GATE | GATE IT 2006 | Question 66

A router has two full-duplex Ethernet interfaces each operating at 100 Mb/s. Ethernet frames are at least 84 bytes long (including the Preamble and the Inter-Packet-Gap). The maximum packet processing time at the router for wirespeed forwarding to be possible is (in micro­seconds)
(A) 0.01
(B) 3.36
(C) 6.72
(D) 8


Answer: (B)

Explanation: Here, we need to maintain the speed of wire, it means the receiving and sending rates both are the maximum possible,i.e., maximum throughput. So, processing time should be at most same as minimum transmission time to maintain the wire speed. As if router will process it in the minimum transmission time even if a packet is transmitted in minimum time it gets processed and the packet doesn’t get delayed for transmission.

Since router has two full-duplex Ethernet interfaces each operating at 100 Mb/s. Therefore, maximum packet processing time at the router,

= minimum transmission time
= (84 bytes) / (2*100 bits per sec)
= (84 * 8 bits) / (2*100 bits per sec)
= 3.36 seconds 

So, option (B) is correct.

This solution is contributed by Shashank Shanker khare

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