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  Hi All,

  What should be the AP and client behavior when there is a change in the channel being used by and AP?
  Should the AP just change the channel and disassociate every client?
  Should the AP inform the clients of a channel change?
  Is there any 802.11 standard for that?
  Any help would be appreciated. 
Thanks in advance ...
[]sRonaldo Afonso

Are you speaking of DFS channels in the 5 GHz (UNII-IIe) range, or every range in general?

There definitely is for the DFS (i.e. the RADAR sensitive) channels.   All sorts of rules and restrictions with tight timing requirements too.

I am not sure about others.

I don't believe the 802.11 standard defines what the AP must do when the channel changes because the administrator changed configuration.  As Wlanman said, DFS defines mechanisms for sending an announcement to the stations (Channel Switch Announcement).  In my opinion, the AP should send this when the configuration is changed.

--@@ron

Here's hoping the least that happens is that the AP deauthenticates or disassociates the devices first.    That would at least send a signal to the client that it should be searching (scanning) again.  Even if its not spelled out in the standard, this would be a logical thing to do - not that all devices are logical however :-(

I haven't got the time to try this right now, but I am betting that this is what the better AP's do.

Maybe I'll be able to try this next week.

Ive seen in the past when certain changes are made on the CiscoWLAN  equipment it will send a deauthentication or dissociation. I forget exactly what these changes were. I seen this through some testing I was doing. Nothing I've seen documented. 

One method is (forgetting DFS for now), if a controller wants to change the channel of an AP due to excessive noise, interference, whatever, it can do it politely by waiting until clients disassociate. Once they're gone, it will change. (yes, this could take forever).
In the meantime, the AP wanting to change channels will no longer respond to probe requests for any SSID it is servicing, to ensure no new clients join while it's waiting for the old ones to leave.