Q signaling (abbreviated QSIG), a protocol for Integrated Services Digital Network (ISDN) communications based on the Q.931 standard, is used for signaling between digital private branch exchanges (PBXs). QSIG is employed in voice over IP (VoIP) networks, virtual private networks (VPNs), and high-speed, multi-application networks for corporations, educational institutions, and government agencies.
QSIG ensures that the essential functions in Q.931 are carried from node to node in networks containing equipment from different vendors. These functions include setup (a signal indicating the establishment of a connection), call-proceeding (a signal indicating that the call is being processed by the destination terminal), ring-alert (a signal that tells the calling party that the destination set is ringing), connect (a signal sent back to the source indicating that the intended destination phone set has received the call), and release/complete (a signal sent by either the source or the destination indicating that the call is to be terminated). QSIG has two layers, called BC (basic call) and generic function (GF). QSIG BC ensures that signaling is transparent among nodes from multiple vendors. QSIG GF provides additional functions for large-scale corporate, educational, and government networks, such as line identification, call intrusion, call diversion, and support for multiple applications.