--- title: "Channel in communication system and its classification" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:channel-classification" source: https://www.bctengineer.com/q/semester_6/communication_systems/channel-classification site: BCT Engineering --- # Channel in communication system and its classification Communication Systems · Chapter 1: Introduction - Asked in: 2073 Magh (CS I), 4+6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/channel-classification - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question What do you mean by channel in communication system? Classify the channel with example. ## Short answer A channel is the physical medium carrying the signal from transmitter to receiver. Channels are classified by medium (wired/wireless), input-output nature (analog/digital), time behavior (time-invariant/fading), linearity, noise model (AWGN), and transmission direction (simplex, half-duplex, full-duplex). ## Answer #### Channel A **channel** is the physical medium (and associated equipment) that carries the signal from the transmitter to the receiver. It may be a pair of wires, a coaxial cable, an optical fibre, a waveguide, or free space (radio). Every channel has a limited **bandwidth**, causes **attenuation** and **distortion**, and adds **noise** and **interference**; these characteristics set the maximum data rate (Shannon capacity $C = B\log_2(1+S/N)$) and decide which modulation is suitable. #### Classification of channels **1. Based on physical medium** - **Wired (guided) channels:** the signal is guided along a physical path. - *Twisted pair:* telephone local loop, LAN (Cat-5/6); low cost, limited bandwidth. - *Coaxial cable:* cable TV, older Ethernet; larger bandwidth, better shielding. - *Optical fibre:* backbone networks; very wide bandwidth (THz), very low loss, immune to EMI. - *Waveguide:* microwave links inside equipment. - **Wireless (unguided) channels:** signal propagates through free space by electromagnetic waves. - *Ground wave* (LF/MF, AM broadcasting), *sky wave* (HF, ionospheric reflection), *line-of-sight / space wave* (VHF and above: FM, TV, mobile, microwave links), *satellite channels*. - Underwater acoustic channels and optical free-space links are also examples. **2. Based on the nature of input/output** - **Continuous (analog) channel:** input and output are continuous waveforms, e.g. a telephone voice channel. - **Discrete (digital) channel:** input and output are symbols, e.g. the binary symmetric channel (BSC), a channel of error probability $p$. **3. Based on time behaviour** - **Time-invariant channel:** characteristics do not change with time, e.g. coaxial cable, fibre. - **Time-varying (fading) channel:** characteristics change with time, e.g. mobile radio with multipath fading, ionospheric HF links. **4. Based on linearity** - **Linear channel:** obeys superposition, e.g. cables. - **Non-linear channel:** e.g. satellite transponder with TWT amplifier near saturation. **5. Based on noise model** - **AWGN channel:** only additive white Gaussian noise; a basic model for satellite and deep-space links. - **Band-limited channel:** limited bandwidth causing ISI, e.g. telephone line. - **Multipath fading channel:** e.g. cellular radio. **6. Based on direction of transmission** - **Simplex:** one direction only, e.g. radio/TV broadcasting. - **Half-duplex:** both directions but one at a time, e.g. walkie-talkie. - **Full-duplex:** both directions simultaneously, e.g. telephone, mobile phone. | Channel | Bandwidth | Typical use | |---|---|---| | Twisted pair | Up to ~100 MHz | Telephone, LAN | | Coaxial cable | Up to ~1 GHz | Cable TV | | Optical fibre | THz range | Internet backbone | | Radio (free space) | kHz to GHz | Broadcast, mobile, satellite | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A communication channel is: A. The physical medium carrying the signal from transmitter to receiver B. The output transducer C. The message source D. The modulator only Answer: A. A channel is the physical path (wire, fibre, free space) and equipment. (Key point: Channel definition) 2. A telephone call, with both directions at the same time, is an example of: A. Broadcast B. Full-duplex C. Simplex D. Half-duplex Answer: B. Full-duplex is simultaneous two-way; walkie-talkie is half-duplex and radio/TV is simplex. (Key point: Duplex types) 3. A mobile radio channel with multipath fading is classified as: A. Simplex only B. Linear cable C. Time-invariant D. Time-varying (fading) Answer: D. Its characteristics change with time. (Key point: Time behaviour) 4. The binary symmetric channel (BSC) is an example of: A. Wireless ground wave B. Non-linear channel C. Continuous channel D. Discrete (digital) channel with error probability p Answer: D. BSC has symbol input/output with error probability p. 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