Random Back-off Procedure (802.11)

The random back-off count is selected as a pseudo-random integer drawn from a uniform distribution over the interval [0,CW] , where CW, an integer value, is the contention window.

 Update procedure of CW value
1. CW parameter takes the initial value CWmin.
2. Every unsuccessful MPDU transmit, for example each time an ACK response is not received for a data frame.CW parameter doubles.
3. Every successful MPDU transmit, CW parameter reset.
4. If CW parameter increase due to unsuccessful MPDU transmit and reach the value CWmax , CW parameter keep CWmax until reset.















Random back-off procedure
1. As shown in the figure above, STA3 which select the smallest back-off count win the contention and transmit first.
2. STA2 and STA4 suspend their back-off and resume DIFS after the medium goes to idle.
3. STA4, which select the second smallest back-off count win the 2nd contention.
4. And it goes STA2 and finally STA1 transmit. 

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