In data communications, which device performs both modulation and demodulation functions to interface digital equipment with analog transmission channels?

Introduction / Context:
Many legacy and some current communication links carry analog waveforms over copper or radio, yet endpoint computers produce digital bits. A device is needed to convert (modulate) outgoing bits into analog signals and to recover (demodulate) incoming bits from those signals. This conversion underpins dial-up, DSL variants, and some radio systems.

Given Data / Assumptions:

• Endpoints are digital (computers, routers).
• The channel is analog bandpass (telephone line, RF link).
• We seek a single device performing modulation and demodulation.

Concept / Approach:
A modem (MOdulator/DEModulator) maps digital symbols to analog waveforms (for example, QAM) and reconstructs them at the receiver with synchronization and equalization. Other listed terms do not perform this waveform conversion: a multiplexer combines channels, a gateway translates protocols/routing domains, and protocols are rule sets, not hardware devices.

Step-by-Step Solution:

Verification / Alternative check:
Review dial-up standards like V.34/V.90 using QAM/Trellis coding: the hardware at each end is a modem handling symbol mapping and recovery. DSL CPE similarly modulates/demodulates over copper using DMT.

Why Other Options Are Wrong:

• Protocols: Define communication rules; not a device.
• Gateway: Translates address/protocol domains; does not modulate waveforms.
• Multiplexer: Aggregates multiple signals; no baseband-to-passband mapping.

Common Pitfalls:
Confusing modulation/demodulation with mere encoding (for example, line coding), or assuming multiplexers inherently perform conversion—they do not.

Final Answer:
modem