---
title: "Modem and its modes of operation"
subject: "Communication Systems"
chapter: 7
key: "semester_6/communication_systems:modem-modes"
source: https://www.bctengineer.com/q/semester_6/communication_systems/modem-modes
site: BCT Engineering
---

# Modem and its modes of operation

Communication Systems · Chapter 7: Digital Modulation Techniques

- Asked in: 2070 Asar (CS II), 4 marks
- Video: [Modem](https://www.youtube.com/watch?v=7uXmDntmch8) — WIT Solapur - Professional Learning Community
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/modem-modes
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-7

## Question

What is modem? Discuss the modes of operation of modems.

## Short answer

A modem converts digital data to analog signals and back. Modes include simplex (one-way), half-duplex (two-way but one at a time), and full-duplex (simultaneous two-way over separate channels or echo cancellation). Asynchronous mode frames characters with start/stop bits; synchronous mode transmits clocked blocks for higher efficiency.

## Answer

A **modem (MOdulator-DEModulator)** is a device that converts digital data from a computer into an analog signal suitable for a telephone line or other analog channel (modulation, using ASK/FSK/PSK/QAM), and converts the received analog signal back into digital data (demodulation).

```text
 [PC]--digital--[Modem]~~analog~~[Modem]--digital--[PC]
```

#### Modes of operation

1. **Simplex:** data flows in one direction only, e.g. a remote sensor sending readings.
2. **Half-duplex:** both directions, but one at a time; the modems switch between send and receive (line turnaround), e.g. older fax modems.
3. **Full-duplex:** both directions at the same time, either over a 4-wire line or over a 2-wire line by splitting the band into two frequency channels (e.g. Bell 103: 1070/1270 Hz one way and 2025/2225 Hz the other) or by echo cancellation (V.32).

Modems also work in:

- **Asynchronous mode:** each character is framed with start and stop bits; no common clock; used for low speeds.
- **Synchronous mode:** blocks of data are sent with a recovered clock; no start/stop bits, so higher speed and efficiency.

## Check yourself

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

1. What are the two functions of a modem?
   A. Encode and decode source data
   B. Modulate digital data to an analog signal and demodulate it back
   C. Amplify and attenuate signals
   D. Multiplex and demultiplex channels
   Answer: B. MOdulator-DEModulator: it converts digital data to an analog form suitable for the line and the received analog signal back to digital. (Key point: What a modem does)

2. How does half-duplex operation differ from full-duplex?
   A. Half-duplex sends in one direction only
   B. Half-duplex works in both directions but one at a time (line turnaround); full-duplex works both ways at once
   C. Full-duplex needs exactly one wire pair and one frequency
   D. Half-duplex sends both directions at the same time at half the speed
   Answer: B. Half-duplex modems switch between send and receive; full-duplex allows simultaneous transmission both ways. (Key point: Half-duplex vs full-duplex)

3. How can full-duplex be achieved over a 2-wire line?
   A. Only by using 4 wires
   B. By splitting the band into two frequency channels or by echo cancellation
   C. By using start and stop bits
   D. By lowering the baud rate to zero
   Answer: B. E.g. Bell 103 uses separate frequency pairs for each direction; V.32 uses echo cancellation. (Key point: Full-duplex on 2 wires)

4. Which is true of synchronous mode?
   A. Each character has start and stop bits
   B. Blocks of data are sent with a recovered clock and no start/stop bits, giving higher speed and efficiency
   C. There is no clock at all
   D. It is used for low-speed links only
   Answer: B. Asynchronous mode frames each character with start/stop bits (low speed); synchronous sends blocks with a recovered clock. (Key point: Asynchronous vs synchronous)

## Related in other subjects

- [ADSL operation, modem connection and topology](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aadsl-modem-topology) — Telecommunications, chapter 8, asked 3×
- [Modes of operation of 8255](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3A8255-modes) — Microprocessor, chapter 4
- [Dual processor modes in centralized SPC](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adual-processor-modes-spc) — Telecommunications, chapter 4, asked 4×
- [LD instruction under addressing modes](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Ald-addressing-modes) — Computer Organization and Architecture, chapter 2