Q.931

Q.931 It is the ISDN connection control protocol which is somehow parallel to TCP in the Internet Protocol Stack. Q.931 does not facilitate flow control or perform retransmission as the underlying layers are thought to be reliable and the circuit-oriented nature of ISDN allocates bandwidth in fixed increments of 64 kbps. Q.931 also manages connection setup and breakdown.  In H.323 scenario, this typical protocol is encapsulated in TCP and transmitted to port 1720.