- 2081 Chaitra · 5 marks
Short note: 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.
Write a short note on FDM vs TDM.
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.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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