---
title: "FDM hierarchy in telephone system"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:fdm-hierarchy-telephone"
source: https://www.bctengineer.com/q/semester_5/data_communication/fdm-hierarchy-telephone
site: BCT Engineering
---

# FDM hierarchy in telephone system

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2074 Chaitra, 5 marks
- Video: [What is Basic Group,Super Group,Master Group,Jumbo Group](https://www.youtube.com/watch?v=nrHJU7n8ruw) — HET
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/fdm-hierarchy-telephone
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain 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.

## Answer

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.

```text
 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
1. **Voice channel**: a 4 kHz channel (300–3400 Hz speech plus guard).
2. **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.
3. **Supergroup**: 5 groups are shifted by carriers 420–612 kHz, giving 5 × 48 = 240 kHz (312–552 kHz), 60 voice channels.
4. **Mastergroup**: 10 supergroups give 600 channels. With guard bands the bandwidth is 2.52 MHz.
5. **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 This analog hierarchy has been replaced by digital TDM (T-carriers/E-carriers).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is a group formed in the FDM telephone hierarchy?
   A. 5 supergroups combined
   B. 12 voice channels of 4 kHz shifted by carriers, giving 48 kHz (60-108 kHz)
   C. 10 groups combined into 480 kHz
   D. 60 voice channels shifted to 312-552 kHz
   Answer: B. 12 voice channels x 4 kHz = 48 kHz placed in the 60-108 kHz band using SSB. (Key point: Group formation)

2. What is a supergroup?
   A. 5 groups, 60 voice channels, 240 kHz (312-552 kHz)
   B. 12 groups, 144 channels
   C. 10 groups, 120 channels
   D. 6 groups, 72 channels
   Answer: A. 5 groups x 12 channels = 60 channels, 5 x 48 = 240 kHz. (Key point: Supergroup)

3. Which sequence of channel counts is correct from mastergroup to jumbogroup?
   A. 300 then 1800
   B. 600 then 3600
   C. 60 then 600
   D. 1200 then 7200
   Answer: B. A mastergroup is 10 supergroups = 600 channels; a jumbogroup is 6 mastergroups = 3600 channels. (Key point: Mastergroup and jumbogroup)

4. What has largely replaced the analog FDM hierarchy?
   A. Wavelength division multiplexing only
   B. Digital TDM (T- and E-carriers)
   C. Spread spectrum
   D. Larger guard bands
   Answer: B. The analog hierarchy has been replaced by digital TDM carriers. (Key point: Fate of analog hierarchy)

## Related in other subjects

- [FDM vs TDM; FDM telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-vs-tdm-fdm-hierarchy) — Communication Systems, chapter 5
- [FDMA; FDM in telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-fdm-telephony-hierarchy) — Communication Systems, chapter 5, asked 2×
- [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2×
- [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×