Non destructive scanning while connected to current AP.

Ajeet Nankani fromkth+hostap
Mon Feb 28 08:45:22 PST 2005

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,


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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