Specifications

Cisco Unified Wireless IP Phone 7921G Series Deployment Guide 26
Call Admission Control
Inbound and outbound call admission control can be enabled on the access point.
• Enable Call Admission Control / Wi-Fi MultiMedia Traffic Specifications (TSPEC)
• Set the desired maximum RF bandwidth that is allocated for voice traffic (default = 75%)
• Set the bandwidth that is reserved for roaming clients (default = 6%)
The minimum PHY rate can be configured for which the phone is to use when Call Admission Control (CAC) is enabled.
• Enable a data rate that is enabled on the access point. (Default setting is 12 Mbps)
• Cisco Access Points will only accept a minimum PHY rate of 5.5, 6, 11, 12 or 24 Mbps, so ensure that one of these
rates are enabled.
As of the 1.3(3) release, the Cisco Unified Wireless IP Phone 7921G will auto-negotiate the minimum PHY rate to be used for
TSPEC. By default it will try the locally configured minimum PHY rate (i.e. 12 Mbps) first, but if that data rate is not enabled
on the access point, then it will try the next highest enabled data rate on the access point. If there is not a higher data rate
enabled, then it will then try the next lowest data rate as the minimum PHY rate.
In releases prior to 1.3(3), the Cisco Unified Wireless IP Phone 7921G would use the static minimum PHY rate configured
locally.
If 12 Mbps is not enabled on the access point, then the next highest enabled data rate must be 24 Mbps. For example, if 12
Mbps is disabled but 18 Mbps is enabled, the phone will try the next highest rate of 18 Mbps and fail because that minimum
PHY rate for CAC is not supported by the Cisco access point.
The dynamic minimum PHY rate is useful for deployments that require higher capacity where 24 Mbps and higher data rates
are only enabled. For this high capacity deployment configuration and with release 1.3(3), the minimum PHY rate would be
adjusted to 24 Mbps automatically even if the phone is configured statically for a minimum PHY rate of 12 Mbps. In releases
prior to 1.3(3), the minimum PHY rate would have to be changed to 24 Mbps manually from the default of 12 Mbps in order for
CAC to work correctly for this deployment configuration.
If an 802.11b AP is used, the highest available date rate would be 11 Mbps, so 12 Mbps can not be used as the minimum PHY
rate. For this 802.11b (11 Mbps) deployment configuration and with release 1.3(3), the minimum PHY rate would be adjusted
to 11 Mbps automatically even if the phone is configured statically for a minimum PHY rate of 12 Mbps. In releases prior to
1.3(3), the minimum PHY rate would have to be changed to 11 Mbps manually from the default of 12 Mbps in order for CAC
to work correctly for this deployment configuration.
There is no support for load-based CAC or multiple streams on the autonomous access points therefore it is not recommended
to enable CAC on autonomous access points.
If CAC is enabled on the autonomous access point, then SRTP and barge calls will fail.
Pre-Call Admission Control
If Call Admission Control (TSPEC) is enabled on the access point, the Cisco Unified Wireless IP Phone 7921G sends an Add
Traffic Stream (ADDTS) to the access point to request bandwidth in order to place or receive a call. If the AP sends an ADDTS
successful message then the Cisco Unified Wireless IP Phone 7921G establishes the call. If the call is rejected by the access
point and the wireless IP phone has no other access point to roam to, then phone displays “Network Busy”. If the admission is