Once a station is associated to an AP, either side can terminate the association at any time by sending a disassociation frame (see the following picture). A station would send such a frame, for example, because it leaves the cell to roam to another AP. An AP could send this frame for example because the station tries to use invalid parameters or for reasons related to the AP itself (configuration change, and so on).
A disassociated station is still authenticated. It can try to reassociate by sending a new association request frame, keeping its authenticated status. For this reason, disassociation frames are typically used when parameters change and the station or the AP needs to renegotiate the communications parameters. A station roaming to another cell may also choose to use a disassociation frame, to be able to keep its authenticated status and accelerate the process when roaming back to the same cell before its authentication timeout expires.
The station or AP can also send a deauthentication frame. This frame is used when all communications are terminated, for example, because the AP has to reboot or because the station stops its WiFi communications. It is also used when a frame is received before authentication has completed
above picture from CWAP
The disassociation frame DA can be the unicast MAC address of the station to disassociate or a broadcast address if the AP needs to disassociate all the stations in its cell. When the disassociation frame is unicast, it is acknowledged by the receiving station. Broadcast frames are not acknowledged.
The station or AP can also send a deauthentication frame. This frame is used when all communications are terminated, for example, because the AP has to reboot or because the station stops its WiFi communications. It is also used when a frame is received before authentication has completed
Here is the complete list of reason code as per IEEE 802.11-2012 standard.
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