Non destructive scanning while connected to current AP.

Andrea G Forte andreaf
Mon Feb 28 15:01:21 PST 2005

Actually the scenario I was referring to is different from the one you 
described. This buffering is particularly useful (if we talk about NOT 
real-time apps) when you want to do a pre-scanning. Meaning that you may 
want to do an active scanning before the time you may actually perform 
an handoff. The STA will send the null function frame to the AP (start 
buffering), it will then scan the channels and ultimately go back to the 
old AP. It will then send another null function (stop buffering and send 
me the buffered frames). The handoff process is not performed.
When the handoff process is performed, buffered frames are not very 
useful. SOME of the buffered frames can be sent to the STA between the 
last probe response and the auth request by the old AP. An alternative 
is that the buffered frames can also be sent by the old AP to the new AP 
via IAPP if available. However, when the handoff is performed, the AP 
cannot assume that buffered frames will be delivered (unless IAPP is 
used for this).


Ajeet Nankani wrote:

> Thanks Forte for the detailed answer, but still i have few more 
> question which are not explained in this or the old threads.
> Discussion does not mention that actually at what point STA sends 
> authentication and then Re-Association request to new AP. I mean when 
> it is currently attached to the AP, then first it sends Null Data 
> Frame to curent AP to indicate start of buffering, then it scans, then 
> it again send Null Data Frame to current AP to indicate stop of 
> buffering, after that i am not sure what happens?
> I guess, then STA gets all buffered frames from AP(but does STA sends 
> its buffered frames to the current AP or not??), then send 
> De-Authentication Frame to current AP, then Sends Authentication Frame 
> to the new selected AP from the Scan-Results, then upon successful 
> authentication sends re-association frame.
> I guess Forte has a log of captured frames, can you look into your 
> frame captures log and see, if it happens like what i described in the 
> above para or not or something different?
> Best Regards,
> -ajeet.
> Andrea G Forte wrote:
>> It seems that everytime a handoff occurs, the STA sends Null function 
>> packets to the AP, one at the beginning of the scanning process and 
>> one at the end of the scanning process. These packets tell the old AP 
>> when to start and stop buffering packets for the STA. I had a thread 
>> earlier on the meaning of these frames and Jouni explained what I 
>> just told you. However, these packets can introduce a significant 
>> delay in the handoff process. This means that even though the packets 
>> are buffered, if the delay introduced by these null function frames 
>> is too big, the buffered packets are useless (at least for VoIP and 
>> other real-time applications).
>> It would be better to not have them at all when using real-time 
>> applications. Unfortunately these frames are controlled by the 
>> firmware and not the driver.
>> Furthermore if you read the 802.11 standard the particular mechanism 
>> that takes care of buffering is "out of the scope" of the standard, 
>> so I am not sure if using the null function frames is the "standard" 
>> way to do it.
>> Regards,
>> Andrea
>> Ajeet Nankani wrote:
>>> I want to know that when a STA is connected to AP and is actively 
>>> transferring and receiving data from AP, and during that when STA 
>>> tries to scan network non-destructively then what happens to current 
>>> data transfer while scanning, because for scanning, channel needs to 
>>> be changed for active probes, so what happens with the current data 
>>> frames from current channel?
>>> are they lost? or buffered at STA and at AP both? and if buffered, 
>>> do STA indicates AP to buffer frames by sending PS frame to AP or 
>>> some other procedure?
>>> -ajeet.

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