---
title: "Short note: FDM vs TDM"
subject: "Communication Systems"
chapter: 5
key: "semester_6/communication_systems:short-note-fdm-vs-tdm"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-fdm-vs-tdm
site: BCT Engineering
---

# Short note: FDM vs TDM

Communication Systems · Chapter 5: Multiplexing Techniques

- Asked in: 2081 Chaitra, 5 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_6/communication_systems/short-note-fdm-vs-tdm
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5

## Question

Write a short note on FDM vs TDM.

## Short answer

FDM assigns each signal a separate frequency band with guard bands, used for analog signals like radio. TDM assigns each signal a time slot in a frame with guard times, used for digital signals like T1/E1 telephony. FDM needs modulators/filters, TDM uses samplers and synchronisation.

## Answer

**Frequency division multiplexing (FDM)** sends several signals at the same time over one channel by giving each signal a different frequency band. **Time division multiplexing (TDM)** sends several signals over one channel by giving each signal a different time slot, one after another.

#### FDM
- Each message is band-limited, then modulates its own sub-carrier (usually SSB in telephony).
- Guard bands between adjacent channels let the receiver separate them with band-pass filters.
- Example: 12 telephone channels in the 60–108 kHz basic group, and AM/FM broadcasting.

#### TDM
- Each message is sampled. A commutator takes one sample from each channel in turn to form a frame.
- The receiver separates the channels with a synchronised decommutator.
- Example: a T1 frame carries 24 PCM voice channels at 1.544 Mb/s.

```text
 FDM: frequency axis          TDM: time axis
 | ch1 | ch2 | ch3 |  f       |1|2|3|1|2|3|1|2|3|  t
  all at the same time         one band, shared in time
```

#### Comparison

| Point | FDM | TDM |
|---|---|---|
| Sharing basis | Each signal gets its own frequency band | Each signal gets its own time slot |
| Time use | All signals sent at the same time | Signals sent one after another |
| Signal type | Mainly analog | Mainly digital (PAM/PCM samples) |
| Hardware | Modulators, sharp band-pass filters, oscillators per channel | Sampler, commutator, mostly digital circuits |
| Guard | Guard bands between channels | Guard times between slots |
| Crosstalk cause | Non-linearity, poor filters | Pulse spreading (ISI), poor sync |
| Synchronisation | Not critical | Frame sync is essential |
| Fading effect | Narrow-band fading hits only some channels | Fading during a slot hits one channel |
| Example | Radio/TV broadcast, old analog telephone trunks | T1/E1 digital telephony, GSM slots |

FDM suits analog signals. TDM suits digital signals, and it is used in modern telephone networks because digital hardware is cheap and regenerative repeaters stop noise from building up along the link.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How do FDM and TDM share a channel?
   A. FDM by time slots, TDM by frequency bands
   B. FDM by frequency bands at the same time, TDM by time slots one after another
   C. Both by frequency but with different filters
   D. Both by time but with different carriers
   Answer: B. FDM gives each signal its own band, all sent simultaneously; TDM gives each its own slot in time. (Key point: Sharing basis)

2. Which system critically needs frame synchronisation?
   A. FDM
   B. TDM
   C. Neither
   D. Both equally
   Answer: B. TDM needs a synchronised decommutator to separate channels; FDM does not. (Key point: Synchronisation)

3. Which hardware is typical for FDM?
   A. Commutator and sampler
   B. Modulators, sharp band-pass filters and per-channel oscillators
   C. Only digital counters
   D. Regenerative repeaters only
   Answer: B. FDM needs a modulator, oscillator and sharp filters for each channel; TDM uses a sampler and commutator. (Key point: Hardware)

4. Why is TDM used in modern telephone networks?
   A. It needs less bandwidth than FDM
   B. Digital hardware is cheap and regenerative repeaters stop noise building up
   C. It uses guard bands
   D. It does not need sampling
   Answer: B. Regenerative repeaters remove noise at each hop and digital circuits are cheap. (Key point: Why TDM in modern telephony)

## Related in Communication Systems

- [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
- [FDM vs TDM; FDM telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-vs-tdm-fdm-hierarchy) — Communication Systems, chapter 5

## Related in other subjects

- [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×
- [Compare FDM, WDM and TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acompare-fdm-wdm-tdm) — Data Communication, chapter 6
- [FDM and TDM with block diagrams](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-block) — Telecommunications, chapter 3
- [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×