--- title: "Block diagram of digital communication system" subject: "Data Communication" chapter: 1 key: "semester_5/data_communication:block-diagram-digital-comm" source: https://www.bctengineer.com/q/semester_5/data_communication/block-diagram-digital-comm site: BCT Engineering --- # Block diagram of digital communication system Data Communication · Chapter 1: Introduction - Asked in: 2076 Chaitra, 6 marks; 2075 Chaitra, 8 marks; 2074 Chaitra, 8 marks; 2073 Chaitra, 6 marks; 2073 Shrawan, 5 marks; 2071 Shrawan, 6 marks; 2069 Chaitra, 6 marks; 2068 Chaitra, 8 marks (asked 8 times) - Video: [Block Diagram of Digital Communication System | Objectives of Digital Communication System](https://www.youtube.com/watch?v=Z0Ylnk8zXRo) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/data_communication/block-diagram-digital-comm - Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-1 ## Question Draw a generic block diagram of a digital communication system (full duplex mode) and explain the function of each block. ## Short answer The transmitter chain: source encoder digitizes and compresses, channel encoder adds error-control redundancy, modulator converts to channel signal. Receiver chain reverses the process. Full-duplex uses separate frequency bands or wire pairs to send and receive simultaneously. ## Answer A digital communication system moves information from a source to a destination as a stream of bits. In full-duplex mode each end has both a transmitter and a receiver, so both sides send and receive at the same time. ```text Side A Side B +--------+ +-------+ +-------+ +-------+ +-------+ |Source/ |->|Source |->|Channel|->|Modula-|->| | | Info | |encoder| |encoder| | tor | | | +--------+ +-------+ +-------+ +-------+ | Chan- | +--------+ +-------+ +-------+ +-------+ | nel | |Destin. |<-|Source |<-|Channel|<-|Demodu-|<-| | | | |decoder| |decoder| | lator | | (+ | +--------+ +-------+ +-------+ +-------+ | noise)| +-------+ (Side B has the same chain in the opposite direction; two separate paths/frequencies give full duplex.) ``` #### Function of each block 1. **Information source / input transducer**: produces the message (speech, video, text) and converts it to an electrical signal. 2. **Source encoder**: digitises the signal (sampling, quantization, PCM) and removes redundancy (compression) so fewer bits are sent. 3. **Channel encoder**: adds controlled redundancy (parity, CRC, Hamming, convolutional codes) so the receiver can detect and correct errors. 4. **Modulator (line coder)**: converts the bits into a signal suited to the channel: baseband pulses, or a carrier modulated by ASK/FSK/PSK/QAM. 5. **Channel**: the physical medium (twisted pair, coaxial, fibre, air). It adds attenuation, distortion and noise. 6. **Demodulator**: recovers the bit stream from the received, distorted signal. 7. **Channel decoder**: uses the redundant bits to detect/correct errors and removes them. 8. **Source decoder**: rebuilds the original message (decompression, D/A conversion). 9. **Destination / output transducer**: delivers the message to the user (speaker, screen, computer). In full duplex, a **hybrid or separate paths** (two wire pairs, or two frequency bands) keep the two directions from interfering. A telephone call is a common example. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the job of the source encoder? A. Digitise the signal and remove redundancy so fewer bits are sent B. Add controlled redundancy to correct errors C. Convert bits into a carrier-modulated signal D. Rebuild the original message at the receiver Answer: A. The source encoder samples, quantizes and compresses the message; adding redundancy is the channel encoder's job. (Key point: Source encoder) 2. Why does the channel encoder add extra bits (parity, CRC, Hamming)? A. To compress the message B. So the receiver can detect and correct errors C. To amplify the signal against attenuation D. To convert analog data to digital Answer: B. Controlled redundancy lets the channel decoder detect and correct errors caused by channel noise. (Key point: Channel encoder) 3. What does the modulator do? A. Removes noise from the channel B. Digitises the voice signal by PCM C. Converts bits into a signal suited to the channel, such as baseband pulses or a modulated carrier D. Detects errors using redundant bits Answer: C. The modulator (line coder) maps bits to a waveform suitable for transmission, using ASK/FSK/PSK/QAM or baseband pulses. (Key point: Modulator) 4. How is full-duplex operation achieved in the block diagram? A. Both ends share one path and take turns B. Only one end has a transmitter C. A single encoder serves both directions at different times D. Each end has a transmitter and receiver, with separate paths or frequency bands for the two directions Answer: D. Separate wire pairs or two frequency bands (or a hybrid) stop the two directions interfering, so both sides send at once. (Key point: Full duplex) ## Related in Data Communication - [Compare analog and digital communication](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adigital-vs-analog-comparison) — Data Communication, chapter 1, asked 4× - [Data representation; analog vs digital system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adata-representation-analog-digital) — Data Communication, chapter 1 - [Short note: Multi mode optical fiber](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amultimode-fiber-short-note) — Data Communication, chapter 4 - [Fourier series of full-wave rectified sine](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afs-full-wave-rectified) — Data Communication, chapter 3 - [Advantages of digital over analog systems](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adigital-vs-analog-advantages) — Data Communication, chapter 1, asked 5× - [PCM and analog-to-digital conversion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apcm-working) — Data Communication, chapter 5, asked 4× ## Related in other subjects - [Reasons for modulation; digital communication block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-reasons-digital-adv-duplex) — Communication Systems, chapter 1 - [Duplex digital communication system vs analog](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-comm-duplex-vs-analog) — Communication Systems, chapter 1 - [Digital vs analog communication; DCS block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-vs-analog-dcs-block) — Communication Systems, chapter 1 - [Function of basic components of a digital communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adcs-basic-components) — Communication Systems, chapter 1