Monitoring mode gets frames from more than one channel!

Ajeet fromkth+hostap
Fri Feb 11 05:17:27 PST 2005

eduardgv wrote:
>>>>Even though i am monitoring on particular channel(channel 1), but i also
>>>>get frames(not all but most) from AP operating on other channel(channel
>>>>Why is like that?
>>>That's the way IEEE 802.11b works.. Different channels are using
>>>different center frequencies, but they are overlapping and radios can
>>>receive frames from other channel, although with limited signal
> I think 5 channel separation is enough not to allow packet
> decodification on physical layer, even though it is not enough to
> avoid interferences. I suspect it is a firmware issue. The card keeps
> collecting statistics from other channels in orther to perform future
> handovers.

I think it is only in "iwconfig wlan0 monitor" mode when it collects 
frames from channels other than it is currently on. But when you are in 
managed mode it does not or it should not collects frames from "the 
other" channel if that "other channel" is 5 channel apart. Like if STA 
is on channel 1 and the other channel being 6.
Ofcourse when the STA makes decision to search for other APs becuase of 
weak signal(or some other reason) with current APs then it scans, hence 
collects frames from every other channel.
So it is likely that when that card is in manged mode you found it 
collecting statistics(frames) from "the other" channels (channels which 
are 5 or more channels apart from current associated channel) because it 
made decision to scan.

> Setting mode 2 roaming (iwpriv wlan0 host_roaming 2) still produces
> the same behaviour?

but here if you have instructed the card not to scan or the other case 
that STA is being stationary and not moving to get signal quality 
degraded, and even if after that it is collecting frames from 5 channels 
apart other channels, then i suspect it might be the issue with how fine 
tuned is the receiver to the current channel, but i dont have any radio 
knowledge so someone here with good radio knowledge can explain better 
if that is the issue.

One more question to eduardgv that how you were able to see 802.11 
packets on the card in managed mode, beacuse in my setup i can see 
802.11 header in ethereal only when i set the card in monitor mode, in 
other modes(master or manged) it only shows ethernet header.



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