Call control is a process that is used in telecommunications networks to monitor and maintain connections once they have been established. In voice over Internet Protocol (VoIP) systems, call control is used to control connections between H.323 endpoints, or between an endpoint and a gatekeeper. H.323 is an International Telecommunication Union (ITU) standard for digital communication between terminals, network equipment and services.
In a VoIP network, call control is one of three major categories of communications traffic, the other two being call signaling and media communications. Call control uses Q.931, a connection protocol for digital networks, especially VoIP systems. Messages are transmitted as octets as specified in ITU H.245, which resolves the type of call media to be used (for example, conventional call, videoconferencing, or VoIP), and then manages the connection after it has been established. Call control functions include, but are not limited to, the determination of master/slave status for the endpoints, monitoring of the status of the endpoints, modification of the parameters of a connection, termination of a connection, and restarting a terminated or failed connection.