Helpful information from Cisco TAC:
• Apple does not support OKC. Cisco has developed code to support their “sticky key caching”. This is an enhancement for iDevices doing 802.1x – this cuts the roam time since keys are available at the AP.
• iDevices have a built-in 20 minute timer for Internet access. If the device is not active the timer begins a countdown. If it does not connect to the Internet during the countdown time, the device will jump off the network and require reauth.
• iDevices have different max Tx Power level
o 20mW device - iPhone/iPod/iPod Touch
o 40mW device - iPad(1/2/3)
o iDevices need to run “hotter” than other wireless devices - more tx power
• iDevices prefer Broadcast SSID’s
o If the device has multiple “saved” WLANs and profiles, it will always prefer the Broadcast WLAN; the device will change to the Broadcast SSID even if you select a closed/secure SSID
• iPhone requires that 11M data rate be set to MANDATORY. This will slow your overall network, but this has been thoroughly tested by our TME group in San Jose. If the 802.11b rates are completely disabled, the iPhone will have connectivity problems
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In your specific case, it looks like your device jumped off the network and require reauth at the end of the 20 minute timer.