802.11h in action

As you may or may not know, part of 802.11-2012 standard specifies the DFS or Dynamic Frequency Selection due to regulations that apply in most regulatory domains for RLANs in the 5GHz spectrum. DFS is there so that 802.11 radios don't interfere with other (more important) radar systems in the same radio vicinity. These are usually weather radars used by airplanes and having such a capability is a really good idea. DFS was originally part of 802.11h amendment which in turn is now part of the 802.11-2012 standard, but I'll refer to it as 11h.

In very short, the .11 radio in an AP wishing to operate on one of the UNII2e channels must continuously scan that channel for any presence of radar and must cease transmission on that channel if it detects a radar source. Some of the rules and procedures are written in this document from Cisco. 

The APs objective in changing channels is that disruption to the BSS is minimal and one of DFS procedures to help with this is the setting of the channel switch announcement element (CSA) in Beacon, Probe Response and/or Action management frames which tells it's associated STAs to which channel the AP is hopping and when. Below is an example of a beacon frame with a set CSA element. You can filter these frames out of a capture with "wlan_mgt.csa.channel_switch_mode" filter in wireshark.

The count value is the value of remaining beacons that will be broadcast on the current operating channel. The number here is 20 which indicates that 20 beacons (about 2 seconds) including this one are left before the channel change to channel 128 will be done. This number is decreasing with every sent beacon and when it reaches 1 you won't see another beacon or any other frame from the AP broadcasting BSS on the channel.

This assisting of changing channels isn't a guarantee that the STAs will actually accept the change and follow the AP to the new channel but most STAs will. STAs following the AP is only logical to do since the AP is handing the STA a new pipe to the net on a plate, but not all are made the same and some can switch to a new or even different BSS if they so choose.

If you've read this far first a thank you, and if you're asking your self why am I writing about this or who cares about channel changes I just wanted to  point out one vendors clever use of this function. Ruckus Wireless APs employ what their marketing calls ChannelFly. What their APs do is basically periodically hop through available channels in search of the one with best throughput characteristics. 'Everybody uses that. It's called background scanning' I hear you saying. Well CF differs from background scanning in that it doesn't go off channel to scan a different channel, but it just changes channels and operates on a new channel and takes measurements on that one and then hops again with the point of finding the one with the best characteristics of throughput and capacity. Each time it hops it uses the CSA element in it's beacons to hopefully take all it's associated STAs with it. 'Hopefully!?!', well for the most it does so without a problem. I've found many 5GHz STAs follow the AP without a problem, but some STAs might cause some problems. I have an HP laptop that operates in the 2,4GHz only and is supporting 11h, but upon a channel change it just gets lost and I have to disable/enable the NIC to get it running again.

My recommendation for CF would be to try it and see, but I consider it pretty safe when enabled on the 5GHz. For the 2,4GHz I suggest you try it and see if any STAs have problems when the AP is changing channels.