Wireless Access

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Access network design for branch, remote, outdoor and campus locations with Aruba access points, and mobility controllers.
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Air time Fairness Explanation here

  • 1.  Air time Fairness Explanation here

    Posted Oct 07, 2012 10:15 AM

    Okay i have not seem somewhere the explanation of why this is important and how does it affect not having it  so ill give it a try.

     

    Now it is important to know how does this work without airtime fairness, and to know WHY you always read well turn on the airtime fairness so the 802.11b or 802.11g monopolize the BW.   But did you ever ask why? because i did  a whlie ago and started searching.

     

    No airtime fairness how it works?

     

    Well  let say you got  1 AP 105 ap model  which is 802.11n

    Connected to it you got 1 802.11g which can transmit theorically to 54mbps

    Connected to it you got a 802.11n which can transmit theorically to 300mbs

     

    Now how will this work?

    As you know just one client can access the wireless medium at time, if one is transmitting noone else cant, so just one can be on the channel at time ok?

    Now  without the fairness the AP¨send a packet let say of 1500 to 802.11n station and then it send another packet 1500 packet to the 802.11g station(This is packet based! with no airtimefairness on)(its like a packet level fairness)

    This is what how it normally works without the airtimefariness on

    Now whats happening here?

    if you see 802.11g will need more time int the channel to get those 1500 packet  while the 802.11n does not need too much time.

    As you see this is awful for the 802.11n!

    why? because its making it goes slower, as the 802.11g client spend more time on the channel!!  and thats why when you read
    "turn on the airtime fairness so slower clients like 802.11b or 802.11g do not monopolize the BW" now you know this it does make sense....

     

    Now what happens when you turn the airtime fairness?

    Simple

    Instead of sending a 1500 packet for example to the 802.11n and one packet of 1500g, it will assgn a time! for example it will assign i dunno for example 200micro second to the 802.11n station and also 200ms to the 802.11g station so as you see now it "FAIR"  the 802.11g will send yes less data than 802.11n but now its not slowering the 802.11n  and now the 802.11g is not making the 802.11n going slower.  

    Thats why its called Airtime Fairness because it give fair time in the air to both clients...

     

    Hope this will be informative for you all thanks for reading :)

     



  • 2.  RE: Air time Fairness Explanation here

    Posted Oct 08, 2012 06:56 AM

    Older clients, due to the standard take longer to transmit frames to the access point.  Older clients, either due to lower negotiated rates or protection mechanisms will take longer to transmit a frame than newer clients.  Airtime fairness in general would simply service newer clients more to give them more airtime.



  • 3.  RE: Air time Fairness Explanation here

    Posted Feb 20, 2016 05:57 PM

    Interetsing Artical, quick questin, when a client win the contention (lets sta 802.11b client) it can transmit the whole frame that it was intending to transmit, how does airtime fairment stop this client transmitting the whole frame ? and assign some of that time to another client.



  • 4.  RE: Air time Fairness Explanation here

    Posted Feb 20, 2016 10:26 PM

    An access point can choose to service as many or as little of a client's frames as it is programmed.  A client transmitting is one side of the equation.  An access point choosing to receive that traffic is the other side.



  • 5.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 12:24 PM

    Hi guys,

     

    One question about this. Let's say airtime fairness is disabled and a client access the medium, will this client transmit only one frame or all its data buffered?

     

    Regards,

    Julián



  • 6.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 12:39 PM

    There is no "disabled" option.  There is a default option, where the access point services clients the way it would typically.



  • 7.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 12:51 PM

    Hi Colin,

     

    Then I reformulate my question. Let's say airtime fairness is in default mode and a client access the medium, will this client transmit only one frame or all its data buffered?

    I ask this because if clients transmit only one frame at a time, legacy clients will harm new clients. But if clients can transmit all their data buffered, legacy clients could completely monopolize the BW of the network.

     

    Regards,

    Julián



  • 8.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 01:24 PM

    @fjulianom@hotmail.com wrote:

    Hi Colin,

     

    Then I reformulate my question. Let's say airtime fairness is in default mode and a client access the medium, will this client transmit only one frame or all its data buffered?

    I ask this because if clients transmit only one frame at a time, legacy clients will harm new clients. But if clients can transmit all their data buffered, legacy clients could completely monopolize the BW of the network.

     

    Regards,

    Julián


    The AP will handle traffic the way all APs handle traffic in default mode; using contention.  There is nothing special there.

     

    Legacy clients will only monopolize the network if they are the majority in both number and volume of traffic.  As time goes on, there are less and less legacy devices, making this less of an issue when the setting is on "default".



  • 9.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 01:32 PM

    Hi Colin,

     

    Thanks. The last question regarding Airtime fairness. When the mode is set to Fair Access, the AP allocate the same amount of air time to all clients. When the mode is set to Preferred Access, the AP allocate the air time based on client capabilities and giving less priority to legacy clients based on a ratio. Considering there are legacy and new clients on a WLAN, which one of this two modes is more "fair" or "reasonable"? I would go for Preferred Access.

     

    Regards,

    Julián



  • 10.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 01:46 PM

    I would say that the majority of WLANs that I see run on "default".  Sometimes you will have to experiment to see which setting provides the best throughput, because clients are dynamic, so you do not know the client mix that would be best served by each setting by default.  Changing to anything besides default is best only when you know the client mix is going to be fixed.  Why?  Because later if you have problems with connectivity and client throughput, you would have to turn this off to ensure that Airtime fairness is not affecting the numbers you are seeing.  It is best to do channel and power optimization first, and then when those are optimal, consider changing airtime fairness when you absoultely know that legacy clients are consuming more of your airtime that they would typically.  If the RF is not optimal, retries from legacy clients will also consume alot of bandwith; that is why it is important to ensure the RF is working right even before you consider airtime fairness.



  • 11.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 02:03 PM

    Hi Colin,

     

    I understand your view and makes a lot of sense. Also I know that every day there are less and less legacy clients. But let's say there are new and still legacy devices, and RF parameters are optimal. Then, as you said, changing to anything besides default is best. Legacy clients would always consume more airtime than new clients. If the majority of WLANs have airtime fairness set to "default", all the grace and wisdom of Airtime Fairness, and all what cdelarosa tells at the beginning of this post falls on deaf ears.

     

    Regards,

    Julián



  • 12.  RE: Air time Fairness Explanation here

    Posted Apr 11, 2017 02:37 PM

    Julián,

     

    This is why we try to close old posts. Cdelarosa started this post 5 years ago.....I am not sure the same things are relevant today as they were 5 years ago....