Question about CSMA/CD

[newk]

Hi. Please help understand the theory of CSMA/CD.

Why doesn't a host which wants to transmit just wait for free wire? Why does it send jam instead of waiting for absence of a digital signal on the wire?

Thanks.

[Big Evil]

It does listen to see if the wire is free, it does this first.

But if two signals simultaneously try transmit, then both could see an idle physical medium (i.e. neither will see the other's carrier signal), and both will conclude that no other is currently using the medium. In this case, both will then decide to transmit and a collision will occur.

Collision detection is used to improve CSMA performance by terminating transmission as soon as a collision is detected (jam signal), and reducing the probability of a second collision on retry.


Hope that helps.

[newk]

Thanks. I knew about listening first but I can't understand how collision detection works and how precise is simultaneously (I think they can't begin to transmit ABSOLUTELY simultaneously so a first signal could be considered not broken). I understand that this mechanism works and works well but maybe I needn't know about physical part of CD?

[Big Evil]

Ieee's website will tell you all the physical details of CD.
It really depends on how deep a knowledge you want, and for what reason.
As most people are using Full Duplex now, for me a brief overview is all i have ever needed.

[newk]

Thank you very much for the answers!