Channel capture effect

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The channel capture effect is a phenomenon where one user of a shared medium "captures" the medium for as long as it needs before other users can use the medium. This effect was first seen in networks using the Ethernet protocol. This happens in ethernet links because of the way nodes "backoff" from the link and attempt to reaccess it. In the ethernet protocol, when a communication collision happens (when two users of the medium try to send at the same time), each user waits for a random period of time before reaccessing the link. However, a user will wait ("backoff") for a random amount of time proportional to the number of times it has successively tried to access the link. The channel capture effect happens when one user continues to "win" the link.

For example, user A and user B both try to access a quiet link at the same time. Since they detect a collision, user A waits for a random time between 0 and 1 time units and so does user B. Let's say user A chooses a lower backoff time. User A then begins to use the link and B allows it to finish sending its frame. If user A still has more to send, then user A and user B will cause another data collision. A will once again choose a random backoff time between 0 and 1, but user B will choose a backoff time between 0 and 4 - because this is his second time colliding in a row. Chances are A will "win" this one again. If this continues on, A will most likely win all the collision battles, and after 16 collisions (the number of tries before a user backs down for an extended period of time), user A will have "captured" the channel.