Cisco Cisco Customer Voice Portal 8.0(1) Design Guide

Router requery is not supported when using SIP Refer with Unified CVP Comprehensive Call Flow, and 
the survivability service is handling the REFER message from Unified CVP. Router requery with Refer 
can be supported in other call flows when IOS is handling the REFER without the survivability service, 
or else Unified CM is handling the REFER. For third party SIP trunks, the support of router requery with 
REFER is dependent on their implementation and support for SIP REFER itself.

Call admission control must also be considered from a solution perspective, not just a Unified CVP 
perspective. These considerations are most evident in the distributed branch office model where there 
are other voice services, such as Cisco Unified CM, sharing the same gateways with Unified CVP and 
the amount of bandwidth between the sites is limited.   The most important item to consider in this case 
is which call admission control mechanisms are in place on the network so that a consistent call 
admission control mechanism can be used to account for all the calls traversing the WAN from that site. 
If two call admission control mechanisms can admit four calls each and the WAN link is able to handle 
only four calls, then it is possible for both call admission control entities to admit four calls onto the 
WAN simultaneously and thereby impair the voice quality. If a single call admission mechanism cannot 
be implemented, then each call admission control mechanism must have bandwidth allocated to it. This 
situation is not desirable because it leads to inefficient bandwidth over-provisioning.

There are three call admission control mechanisms that can be used in a Unified CVP environment: 
gatekeeper call admission control, Unified CM Locations, and Unified CM RSVP Agent. In a single-site 
deployment, call admission control is not necessary.

Unified CM performs call admission by assigning devices to certain locations and keeping track of how 
many calls are active between these locations. Because Unified CM knows how many calls are active 
and what codec is in use for each call, it is able to calculate how much bandwidth is in use and to limit 
the number of calls allowed.

A thorough conceptual understanding of call admission control mechanisms is important. These 
mechanisms are explained in the Cisco Unified Communications SRND Based on Cisco Unified 
Communications Manager, available at 

In a pure TDM environment where Unified CVP is switching calls from an ingress gateway to an egress 
gateway attached to a TDM ACD/IVR, the gatekeeper can handle the call admission control 
functionality.

If Unified CM is the egress gateway, gatekeeper call admission control can be used only if the ingress 
Unified CVP gateways and the IP phones are at different sites. Note that gatekeeper dial-plan resolution 
is still in use.

Because Unified  CM locations-based call admission control is used between the remote sites of a cluster, 
a gatekeeper typically is used for dial-plan resolution only. Understanding the routing of calls in the dial 
plan and the gatekeeper resolution is important because call routing situations might occur in which it is 
necessary to use more than one set of gatekeepers in the implementation. This is particularly common