Cisco 8821 Manual page 50

DFS (802.11h)
In the DFS (802.11h) configuration, channel announcement and quiet mode should be enabled.
Power Constraint should be left un-configured or set to 0 dB as DTPC will be used by the Cisco Wireless IP Phone 8821 and
8821-EX to control the transmission power.
In later versions of the Cisco Wireless LAN Controller it does not allow both TPC (Power Constraint) and DTPC (Dynamic
Transmit Power Control) to be enabled simultaneously.
Channel Announcement and Channel Quiet Mode should be enabled.
High Throughput (802.11n/ac)
The 802.11n data rates can be configured per radio (2.4 GHz and 5 GHz).
802.11ac data rates are applicable to 5 GHz only.
Ensure that WMM is enabled and WPA2(AES) is configured in order to utilize 802.11n/ac data rates.
The Cisco Wireless IP Phone 8821 and 8821-EX support HT MCS 0 - MCS 7 and VHT MCS 0 - MCS 9 data rates only, but
higher MCS rates can optionally be enabled if there are other 802.11n/ac clients utilizing the same band frequency that include
MIMO antenna technology, which can take advantage of the those higher data rates.
It is recommended to disable MCS 0.
Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide
50