# Forum

## In an 802.11 STA, when is the contention window equal to zero?

• I think I may have a good explanation as to why the contention window value will never be 0. If a STA were to select a value of 0 for its backoff timer it would immediately begin to transmit, considering the STA has already determined the medium is idle using CCA and checking the NAV, thereby defeating the purpose of a backoff timer. This leads to the potential for collisions, or at least increases the chance dramatically. Therefore, although the range may begin at 0 and increase to aCWmin or aCWmax, the value chosen must be a positive integer. Thoughts?

• I think is is 0-31 for non QoS STA.

I think Spectralink SVP sets the Random Backoff timer to 0 in their phones and thus will always win the contention as soon as the channel is idle

For QoS base station it will start at the lowest minimum of 2 for Voice.

• By nw80211 - edited: June 25, 2014

-Backoff procedure includes randomly choosing an integer from the contention window (CWmax).

-Randon backoff range (0 to CWmax) CWmin = 2expx-1

-The contention window (CW) is never equal to zero

for DSSS: X=5 CWmin =2exp5-1= 31

for OFDM: X=4 CWmin =2exp4-1= 15

For each retry X value will be increment by 1.

Definitely CWmin and CWmax value will be different for QoS.... based on AC.

So Answer "The contention window has a minimum and a maximum for a given transmission or retransmission, but it is always a positive integer." is correct.

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