In data communications, a circuit that permits data transmission in both directions but not simultaneously is operating in which mode?

Introduction / Context:
Duplexing determines whether communication endpoints can send and receive at the same time. Selecting the right duplex mode matters for throughput, latency, and protocol design. Many legacy and wireless systems rely on half-duplex operation to share a medium efficiently.

Given Data / Assumptions:

• The link supports bidirectional transmission.
• Simultaneous two-way transmission is not allowed.
• We need the standard term that describes this constraint.

Concept / Approach:
Simplex allows transmission in only one direction. Half-duplex allows both directions but requires taking turns (time-sharing the medium). Full-duplex allows simultaneous transmission and reception, typically over separate channels or using echo cancellation. Therefore, “both directions but not at the same time” precisely defines half-duplex.

Step-by-Step Solution:
Interpret the phrase “both directions but not simultaneously.”Map to “half-duplex” by definition.Exclude simplex (one-way only) and full-duplex (simultaneous two-way).

Verification / Alternative check:
Analogy: A walkie-talkie Push-to-Talk system is half-duplex—users alternate speaking. A telephone call is full-duplex—both parties can talk and hear concurrently.

Why Other Options Are Wrong:
Simplex: Unidirectional only, not bidirectional.

Full-duplex: Allows simultaneous two-way, contradicting the question.

Asynchronous: Refers to timing without a shared clock, not duplex capability.

None of the above: Incorrect because half-duplex fits exactly.

Common Pitfalls:
Confusing duplex mode with synchronous/asynchronous timing; they are orthogonal properties of a link.

Final Answer:
a half-duplex mode