Cisco 8821 Manual page 46

Auto RF (RRM)
When using the Cisco Wireless LAN Controller it is recommended to enable Auto RF to manage the channel and transmit
power settings.
Configure the access point transmit power level assignment method for either 5 or 2.4 GHz depending on which frequency band
is to be utilized.
If using automatic power level assignment, a maximum and minimum power level can be specified.
If using 5 GHz, it is recommended to enable up to 12 channels only to avoid any potential delay of access point discovery due
to having to scan many channels.
Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide
46