Cisco 8821 Manual page 63

The access point has different methods for Call Admission Control when using SIP CAC depending on whether the client uses
TCP or UDP for SIP communications.
If the client uses TCP for SIP, then the access point will snoop the SIP packets when media session snooping is enabled on the
WLAN and will not forward the SIP frames upstream or downstream if there is not bandwidth available for the new voice
stream. This could potentially result in loss of registration to the Cisco Unified Communications Manager.
If the client uses UDP for SIP, then the access point will snoop the SIP packets when media session snooping is enabled on the
WLAN and will sent a 486 busy message to the client, which in turn can be interpreted as a Network Busy message and the
client could either roam to another access point or simply terminate the call setup for that session.
The Cisco Wireless IP Phone 8821 and 8821-EX use TCP for SIP communications, therefore if the channel is busy where
another call can not be allowed, then the Cisco Wireless IP Phone 8821 and 8821-EX could potentially lose registration to the
Cisco Unified Communications Manager if SIP CAC is enabled.
Admission Control Mandatory for Video should be disabled.
Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide
63