This module describes the steps involved in creating an H.323 call, establishing media communication, and releasing the call. The example network contains two H.323 terminals (T1 and T2) connected to a gatekeeper. Direct call signaling is assumed. It is also assumed that the media stream uses RTP encapsulation. Figure 6 illustrates H.323 call establishment.

Figure 6. H.323 Call Establishment

1. T1 sends the RAS ARQ message on the RAS channel to the gatekeeper for registration. T1 requests the use of direct call signaling.
2. The gatekeeper confirms the admission of T1 by sending ACF to T1. The gatekeeper indicates in ACF that T1 can use direct call signaling.
3. T1 sends an H.225 call signaling setup message to T2 requesting a connection.
4. T2 responds with an H.225 call proceeding message to T1.
5. Now T2 has to register with the gatekeeper. It sends an RAS ARQ message to the gatekeeper on the RAS channel.
6. The gatekeeper confirms the registration by sending an RAS ACF message to T2.
7. T2 alerts T1 of the connection establishment by sending an H.225 alerting message.
8. Then T2 confirms the connection establishment by sending an H.225 connect message to T1, and the call is established.

   Figure 7 illustrates H.323 control signaling flows.

   
   Figure 7. H.323 Control Signaling Flows

9. The H.245 control channel is established between T1 and T2. T1 sends an H.245 TerminalCapabilitySet message to T2 to exchange its capabilities.
10. T2 acknowledges T1's capabilities by sending an H.245 TerminalCapabilitySetAck message.
11. T2 exchanges its capabilities with T1 by sending an H.245 TerminalCapabilitySet message.
12. T1 acknowledges T2's capabilities by sending an H.245 TerminalCapabilitySetAck message.
13. T1 opens a media channel with T2 by sending an H.245 openLogicalChannel message. The transport address of the RTCP channel is included in the message.
14. T2 acknowledges the establishment of the unidirectional logical channel from T1 to T2 by sending an H.245 openLogicalChannelAck message. Included in the acknowledge message are the RTP transport address allocated by T2 to be used by the T1 for sending the RTP media stream and the RTCP address received from T1 earlier.
15. Then, T2 opens a media channel with T1 by sending an H.245 openLogicalChannel message. The transport address of the RTCP channel is included in the message.
16. T1 acknowledges the establishment of the unidirectional logical channel from T2 to T1 by sending an H.245 openLogicalChannelAck message. Included in the acknowledging message are the RTP transport address allocated by T1 to be used by the T2 for sending the RTP media stream and the RTCP address received from T2 earlier. Now the bidirectional media stream communication is established.

    Figure 8 illustrates H.323 media stream and media control flows.

    
    Figure 8. H.323 Media Stream and Media Control Flows

17. T1 sends the RTP encapsulated media stream to T2.
18. T2 sends the RTP encapsulated media stream to T1.
19. T1 sends the RTCP messages to T2.
20. T2 sends the RTCP messages to T1.

    Figure 9 illustrates call release flows.

    
    Figure 9. H.323 Call Release

21. T2 initiates the call release. It sends an H.245 EndSessionCommand message to T1.
22. T1 releases the call endpoint and confirms the release by sending an H.245 EndSessionCommand message to T2.
23. T2 completes the call release by sending an H.225 release complete message to T1.
24. T1 and T2 disengage with the gatekeeper by sending an RAS DRQ message to the gatekeeper.
25. The gatekeeper disengages T1 and T2 and confirms by sending DCF messages to T1 and T2.