About Call Admission Control
Use call admission control (CAC) to regulate voice quality over a WAN link.
Voice quality can degrade when too many active calls exist on a link and the amount of bandwidth is oversubscribed. Call admission control regulates voice quality by limiting the number of calls that can be active at the same time on a particular link. Call admission control does not guarantee a particular level of audio quality on the link, but it does allow you to regulate the amount of bandwidth that active calls on the link consume.
Call admission control operates by rejecting a call for bandwidth and policy reasons. When a call is rejected due to call admission control, the phone of the called party does not ring, and the caller receives a busy tone. The caller also receives a message on their phone, such as “Not enough bandwidth.” If you have enabled automated alternate routing (AAR), call admission control automatically diverts calls to alternate public switched telephone network (PSTN) routes when WAN bandwidth is not available.