80211 Topic-1: How BackOff Window or Contention Window will work

Hi,

Till now we are hearing that if any CSMA/CD is used in 80211 pkt transmission. When there is a collision in pkt transmission between 2 or more than 2 stations this backoff window will be used to select the station to transmit the data first.

Let's see how it works,

For Example, There are 2 stations [station A, B] and both found that medium is free and started transmitting data after DIFS [Discrete inter-frame spacing]. After collision, both stations will be in the race at the contention window to get access for the medium.

wait time for each station will be driven as below,

wait time of station  = K * T slots

Here K = [0, 2ⁿ − 1]   ; n = number of collisions

Example:

If there is a collision between A and B stations for the first time then n=1

k=[0, 1]. A and B can pick any one digit from [0,1]

If A selected 0 and B selected 1 then wait time of A will be 0 T slots, B will be 1 T slot. Means A got the access first.

If A and B selected 0 then wait time for A and B stations will be 0 T slots and there is collision again.

Now n=2 for A and B.

So k=[0,1,2,3] for A and B. So this will continue until it resolves the collision. 

If A got the access it will send the duration filed and B will update it in it's NAV field. Once NAV is 0 B will try to send the data.

Note: Maximum size of contention window size is 1023.