Call signaling is a process that is used to set up a connection in a telephone network. Call signaling processes include: authentication, to verify the identities of the calling and called parties; authorization, to ensure that the network is not misused and to minimize the risk of denial-of-service (DOS ) attacks; and communications security, to help prevent eavesdropping or manipulation of message content.
In a VoIP network, call signaling is one of three major categories of communications traffic, the other two being call control and media communications. Call signaling is used in voice over IP (VoIP ) systems to establish connections between H.323 endpoints, or between an endpoint and a gatekeeper. H.323 is an International Telecommunication Union (ITU) standard that defines how digital communications occur between terminals, network equipment and services.
Call signaling using Q.931 (a connection protocol for digital networks, especially VoIP systems, in which messages are transmitted as octets) is done according to the ITU H.225.0 specification, which defines message formats for digital multimedia and VoIP communications.