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Contributor II

What would the STA do before send data to AP?

Hi guys,

 

I'm a newbie here, and I  have a silly questions about what would the STA do before send data to AP.

 

I had 2 theories about it:

 

1.The STA will wait a pried of time which the duration tell it to wait.When NAV  is equal to 0, then starts the random back-off time and assesses the medium.

Finally,when random back-off time is equal to 0 and there is no medium in the air, then sends the data to AP.

 

2.The random back-off time and the NAV work independently. every time the random back-off time is equal to 0,the STA will check the virtual carrier sense and physical carrier sense. If the virtual carrier sense said you had to wait a DIFS or the physical carrier sense said there was some medium in the air, then then random back-off time would reset and double its time.

 

I wish the first theory was right, because if the second one was right, then the random back-off time would do a lot of useless works. Why they wouldn't just hold there and wait the NAV finished first?

 

If I posted the wrong place, please tell me.

Valued Contributor II

Re: What would the STA do before send data to AP?

HI,

 

Yes, it will start the RBT decrementing once the NAV timer reaches Zero.

here is how it works,

1. When a client wins the medium, it will send RTS with DID (Duration id, Approx time it want to hold the medium)

2. All other clients those are in the same BSA will set their NAV time equal to DID and decrement by one for every slot time. (Which is called the Virtual carrier sensing )

3. Once the NAV timer becomes Zero, All clients will do CCA (Physical carrier sensing) means will sens whether any frames are going or coming from the same BSSID where they are connected with.

4. If CCA is clear then they will decrement RBT by one and again all clients will do the step 3 (CCA). if CCA is clear then they will decrement the RBT by one and continues till the RBT=0

5. Once the RBT becomes Zero (Any one client's RBT will becomes Zero) and the CCA is clear then that particular client will send RTS with the DID .

6. Once RTS is seen in the air and the CCA will fail then other clients will pause their RBT and will set their NAV with the new DID.

 

the above cycle will repeat.

 

hope you got some clarity on this.

 

For your ref :

 

DCF1.png

Cheers,
Venu Puduchery,
[Is my post helped you ? Give Kudos :) ]
Contributor II

Re: What would the STA do before send data to AP?

Hi 

 

Accrodin to your picture which from the CWNA 100, it seems like the STA will check the NAV twice.

The frist time is before RBT, the second time is during the CCA, the STA will check the CCA and the NAV at the same time.

 

PS: How to give people Kudos? I can't click the Kudos button.

Valued Contributor II

Re: What would the STA do before send data to AP?

HI,

 

In fact, once the NAV timer = 0 then only clients will go for Physical carrier sensing (CCA) . Decrementing RBT and CCA will be in loop until the RBT=0. then it will go with final CCA before sending RTS.

 

Hope you got some more clarity on this.

Cheers,
Venu Puduchery,
[Is my post helped you ? Give Kudos :) ]
Contributor II

Re: What would the STA do before send data to AP?

Hi,

 

I prefer that the STA only checks RBT and CCA, but your picture shows that when RBT > 0, the STA will check both CCA and NAV.

 

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