Call admission control (CAC) is a form of admission control that prevents or mitigates oversubscription of VoIP networks.
Integrated services with RSVP (which reserve resources for the flow of packets through the network) using controlled-load service ensures that a call cannot be set up if it cannot be supported.
CAC rejects calls when either there is insufficient CPU processing power, the upstream and downstream traffic exceeds prespecified thresholds, or the number of calls being handled exceeds a specified limit.
[1] CAC can be used to prevent congestion in connection-oriented protocols such as ATM.
[2] However, VoIP differs in that it uses RTP, UDP and IP, all of which are connectionless protocols.