Call admission control (CAC) is the practice or process of regulating traffic volume in voice communications, particularly in wireless mobile networks and in VoIP (voice over Internet Protocol, also known as Internet telephony). Call admission control can also be used to ensure, or maintain, a certain level of audio quality in voice communications networks, or a certain level of performance in Internet nodes and servers where VoIP traffic exists.
Most CAC algorithms work by regulating the total utilized bandwidth, the total number of calls, or the total number of packets or data bits passing a specific point per unit time. If a defined limit is reached or exceeded, a new call may be prohibited from entering the network until at least one current call terminates. Alternatively, a graceful degradation methodology can be implemented. This means that the audio quality of individual calls can deteriorate to a certain extent before new calls are denied entry. Another method involves the regulation of calls according to defined characteristics such as priority descriptors. Still another method prevents new calls from entering the network only if the resources of the central processing unit (CPU) of a particular computer or server would be overburdened by such calls.
Call admission control can be tricky, because the volume of traffic in communications networks is inherently chaotic or "bursty," and traffic bursts are virtually impossible to predict. Another problem is that the actual content of a call may not conform to its descriptor.