Re: passive scan / monitoring on multiple channels

You've some misconceptions about active/passive scan.

If you want to do ANY scan, you have to switch your WLAN receiver 
to the channel where you expect access-points (or 
adhoc-partners).

Once you're on this channel, you can now either passive listen 
and wait for up to 100 ms to be sure that you'll get every 
beacon. Or you actively send "probe requests" and wait 
for "probe responses". This way you can scan faster and don't 
need to stay for 100 ms on one channel, but maybe only 30-40 ms.

(the milliseconds are now out of my mind, they might be a bit 
different in reality).



Keep in mind that active scan might not be allowed on every 
channel, e.g. with cfg80211-enabled WLAN driver, "iw list" and a 
properly install wireless-crda, you'll properly see on which 
channel you can do active scans and on which not. The scan logic 
inside mac80211 (or cfg80211-enabled drivers) will automatically 
do an active scan if permissive for you regulatory domain.


You cannot be connected to one AP on 2412 MHz and at the same 
time scan at 2467 MHz. That's simply impossible with one WLAN 
card.

That is, you can by using a trick. The WLAN card can send a "I'm 
in power-save mode" frame to the AP. This will make the AP keep 
the connection, but buffer packets for this station. Now the 
station can change to 2467 MHz, do the scan, and return to 2412 
MHz. Now it sends a "I'm no longer in power-safe mode" frame to 
the AP. This will just degrade performance, but it will keep the 
current connection perfectly usable.

mac80211-based WLAN drivers do this automatically. I don't know 
currently about madwifi, but others on this mailing list may 
tell you this.

However, if you plan to invest considerable effort into this, 
then please first test the mac80211-enabled ath5k driver from 
wireless-testing or compat-wireless. That way you'll invest 
time/effort into something that is future proof.


-- 
http://www.holgerschurig.de

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