In this document Cisco TAC engineer "Siddharth Vij" has explained "iPads running IOS 7 unable to join WLAN automatically after waking up from Sleep mode".
Wireless Services Module 2 (WS-SVC-WISM2)
WISM 2 running 220.127.116.11
Wireless LAN - WS-SVC-WISM2
iPad on ios 7 unable to rejoin wireless network after going to sleep on WISM 2 running 18.104.22.168. Wireless Services Module 2 (WS-SVC-WISM2)
iPads running version iOS 7 and connecting to a guest network on WISM 2 running 22.214.171.124 are experiencing issues while trying to connect automatically to the Wireless network after the devices wake up from sleep.
Took the output of the debug client command for the iPads and found that the session timeout for WebAuthSSID is set to 43200 seconds and the Global user Idle Timeout was set to 300 seconds.
The Client was sleeping for about 10 minutes, during that time the AP sent deauth frame to the iPad after completing the 300 second user idle timeout interval.
When the client wakes up from sleep it tries to join the same session automatically which is no longer valid on the controller.
Increase the User Idle timeout value to 1800 seconds and this resolved the issue. Now the clients are able to join the WLAN session after waking up from sleep.
If clients are active after successful login, they will get de-authenticated and entry can still be removed from the controller after the session timeout period configured on that WLAN (for example,1800 seconds by default and can be changed using this CLI command: configwlan session-timeout <WLAN ID> <seconds>). When this occurs, client entry is removed from the controller. If the client associates again, it will move back in a Webauth_Reqd state.
If clients are in Webauth_Reqd state, no matter if they are active or idle, the clients will get de-authenticated after a web-auth required timeout period (for example, 300 seconds and this time is non-user configurable). All traffic from the client (allowed via Pre-AuthACL) will be disrupted. If the client associates again, it will move back to the Webauth_Reqd state.