Unanswered Question
Mar 16th, 2010

Hi Everyone,

I have a small problem in figuring out the following question

Given that the maximum length of a 10Mb/s Ethernet is 1500m, compute

the worst-case propagation delay (assume that the signal travels through the

medium at 2   108 m/s). With this result in mind, explain why the minimum

Ethernet frame size is 512 bits.

Does any one got any idea

Thanks in advance


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Giuseppe Larosa Tue, 03/16/2010 - 13:44

Hello Mohamed,

the question probably taken from a university book is thought to make you think of collision detection at the MAC layer:

in the worst case you have two hosts with 1500 meters of cable between them (no 10 BaseT bul older coaxial based cable I suppose).

collision happens if both hosts senses the wire and send a frame at the same time.

At circuital level the NIC compares sent signal with received signal on wire to detect a collision.

The time it takes for a bit to travel from host A to host B and back to host A  is 2 * 1500 / (2*10^8) =

3000 / 2*10^8

0,000015 seconds

the time it takes to send 512 bits at 1000000 = 10 Mbps is : 512/10000000 = 0,0000512 seconds

ethernet standard defines collision as a mismatch in bits between Tx and rx on first 64 bytes of frame.

the two times propagation delay and time to transfer 512 bits are comparable (same order)

64 bytes is the minimum frame size of ethernet frame from standards.

the systems need to be able to decide if a frame has been transmitted correctly or not over the longest run possible.

This should be the hint under this question.

to be noted that in current networks we use UTP cables with no more then 100 meters of cable to switch port and the switch port isolates the host from all the others so all collision issues are solved with full duplex mode.

Hope to help


mhmdspy0714925553 Wed, 03/17/2010 - 08:12

Thanks very much,

U are correct its a Uni question. I am a student and It was a question in my paper. Wanted to get the answer. So the best place where experts share their knowledge is the cisco forums.

I understood the first part but could not get the second part I mean why is the minimum ethernet frame size 512 bits


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