The ISO (International Organization for Standardization) mirror standard for the ITU-T Q Signaling (QSIG) standard. Formally known as Q.931, QSIG is a standard that defines services and signaling protocols for interconnecting TDM-based PBXs. QSIG is a Common Channel Signaling (CCS) protocol that runs over the ISDN D-channel for signaling between nodes in a Private Integrated Services Network (PISN). See also ISO, PBX, PISN, and QSIG.