A standard that defines services and signaling protocols for interconnecting TDM-based PBXs based on the ITU-T ISDN standard Q.931. 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). QSIG supports call setup, call teardown, and transparency of features such as message waiting, camp-on, and callback. The ISO (International Organization for Standardization) has adopted QSIG as Private Signaling System No. 1 (PSS1). See also CCS, ISDN, ISO, Q.931, and Q series.