GATE | Gate IT 2008 | Question 63

The minimum frame size required for a CSMA/CD based computer network running at 1 Gbps on a 200m cable with a link speed of 2 × 108m/s is
(A) 125 bytes
(B) 250 bytes
(C) 500 bytes
(D) None of these

Answer: (B)

Explanation: For the CSMA/CD protocol to work, the transmission time of the frame must be more than a minimum value. This minimum value is the worst case Round trip Time.
Therefore, Transmission Time >= RTT
Let the frame size be f bytes.
 \frac{f}{1Gbps} \geq \frac{2*200}{2*10^8}\\ f \geq \frac{2*200*10^9}{2*10^8*8}\\ f \geq 250
The minimum frame size is 250 bytes
Therefore option (B) is correct.

This explanation is provided by Chirag Manwani.

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