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When I use the protocol analyzers to capture packets on a given ESSID, then it shows the frames only on the channels the APs are configured for.

But when I use the protocol analyzers in scan mode, it does show the frames on adjacent channels which the APs are configured for. Like I have an AP configured to use channel 1, the scan mode shows some beacons on channel 2 also.

Is this the result of the channel overlap that makes the protocol analyzers to be able to capture the frames on the wrong channels ?

Thanks,

Vu

Hi Vu:

Yes, your protocol analyzer while tuned to one 2.4 GHz channel will receive frames transmitted on overlapping channels. Only beacon and probe response frames carry the identity of the channel the transmitter was centered on. All other frames are channel anonymous.

While you are at it notice how probe request frames arrive in bunches as the transmitter moves through the channels for successive transmissions.

I hope this helps. Thanks. /criss

Thanks, Criss.

Normally I capture only the specific channel. I first used the scan mode this morning when I was testing the 802.11a radio on the HWIC-AP-AG and I said "What! 802.11a makes all other b/g AP crazy ?" :)

And yes, I do see a lot of probe requests in a row. Interesting to see that.

Another notice is although AiroPeek and Observer show the frames on other channels, the YellowJacket shows only the correct channels. I think YJ checks the channel information in the beacon before displaying the AP list, while the other two display whatever the card captures.

Vu

I am not sure if my analysis is right, just want to add a point here.
Usually in scanner mode radio will allocate equal amount of time on each channel and monitor for beacons, so it can find beacons on all channnels. But in normal mode radio will monitor most of the time on serving channels

Hi Gontias:

When not scanning an IEEE 802.11 receiver is tuned to the channel center of the BSS it has joined. Since 2.4 GHz channels overlap, a receiver may hear beacons transmitted by beacon producers tuned to channel centers on either side. For example a receiver tuned to channel 6 should expect to hear beacons transmitted on channels 2 through 10 (and may faintly hear beacons transmitted nearby on channels 1 and 11 as well).

When scanning a receiver tunes to each of the channel centers and listens for a given amount of time. During that time it may hear beacons transmitted by beacon producers tuned to channel centers on either side. The scanning receiver discerns the channel of a given beacon producer not by the channel of the receiver when it received the beacon, but by the channel number carried in the beacons.

I hope this helps. Thanks. /criss