# Forum

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

• I have a query on one of the practice test. Following is the question and explanation says "The contention window has a minimum and a maximum for a given transmission or retransmission, but it is always a positive integer."

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

(Choose 1)

A : When both the virtual and physical carrier sense report an idle channel

B : After the maximum retry counter resets

C : The contention window is never equal to zero

D : When a STA or AP completely decrements its backoff counter

E : Immediately following every DCF Interframe Space (DIFS)

Explanation:

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

However in the standard it says "the minimum encoded value of CWmin and CWmax is 0, and the maximum value is 32 767" and also in the white paper of Arbitration by Marcus Burton it says " It is important to realize that the minimum value in a CW is always zero. While the maximum value in the CW varies, the minimum value is constant."

Thanks.

• so whats you question here ?

• The practice test says contention windows will never equal to Zero. But standard says minimum value can be Zero. I wanted to clarify whether CW can be 0 or it will always be greater than zero.

• I hate to think the exam is that picky, but both conditions are probably true.
1.   The lowest possible value is zero.
2.   You'll probably never see a value that low - some implementations (ie firmware) probably
won't even allow it to be set that low.

• Sathwick
I also had issues with this CWAP test question.  I believe the authors of the test could clear up this question for us, as to exactly what is meant in the test selection "the contention window is never equal to zero".  It appears that the CW can be zero at times, as is seen in 802.11-2007, 7.3.2.29  "Hence the minimum encoded value of CWmin and CWmax is 0, and the maximum value is 32 767."  Again, I think some clarification is needed for this practice test question.

• When there is no need for contention, contention window can be zero. which is more logical. I believe the answer to be A - when the channel is ldle.

• Mittu,

There is always need for STA to contend the medium. That is how the algorithm works.

The STAs must perform Physical carrier sense all the time. Virtual carrier sense to know how much duration the other STA going to occupy the medium. After that it has to do random back off time etc. If there is no other STA transmitting, the back off timer should kick in to avoid 2 transmitting at same time.

In order to know the channel is idle for transmission and to avoid collision we have to follow the CSMA/CA.

• Sathwik

The STA will only contend for the medium when it has data to send

• Skid,
Totally agree with you. All these contention window everything comes into picture when STA has data to send. When I said 'always' in my previous comment I had assumed STA has data to send. I apologize for the confusion.

• What of during Contention Free Period in PCF.

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