Q.931

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ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates bandwidth in fixed increments of 64 kbit/s. Q.931 does manage connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine. Q.931 has more recently been used as part of the VoIP H.323 protocol stack (see H.225.0) and in some mobile phone transmission systems.

Q.931 defines a number of messages used to establish and tear down connections (typically telephone calls). These include SETUP (indicating the establishment of a connection), CALL PROCEEDING (indicating that the call is being processed by the destination terminal), ALERTING (tells the calling party that the destination terminal is ringing), CONNECT (sent back to the calling party indicating that the intended destination has answered the call), and RELEASE COMPLETE (sent by either the source or the destination indicating that the call is to be terminated).