This is a medium access protocol.
The nodes/stations contending for the medium first listen for an on-going talk over the medium. When somebody is already talking, there will be a carrier signal on the medium which carries the message. If the listening station senses a carrier, that means, the medium is already owned by somebody else. So it backs out for a random period of time and try again.
The principle is, talk when nobody else is NOT talking.
But there is a chance that more than one stations detect NO CARRIER on the medium at the same time and start talking. There occures a collission.
On each medium there is a mechanism to detect collission.
When the talker detects a collission, he backs out and sends a JAM signal, for the other station, in case, he didnt detect the collission. Eventually both parties backs out.
They come back after a random period of time.
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