What is the Ethernet minimum frame size?

• A. 32 bytes
• B. 64 bytes
• C. 96 bytes
• D. 128 bytes

Answer: (B)

In Ethernet networks that rely on CSMA/CD, the frame size is tied to the ability to detect collisions within the time it takes for a signal to propagate to the far end and back. The minimum frame must be large enough so that a transmitter is still sending long enough for a collision with any other node on the same collision domain to be detected before the frame finishes.

A 64-byte frame achieves this because it includes the destination and source MAC addresses (6 bytes each), the EtherType or length field (2 bytes), a minimum payload (46 bytes), and the Frame Check Sequence (4 bytes), totaling 64 bytes. The 8-byte preamble is not counted as part of the frame size.

So, 64 bytes is the smallest frame size that ensures a collision on a 10 Mbps Ethernet network would be detected in time, given typical propagation delays.