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FDM and FDM hierarchy in telephony
Short answer
FDM places signals in non-overlapping frequency bands. Telephony hierarchy: 12 voice channels (4 kHz each) form group (60-108 kHz) via SSB-SC LSB modulation. 5 groups → supergroup (60 ch, 312-552 kHz), 5 supergroups → mastergroup (300 ch, 812-2044 kHz), 3 mastergroups → supermastergroup (900 ch, 8.5-12.4 MHz). Modular equipment, filters at practical frequencies.
Define FDM. Explain FDM hierarchy used in telephony.
FDM
Frequency Division Multiplexing (FDM) is a technique in which several signals share one channel by being placed in different, non-overlapping frequency bands, each modulating its own carrier. All signals are transmitted simultaneously; guard bands between them prevent interference, and band-pass filters separate them at the receiver.
|ch1|g|ch2|g|ch3|g| ... |chN| g = guard band
f1 f2 f3 fN --> frequency
FDM hierarchy in telephony
In analog telephony, each voice signal (300–3400 Hz) is allotted 4 kHz and translated by SSB-SC (LSB) modulation. Channels are combined step by step into larger blocks. The ITU-T (CCITT) hierarchy:
12 voice ch --> Group (60-108 kHz)
5 Groups --> Supergroup (312-552 kHz)
5 Supergrps --> Mastergroup (812-2044 kHz)
3 Mastergrps--> Supermastergroup (8516-12388 kHz)
1. Basic group
- 12 voice channels.
- Carriers at 64, 68, 72, ..., 108 kHz; lower sidebands selected.
- Occupies 60–108 kHz (48 kHz bandwidth).
2. Supergroup
- 5 groups = 60 channels.
- Each group modulates carriers 420, 468, 516, 564, 612 kHz (LSB).
- Occupies 312–552 kHz (240 kHz).
3. Mastergroup
- 5 supergroups = 300 channels (CCITT).
- Occupies 812–2044 kHz, with 8 kHz guard bands between supergroups.
4. Supermastergroup
- 3 mastergroups = 900 channels, occupying 8516–12388 kHz.
| Level | Composition | Channels | Band |
|---|---|---|---|
| Group | 12 voice | 12 | 60–108 kHz |
| Supergroup | 5 groups | 60 | 312–552 kHz |
| Mastergroup | 5 supergroups | 300 | 812–2044 kHz |
| Supermastergroup | 3 mastergroups | 900 | 8.516–12.388 MHz |
AT&T (North American) version: mastergroup = 10 supergroups = 600 channels (564–3084 kHz); jumbo group = 6 mastergroups = 3600 channels (0.564–17.548 MHz).
Why a hierarchy: the same standard equipment (channel banks, group and supergroup translators) is reused; filters at each stage work at practical frequencies; blocks of channels can be routed as a unit.
Requirements: sharp SSB filters, carriers derived from one master oscillator, pilot tones for gain/frequency control, linear wideband amplifiers to limit intermodulation.
✓ Answer checked by Claude Opus 5.5 (high effort)
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