Call Admission Control

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Call Admission Control prevents oversubscription of VoIP networks, CAC is a concept that applies only to real time media traffic and not to data traffic. CAC mechanisms complements the capabilities of QoS tools to protect voice traffic from the negative effects of other voice traffic and to keep excess voice traffic off the network.

Call Admission Control(CAC) is used to prevent congestion in Voice Traffic. It is a preventive Congestion Control Procedure. It is used in the Call Setup phase. CAC can be used to prevent congestion in connection-oriented protocols such as ATM. There are several CAC schemes available, which are detailed in [1].

It can also be used in VoIP in order to ensure there is enough bandwidth for the authorized flows. The difference between the former and the latter, however, is that VoIP uses RTP/UDP/IP, all of which are connectionless protocols. In this case, Integrated Services with RSVP (reserve resources for flow of packets through the network) using Controlled-Load Service is used in order to ensure that the call cannot be setup if it is not possible to support the flow described.

Also, according to the External link specified below, CAC kicks in to reject calls when either there is insufficient CPU processing power, or the Upstream/Downstream traffic exceeds prespecified thresholds, or the number of calls being handled exceeds a prespecified limit.