- 2074 Chaitra · 5 marks
FDM hierarchy in telephone system
Short answer
The analog FDM telephone hierarchy builds from 12 voice channels forming a 48 kHz group (60-108 kHz), then5 groups make a240 kHz supergroup (60 channels),10 supergroups form a mastergroup (600 channels), and6 mastergroups yield a jumbogroup (3600 channels). All stages use SSB modulation with guard bands.
Explain FDM hierarchy in telephone system.
The telephone system carries many 4 kHz voice channels on a high-capacity trunk by combining them in stages with FDM, forming an analog hierarchy. Each stage multiplexes the lower-level bundle.
12 voice --> GROUP (48 kHz, 60-108 kHz)
5 groups --> SUPERGROUP (60 ch, 240 kHz, 312-552 kHz)
10 supergroups --> MASTERGROUP (600 ch, ~2.52 MHz)
6 mastergroups --> JUMBOGROUP (3600 ch, ~16.98 MHz)
Stages
- Voice channel: a 4 kHz channel (300–3400 Hz speech plus guard).
- Group: 12 voice channels are each shifted by carriers from 64 kHz to 108 kHz in 4 kHz steps (using SSB, lower side band) giving 12 × 4 = 48 kHz, in the 60–108 kHz band.
- Supergroup: 5 groups are shifted by carriers 420–612 kHz, giving 5 × 48 = 240 kHz (312–552 kHz), 60 voice channels.
- Mastergroup: 10 supergroups give 600 channels. With guard bands the bandwidth is 2.52 MHz.
- Jumbo group: 6 mastergroups give 3600 channels in 16.984 MHz.
Higher groups are carried over coaxial cable or microwave. Guard bands separate the units to avoid interference. This analog hierarchy has been replaced by digital TDM (T-carriers/E-carriers).
✓ Answer checked by Claude Sonnet 5.5
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