Cisco 9971 Deployment Manual page 69

Load based CAC logic is utilized with SIP CAC, so all 802.11 traffic and energy on the channel is accounted for to determine
available bandwidth.
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 / SSID 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 / SSID 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 Unified IP Phone 9971 uses TCP for SIP communications, therefore if the channel is busy where another call can not
be allowed, then the Cisco Unified IP Phone 9971 could potentially lose registration to the Cisco Unified Communications
Manager.
Admission Control Mandatory for Video should be disabled.
Cisco Unified IP Phone 9971 Wireless LAN Deployment Guide
69