Fragment-free switching mode

Unanswered Question
Feb 10th, 2007

We can read the following statement in the CCNA Intro book:

One interesting fact about Ethernet CSMA/CD logic is that collisions should be detected within the first 64 bytes of a frame.

My question:

What is it in the first 64 byte of any frame that the switch can detect collisions by using this method?



I have this problem too.
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Richard Burts Sat, 02/10/2007 - 10:46


The important thing about the first 64 bytes was that the time to transmit 64 bytes was the amount of time it would take for signal to go from one end of the maximum length cable to the other end. In the early standards for Ethernet (when everything was half duplex) the Ethernet card would listen for incoming signal while it was transmitting outbound. If it detected incoming signal (from some other station) while it was transmitting, then it declared it as a colision. It was important for every frame to be at least 64 bytes so that a station would still be transmitting and able to detect that a station on the far end of the wire had started to transmit.




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