The Channel Switch Announcement element is used by an AP in a BSS, a STA in an IBSS, or a mesh STA in an MBSS to advertise when it is changing to a new channel and the channel number of the new channel. The format of the Channel Switch Announcement element is shown below (source IEEE 802.11-2012)
Length : set to 3 bytes
Channel Switch Mode :
Indicates any restrictions on transmission until a channel switch. An AP in a
BSS or a STA in an IBSS sets the Channel Switch Mode field to either 0 or 1 on transmission. In an MBSS,the Channel Switch Mode Field is reserved
New Channel Number: set to the number of the channel to which the STA is moving.
Channel Switch Count: for nonmesh STAs, this field either is set to the number of TBTTs until the STA sending the Channel Switch Announcement element switches to the new channel or is set to 0. A value of 1 indicates that the switch occurs immediately before the next TBTT. A value of 0 indicates that the switch occurs at any time after the frame containing the element is transmitted.
This Channel Switch Announcement element present in beacons & probe responses. Channel Switch Announcement element also associated with an action frame (spectrum management type or category type =0) that can be sent by the AP between beacons to announce the channel switch.
Here is the frame format of Action Frame that contain the Channel Switch Announcement field.
Here are some important points about Channel Switch Announcement in a BSS.
- An AP shall inform associated STAs that the AP is moving to a new channel and maintain the association by advertising the switch using Channel Switch Announcement elements in Beacon frames, Probe Response frames, and Channel Switch Announcement frames until the intended channel switch time.
- The AP may force STAs in the BSS to stop transmissions until the channel switch takes place by setting the Channel Switch Mode field in the Channel Switch Announcement element to 1.
- The channel switch should be scheduled so that all STAs in the BSS, including STAs in power save mode, have the opportunity to receive at least one Channel Switch Announcement element before the switch.
- The AP may send the Channel Switch Announcement frame in a BSS without performing a backoff, after determining the WM is idle for one PIFS period.
- A STA that receives a Channel Switch Announcement element may choose not to perform the specified switch, but to take alternative action. For example, it may choose to move to a different BSS.
- A STA in a BSS that is not the AP shall not transmit the Channel Switch Announcement element
References
1. CWAP Official Study Guide – Chapter 4
2. IEEE 802.11-2012 Standard
Related Post
1. 802.11 Mgmt : Action Frames
mrn-cciew 
Might as well read about this in the books
will the clients get disconnected and connected again during the channel switch?
Yes, if you change channel, clients has to disconnect & reassociate using new channel
HTH
Rasika
how long is the duration of this disconnection? ms or sec?
(CAC+PNO time) – 240 secs
Hello,
Thank you for the useful and clear documentation!
What is the Element ID corresponding to CSA in a beacon?
Is it possible to use CSA in 80211n networks?
Thank you in advance for help.
Regards,
Mira
Hi Mira,
I know it is too late, however here is the info for reference. You can find great details from below page (including sample DFS event pcaps)
http://thedfsproject.com/
CSA can be announced through Beacon or Action Frames.
In Beacon, Tag Number 37 (Channel Switch Announcement)
This is the best talk on that topic from WLPC2020 by David Coleman
https://wlanprofessionals.com/dfs-the-untold-story-with-david-coleman-a-video-from-wlpc-phoenix-2020/
HTH
Rasika
If there are no clients associated with the AP, should we still expect CSA in beacons during a change in channel ?
It is a good question. Not 100% sure. If you are able to test it out or find the answer let us know. Here is a very good talk on this topic.
https://wlanprofessionals.com/dfs-the-untold-story-with-david-coleman-a-video-from-wlpc-phoenix-2020/
Rasika
Hello,
In case of 802.11n and 802.11ac, we can configure the AP to operate in 20,40,80M channel widths.
How the station will know that the AP is operating in which channel range it is operating?
Thanks in advance
By using Dynamic bandwidth selection procedure STA know about that info
Hello, I am fan of your articles. I find the topics that I want to learn are beautifully explained here. Thanks for your time and effort to spread the knowledge.
I have a question about the CSA element in the beacons. When an AP wants to send the CSA via beacons, does it broadcast the beacon frames with CSA element or does the beacon frame contain the addresses of the associated STAs?
Beacons always be broadcast frames. Pls, go through this WLPC video by David Coleman who covers this topic in more detail.
https://wlanprofessionals.com/dfs-the-untold-story-with-david-coleman-a-video-from-wlpc-phoenix-2020/
HTH
Rasika
Is it possible that CSA can be used as seamless handover method when STA roam from AP1 to AP2?
CSA is for let client know AP is going to change the channel due to DFS radar detection.
In roaming client re-associate to another AP on a different channel. They are two different things
In roaming scenario, you can use 802.11k/v to let client know about the RF neigborhood, that may help client to reduce scan required to discover by itself
HTH
Rasika
CSA is not supported for non-dfs to non-dfs channel change?
Yes, CSA is only for DFS channels (UNII-2 & UNII-2 extended) channels. It is not for non-DFS channels.
Rasika
CSA is useful for Non-DFS channel switch too.