- 2068 Jestha (old course) · 4+6 marks
FDM vs TDM; FDM telephone hierarchy
Short answer
FDM allocates frequency bands to simultaneous analog signals; TDM assigns time slots sequentially to digital signals. The FDM hierarchy: 12 voice channels form a basic group (60–108 kHz), five groups form a supergroup (312–552 kHz), and five supergroups form a mastergroup (812–2044 kHz).
Differentiate between FDM and TDM. Draw spectral details and explain various standard groups of FDM telephone hierarchy.
Difference between FDM and TDM
| 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 telephone hierarchy and spectral details
Telephone FDM uses SSB-SC modulation. Each voice channel (300–3400 Hz) gets a 4 kHz slot that includes its guard band. The standard (CCITT) groups are:
| Level (CCITT) | Made of | Voice channels | Frequency band | Bandwidth |
|---|---|---|---|---|
| Voice channel | — | 1 | 0–4 kHz (0.3–3.4 kHz used) | 4 kHz |
| Basic group | 12 voice channels | 12 | 60–108 kHz | 48 kHz |
| Supergroup | 5 groups | 60 | 312–552 kHz | 240 kHz |
| Mastergroup | 5 supergroups | 300 | 812–2044 kHz | 1232 kHz |
| Supermastergroup | 3 mastergroups | 900 | 8516–12388 kHz | 3872 kHz |
1. Basic group (12 channels, 60–108 kHz). Channel modulates a carrier kHz (, i.e. 108, 104, …, 64 kHz). The lower sideband is kept, so channel occupies to kHz.
2. Supergroup (60 channels, 312–552 kHz). Five groups modulate carriers at 420, 468, 516, 564 and 612 kHz. Taking the lower sideband turns each 60–108 kHz group into a 48 kHz block. For example, group 1 on the 420 kHz carrier occupies to kHz.
3. Mastergroup (300 channels, 812–2044 kHz). Five supergroups are placed with 8 kHz guard bands between them. (The Bell system mastergroup instead uses 10 supergroups = 600 channels in 564–3084 kHz.)
4. Supermastergroup (900 channels, 8516–12388 kHz). Three mastergroups.
Group: 12 channels, carriers 64, 68, ..., 108 kHz
(LSB of each channel kept)
|ch12|ch11|ch10| ch9| ch8| ... | ch2| ch1|
60 64 68 72 76 100 104 108 kHz
|<----------------- 48 kHz ----------------->|
Supergroup: 5 groups, carriers 420, 468, 516,
564, 612 kHz (LSB kept)
| G1 | G2 | G3 | G4 | G5 |
312 360 408 456 504 552 kHz
|<-------- 240 kHz -------->|
Each step needs only one set of modulators and filters for a whole block of channels, which keeps the equipment simple. The wide guard bands at the higher levels make the filters easier to build.
✓ Answer checked by Claude Opus 5.5 (high effort)
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