---
title: "Short note: FDM in telephony"
subject: "Communication Systems"
chapter: 5
key: "semester_6/communication_systems:short-note-fdm-telephony"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-fdm-telephony
site: BCT Engineering
---

# Short note: FDM in telephony

Communication Systems · Chapter 5: Multiplexing Techniques

- Asked in: 2076 Baisakh (CS I), 4 marks; 2076 Bhadra (CS I), 4 marks; 2070 Bhadra (CS I), 5 marks; 2067 Mangsir (CS I), 5 marks; 2065 Kartik (CS I), 4 marks (asked 5 times)
- Video: [Frequency Division Multiplexing (FDM) Explained](https://www.youtube.com/watch?v=UwWmDwbsDBs) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-fdm-telephony
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5

## Question

Write a short note on FDM in telephony.

## Short answer

FDM in telephony combines voice channels (300-3400 Hz) into 4 kHz slots using SSB-SC modulation. Hierarchy: 12 channels form group (60-108 kHz), 5 groups form supergroup (60 channels, 312-552 kHz), 5 supergroups form mastergroup (300 channels, 812-2044 kHz), 3 mastergroups form supermastergroup (900 channels, 8.5-12.4 MHz). Requires sharp filters, stable oscillators, pilot tones.

## Answer

**FDM in telephony** combines many voice channels onto one wideband link (coaxial cable, microwave) by giving each a separate frequency slot. It was the standard in analog long-distance telephone networks.

**Principle**
- Each voice signal is band-limited to 300–3400 Hz and given a **4 kHz slot** (the extra 900 Hz is a guard band).
- Each channel **SSB-SC modulates** (usually LSB) a different subcarrier; SSB halves the bandwidth compared with DSB.
- The outputs are added and sent together; at the receiver band-pass filters separate them and synchronous demodulators recover each voice.

```text
ch1->[SSB mod f1]->[BPF]-+
ch2->[SSB mod f2]->[BPF]-+->(sum)-> line
 ...                     |
ch12->[SSB f12]->[BPF]---+
```

**Hierarchy (ITU-T/CCITT)**

| Level | Made of | Channels | Band |
|---|---|---|---|
| Basic 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 | 8516–12388 kHz |

In the basic group, carriers are 64, 68, ..., 108 kHz with LSB selection.

**Requirements:** sharp SSB filters (crystal/mechanical), stable carriers derived from one master oscillator, pilot tones for level and frequency control, and linear amplifiers to avoid intermodulation crosstalk.

FDM telephony has largely been replaced by digital TDM (E1/T1, SDH), but the principle remains in radio and cable systems.

## Check yourself

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

1. Each voice channel (300-3400 Hz) in FDM telephony is allotted:
   A. 3.1 kHz
   B. 4 kHz
   C. 8 kHz
   D. 48 kHz
   Answer: B. A 4 kHz slot includes a guard band. (Key point: Slot per channel)

2. Which modulation is typically used?
   A. DSB-SC
   B. FM
   C. SSB-SC (usually LSB)
   D. PAM
   Answer: C. SSB halves the bandwidth compared with DSB. (Key point: Modulation)

3. A supergroup consists of:
   A. 12 voice channels
   B. 5 groups, 60 channels
   C. 5 supergroups
   D. 3 mastergroups
   Answer: B. Supergroup = 5 groups = 60 channels (312-552 kHz). (Key point: Hierarchy)

4. The basic group occupies:
   A. 60-108 kHz
   B. 312-552 kHz
   C. 812-2044 kHz
   D. 0-48 kHz
   Answer: A. 12 channels x 4 kHz = 48 kHz, from 60 to 108 kHz. (Key point: Basic group band)

## Related in Communication Systems

- [FDM and FDM hierarchy in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-hierarchy-telephony) — Communication Systems, chapter 5, asked 4×
- [FDM in telephony and uses of filters](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-filters) — Communication Systems, chapter 5
- [FDM and its method in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-method) — 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×