--- title: "FDM and TDM with block diagrams" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:multiplexing-fdm-tdm-block" source: https://www.bctengineer.com/q/semester_8/telecommunications/multiplexing-fdm-tdm-block site: BCT Engineering --- # FDM and TDM with block diagrams Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2080 Baisakh, 2+6 marks - Video: [TDM, Statistical TDM & FDM](https://www.youtube.com/watch?v=fIZhDl35_XY) — Sunny Classroom - 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-block - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question Define multiplexing. Discuss about FDM and TDM with the help of neat block diagrams. ## Short answer Multiplexing combines several independent signals into one composite signal for transmission over a single medium; demultiplexing reverses it. FDM divides bandwidth into frequency bands, each signal modulates its own carrier, sent simultaneously; TDM divides time into frames and slots, each signal sampled and sent in turn. ## Answer **Multiplexing** is the process of combining several independent message signals into one composite signal so that they can be sent over a single transmission medium; the reverse process at the receiver is **demultiplexing**. It saves cost because one high-capacity link replaces many separate links. #### Frequency Division Multiplexing (FDM) The available bandwidth is divided into frequency bands; each signal modulates its own carrier and occupies one band. All signals are sent at the same time. ```text TRANSMITTER ch1 -[LPF]-[Mod f1]-[BPF]-+ ch2 -[LPF]-[Mod f2]-[BPF]-+--(SUM)--> line chN -[LPF]-[Mod fN]-[BPF]-+ RECEIVER +-[BPF f1]-[Demod f1]-[LPF]-> ch1 line -+-[BPF f2]-[Demod f2]-[LPF]-> ch2 +-[BPF fN]-[Demod fN]-[LPF]-> chN ``` **Working:** 1. Each baseband signal is band-limited by an LPF (voice 0.3–3.4 kHz). 2. It modulates a sub-carrier $f_1, f_2, \ldots, f_N$ (SSB-SC in telephony to save bandwidth). 3. A BPF keeps only the wanted sideband; channels are spaced 4 kHz apart, leaving **guard bands**. 4. All outputs are summed and transmitted. 5. At the receiver, BPFs separate each band, demodulators bring it back to baseband and LPFs recover each message. Example: CCITT basic group – 12 voice channels in 60–108 kHz. ```text amplitude | [ch1] [ch2] [ch3] [ch4] | | |g | |g | |g | | +--+---+--+---+--+---+--+---+--> f f1 f2 f3 f4 g = guard band ``` #### Time Division Multiplexing (TDM) The time axis is divided into frames and each frame into time slots; each signal is sampled and sent in its own slot in turn, using the full bandwidth during that slot. ```text ch1 -[LPF]-o o-[LPF]- ch1 ch2 -[LPF]-o \ / o-[LPF]- ch2 (rotating) link (rotating) chN -[LPF]-o / switch ------> \ o-[LPF]- chN commutator decommutator ^ ^ +-- clock / sync ----+ ``` **Working:** 1. Each signal is band-limited by an LPF. 2. A **commutator** (electronic rotating switch) samples channel 1, 2, …, N in turn at the sampling rate $f_s \geq 2f_m$ (8 kHz for voice). 3. The interleaved samples (PAM) are usually quantised and coded (PCM) and sent with frame synchronisation bits. 4. At the receiver a **decommutator**, synchronised to the transmitter by the clock and framing pattern, sends each sample to the correct output, where an LPF rebuilds the signal. ```text |<-------------- one frame ------------->| | S1 | S2 | S3 | ... | SN | S1 | S2 | ... slot slot slot S_i = time slot of channel i ``` Example: E1 – 32 slots × 8 bits × 8000 = 2.048 Mbps carrying 30 voice channels. | FDM | TDM | |---|---| | Shares frequency | Shares time | | Analogue signals | Mostly digital | | Needs filters, guard bands | Needs synchronisation, guard time | | Crosstalk from non-linearity | Little crosstalk | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the order of processing for each channel in an FDM transmitter? A. Modulator, LPF, BPF B. LPF, modulator at its own sub-carrier, BPF, then summing C. BPF, LPF, summer D. Commutator, coder, LPF Answer: B. Band-limit, shift to its own band, filter the wanted sideband, then add all channels. (Key point: FDM transmitter chain) 2. Why is SSB-SC used with guard bands in telephone FDM? A. To save bandwidth (4 kHz per channel) and allow for filter roll-off and reduce crosstalk B. To increase carrier power C. To avoid filters D. To make channels digital Answer: A. SSB-SC halves bandwidth; guard bands allow practical filters. (Key point: Why SSB-SC and guard bands) 3. What does the commutator do in TDM? A. Adds carriers to each channel B. Filters each channel into a band C. Rebuilds the signal at the receiver D. Samples channels 1 to N in turn at fs >= 2fm Answer: D. The rotating switch interleaves samples; the decommutator at the receiver, synchronised by clock and framing, separates them. (Key point: TDM commutation) 4. Which comparison is correct? A. FDM needs synchronisation; TDM needs guard bands in frequency B. FDM shares frequency and suffers crosstalk from non-linearity; TDM shares time and needs synchronisation C. Both share time D. TDM is mainly analog Answer: B. FDM channels sit in different bands at the same time; TDM channels take turns using the full band. (Key point: FDM vs TDM) ## Related in Telecommunications - [Comparison of FDM, TDM and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-wdm) — Telecommunications, chapter 3, asked 2× - [Multiplexing techniques and TDM standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-techniques-tdm-standards) — 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 - [TDM types and pulse stuffing](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-types-pulse-stuffing) — Telecommunications, chapter 3 - [Pulse stuffing and complete TDM PCM system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-pulse-stuffing-tdm-pcm) — Telecommunications, chapter 3 ## Related in other subjects - [T1 and E1 TDM-PCM hierarchy with frame diagrams](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-e1-frame-diagrams) — Communication Systems, chapter 5 - [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× - [Compare FDM, WDM and TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acompare-fdm-wdm-tdm) — Data Communication, chapter 6 - [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