Communication modes: which mode supports transmission of data in both directions at the same time between two endpoints?

Introduction:
Communication systems are often described by their directionality: whether information can flow one way only, both ways but not simultaneously, or both ways at the same time. Knowing these distinctions is foundational for configuring serial links, Ethernet links, and wireless channels. This question asks you to identify the mode that permits simultaneous two-way transmission.

Given Data / Assumptions:

• Two endpoints exchange data.
• We compare simplex, half-duplex, and full-duplex modes.
• Ignore unrelated terms that describe capacity sharing (multiplexing).

Concept / Approach:
Simplex permits one-way flow only (e.g., broadcast paging). Half-duplex allows both directions but not at the same time; the channel alternates (e.g., push-to-talk radios, legacy shared Ethernet). Full-duplex supports concurrent transmission and reception, often by using separate physical pairs/frequencies or echo cancellation (e.g., modern Ethernet over twisted pair, many telephony systems). Therefore, the correct answer is full-duplex.

Step-by-Step Solution:

Verification / Alternative check:
1000BASE-T Ethernet operates full-duplex using echo cancellation across all pairs; serial links can achieve full-duplex by dedicating separate TX/RX conductors, confirming concurrency in both directions.

Why Other Options Are Wrong:

• Simplex: one way only.
• Half-duplex: alternates direction, not simultaneous.
• Multiplex: refers to sharing medium among signals (time/frequency), not directionality.
• None of the above: incorrect because full-duplex is correct.

Common Pitfalls:
Confusing multiple access or multiplexing with duplex modes; assuming “collision-free” implies full-duplex (it does not necessarily).

Final Answer:
Full-duplex.