On a classic shared-medium Ethernet LAN using CSMA/CD (e.g., coaxial bus or hub-based half-duplex), how many pairs of stations can successfully communicate at the same time?

Introduction / Context:
Ethernet has evolved from a shared collision domain (coax or hubs) to modern switched, full-duplex topologies. Understanding the legacy CSMA/CD behavior clarifies why collisions occur and why only one frame can occupy the medium at any given instant on a shared segment.

Given Data / Assumptions:

• We are considering classic half-duplex, shared-medium Ethernet (single collision domain).
• All stations contend for the same channel using CSMA/CD.
• Question asks about simultaneous successful communication.

Concept / Approach:
In a single collision domain, if two transmit at once, a collision occurs and both must back off. Therefore, only one transmitter–receiver pair can be successfully exchanging a frame at a time. Switched, full-duplex Ethernet removes collisions and allows many simultaneous conversations, but that is a different topology than a shared medium.

Step-by-Step Solution:
Model the shared bus/hub as one channel.Apply CSMA/CD: simultaneous transmissions collide and are aborted.Conclude only one conversation can be in progress successfully at a time.

Verification / Alternative check:
Measurement on hub networks shows rising collision rates with load; bandwidth is shared and only one frame can be on the wire at once without collision.

Why Other Options Are Wrong:
2 or 3 or multiple: Would require switches or separate collision domains.

None of the above: Incorrect because “1” is correct for shared segments.

Common Pitfalls:
Confusing shared hubs with modern switches; switched Ethernet supports many simultaneous full-duplex links, but the question targets CSMA/CD on a shared medium.

Final Answer:
1