answer An optical fibre communication (OFC) system converts an electrical message into light, sends it through a glass fibre, and converts it back to an electrical signal at the receiver. #### Block diagram ```text Message +-----------+ +---------+ +--------+ input --->| Encoder / |-->| Driver |-->| Light | | modulator | | circuit | | source | +-----------+ +---------+ +---+----+ LED/laser | v ~~~~~~~~ optical fibre (splices, connectors, repeaters) ~~~~~~~~ | v Message +-----------+ +---------+ +---+----+ output <--| Decoder / |<--| Amp. + |<--| Photo- | | demod. | | regen. | |detector| +-----------+ +---------+ +--------+ PIN / APD ``` 1. **Information source and encoder**: the analog or digital message is coded (e.g. PCM) into an electrical signal. 2. **Driver circuit**: supplies the current that modulates the light source. 3. **Light source**: an LED or laser diode converts the electrical signal into light at 850, 1310 or 1550 nm. 4. **Source-to-fibre coupler**: launches the light into the fibre within its acceptance angle. 5. **Optical fibre (channel)**: carries light by total internal reflection; splices, connectors and repeaters or optical amplifiers (EDFA) are added on long routes. 6. **Photodetector**: a PIN or avalanche photodiode converts light back to current. 7. **Amplifier, regenerator and decoder**: amplify, reshape and decode the signal to recover the message. #### Advantages over cable (copper) communication - Huge bandwidth (THz range), so very high data rates. - Very low loss (0.2 dB/km), so long repeater spacing. - Immune to electromagnetic interference, lightning and crosstalk. - No electrical hazard or sparking; safe in explosive areas. - Small size and light weight. - Difficult to tap, so more secure. - Made from silica (sand), a plentiful raw material. #### Disadvantages - Splicing and connectors need costly tools and skilled workers. - Fibre is fragile; bending it below its minimum bend radius causes loss or breakage. - Cannot carry electrical power to remote repeaters directly. - Optical sources, detectors and test equipment are costly. - Electrical–optical conversion is needed at each end. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the OFC transmitter, what does the driver circuit do? A. Supplies the current that modulates the light source B. Converts light back to current C. Amplifies the optical signal in the fibre D. Couples light into the fibre core Answer: A. The driver circuit supplies the modulating current to the LED or laser, so the electrical message becomes a light signal. (Key point: Role of the driver circuit) 2. Which sequence correctly describes the receiver side of an OFC system? A. Decoder, amplifier, photodetector B. Photodetector, amplifier and regenerator, decoder C. Amplifier, light source, decoder D. Photodetector, driver, encoder Answer: B. A PIN or APD photodetector turns light into current, which is amplified and reshaped, then decoded to recover the message. (Key point: Receiver chain) 3. Why is fibre immune to electromagnetic interference and hard to tap compared with copper cable? A. It carries a larger current B. It uses light in glass, so it is not affected by EM fields and does not radiate C. It is shielded by a metal sheath D. It operates at lower frequency Answer: B. Information travels as light in a dielectric, so EM fields do not induce noise, and no signal radiates outside the fibre. (Key point: Advantage: EMI and security) 4. Which is a genuine disadvantage of OFC over copper cable? A. Higher crosstalk B. Very high attenuation (about 10 dB/km) C. It cannot carry electrical power to remote repeaters D. Low bandwidth Answer: C. Glass cannot carry power, so remote repeaters need a separate supply. Fibre also needs costly splicing and electrical-optical conversion. 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