Communication modes: Which mode supports two-way communication but only one direction at a time (that is, alternating send and receive)?

Introduction / Context:
Communication systems are classified by directionality: one-way only, two-way simultaneously, or two-way but not at the same time. Correctly naming these modes is fundamental for designing links, protocols, and media access strategies.

Given Data / Assumptions:

• Two-way communication is required.
• Only one direction can use the channel at any instant.
• The link alternates between sending and receiving.

Concept / Approach:

Half duplex allows bidirectional traffic, but transmissions do not overlap in time on the same channel. Classic examples include push-to-talk radios and older Ethernet hubs with collision domains where stations take turns. Full duplex, by contrast, allows simultaneous send and receive, and simplex allows only one fixed direction.

Step-by-Step Solution:

Verification / Alternative check:

Review protocol and media examples: walkie-talkies (half duplex), modern Ethernet on switches (full duplex), and broadcast TV (simplex). The alternating talk-listen model confirms half duplex.

Why Other Options Are Wrong:

• Simplex: Supports only a single fixed direction, not two-way.
• Duplex (full duplex): Supports simultaneous bidirectional transfer, unlike the scenario.
• Multiplex: Refers to combining multiple signals on one medium, not directionality.
• None of the above: Incorrect because half duplex fits exactly.

Common Pitfalls:

Confusing multiplexing (a bandwidth sharing technique) with duplex modes (directionality). Also, assuming duplex always means full duplex; half duplex is a distinct, important category.

Final Answer:

half duplex