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Comparison of FDM, TDM and WDM
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).
Define multiplexing. Compare FDM, TDM and WDM with neat diagram.
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.
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.
|<-------------- 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.
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 |
✓ Answer checked by Claude Opus 5.5 (high effort)
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