Cisco 8821 Manual page 100

If enabling the Stream feature either directly or via selecting Optimized Voice for the radio access category in the QoS
configuration section, then use the defaults, where 5.5, 6, 11, 12 and 24 Mbps are enabled as nominal rates for 802.11b/g, 6, 12,
and 24 Mbps enabled for 802.11a and 6.5, 13, and 26 Mbps enabled for 802.11n.
If the Stream feature is enabled, ensure that only voice packets are being put into the voice queue. Signaling packets (SIP)
should be put into a separate queue. This can be ensured by setting up a QoS policy mapping the DSCP to the correct queue.
Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide
100