--- title: "Comparison of FDM, TDM and WDM" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:multiplexing-fdm-tdm-wdm" source: https://www.bctengineer.com/q/semester_8/telecommunications/multiplexing-fdm-tdm-wdm site: BCT Engineering --- # Comparison of FDM, TDM and WDM Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2078 Bhadra, 1+6 marks; 2074 Asoj, 2+6 marks (asked 2 times) - Video: [Types of Multiplexing | FDM TDM WDM | Analog Digital | Computer Networks](https://www.youtube.com/watch?v=WXof7bg_Zys) — Dr. Ankit Verma - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/multiplexing-fdm-tdm-wdm - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question Define multiplexing. Compare FDM, TDM and WDM with neat diagram. ## Short answer Multiplexing combines several independent signals onto a shared transmission medium to use capacity efficiently. FDM separates channels by frequency with guard bands, TDM by time slots in a repeating frame, and WDM by wavelength of light (optical FDM). ## Answer **Multiplexing** is the technique of sending several independent signals together over a single shared transmission medium, so that the high capacity of the medium is used efficiently and the cost per channel falls. #### FDM (Frequency Division Multiplexing) Each signal modulates a different carrier, so the channels sit side by side in frequency with guard bands between them; all channels are transmitted at the same time. Used for analogue telephony carrier, radio/TV broadcasting. ```text amplitude | [ch1] [ch2] [ch3] [ch4] | | |g | |g | |g | | +--+---+--+---+--+---+--+---+--> f f1 f2 f3 f4 g = guard band ``` #### TDM (Time Division Multiplexing) Each signal is sampled and given a short time slot in turn; all channels use the full bandwidth but at different times. Used in digital telephony (T1/E1), GSM. ```text |<-------------- one frame ------------->| | S1 | S2 | S3 | ... | SN | S1 | S2 | ... slot slot slot S_i = time slot of channel i ``` #### WDM (Wavelength Division Multiplexing) Used in optical fibre: each signal is carried on a different wavelength (colour) of light; a prism-like multiplexer combines them and a demultiplexer separates them. It is FDM at optical frequencies. ```text l1 -->|\ /|--> l1 l2 -->| MUX ==== fibre ==== DEMUX|--> l2 l3 -->|/ (l1+l2+...+lN) \|--> l3 lN --> --> lN l = wavelength (lambda) ``` #### Comparison | Point | FDM | TDM | WDM | |---|---|---|---| | Shared resource | Frequency band | Time | Wavelength of light | | Signal type | Mainly analogue | Mainly digital | Optical (digital) | | Channel separation | Guard bands | Guard times / sync | Wavelength spacing | | Medium | Copper, coax, radio | Copper, fibre, radio | Optical fibre | | Key devices | Modulators, filters | Commutator, clock, buffers | Lasers, optical filters, prisms/gratings | | Synchronisation | Not needed | Essential | Not needed between channels | | Main impairment | Crosstalk from filter overlap, intermodulation | Timing jitter, slips | Channel crosstalk, dispersion | | Example | 12-ch group, FM radio | T1 (24 ch), E1 (30 ch) | DWDM 40–96 λ backbones | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is multiplexing? A. Amplifying a signal for a long link B. Sending several independent signals over one shared medium to use its capacity efficiently C. Converting analog to digital D. Splitting one signal into noise and data Answer: B. Multiplexing shares one medium among many signals, lowering the cost per channel. (Key point: Definition) 2. What resource is shared among channels in FDM, TDM and WDM respectively? A. Time, frequency, wavelength B. Wavelength, time, frequency C. Frequency, wavelength, time D. Frequency, time, wavelength Answer: D. FDM divides frequency, TDM divides time slots, WDM divides optical wavelength. (Key point: Shared resource) 3. Which system essentially requires synchronisation between transmitter and receiver? A. TDM B. FDM C. WDM D. None of them Answer: A. TDM must keep slot timing aligned; FDM and WDM separate channels by filters. (Key point: Synchronisation) 4. What is WDM, and where is it used? A. FDM at audio frequencies on twisted pair B. TDM on radio links C. FDM at optical frequencies, carrying each signal on a different wavelength in a fibre D. Digital multiplexing using guard times Answer: C. WDM combines different wavelengths on one optical fibre using a mux and separates them with a demux. (Key point: WDM) ## Related in Telecommunications - [FDM and TDM with block diagrams](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-block) — Telecommunications, chapter 3 - [Multiplexing techniques and TDM standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-techniques-tdm-standards) — Telecommunications, chapter 3 - [Need for multiplexing and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-need-wdm) — Telecommunications, chapter 3 - [Compare multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acompare-multiplexing-techniques) — Telecommunications, chapter 3 - [Working principle of FDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-principle) — Telecommunications, chapter 3 - [Pulse stuffing in TDM synchronization](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apulse-stuffing-tdm-sync) — Telecommunications, chapter 3 ## Related in other subjects - [Compare FDM, WDM and TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acompare-fdm-wdm-tdm) — Data Communication, chapter 6 - [Multiplexing; synchronous vs asynchronous TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-compare-sync-async-tdm) — Data Communication, chapter 6, asked 6× - [TDM vs FDM for voice; E1 and T1 hierarchies](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Atdm-fdm-voice-e1-t1) — Communication Systems, chapter 5 - [Short note: FDM vs TDM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm-vs-tdm) — Communication Systems, chapter 5