The ITU-T Recommendation for the user network interface (UNI) for integrated services digital network (ISDN) basic call control, such as call setup and teardown.The specifications include user-touser and network-to-network call control messages for both circuit-switched and packet-switched networking. ISDN signaling and control takes place over the D channel. Q.931 also is included in the ITU-T H.323 protocol suite for multimedia communications over packet networks. See also D channel, H.323, ISDN, and ITU-T.