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 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
1. CWAP Official Study Guide – Chapter 4
2. IEEE 802.11-2012 Standard