- 2070 Asar (CS II) · 4 marks
Modem and its modes of operation
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.
What is modem? Discuss the modes of operation of modems.
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).
[PC]--digital--[Modem]~~analog~~[Modem]--digital--[PC]
Modes of operation
- Simplex: data flows in one direction only, e.g. a remote sensor sending readings.
- Half-duplex: both directions, but one at a time; the modems switch between send and receive (line turnaround), e.g. older fax modems.
- 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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