4. Subscriber A gateway confirms the reception of reply from the forwarding server.
5. Subscriber A rings up Subscriber B directly.
6. Subscriber B gateway receives the command for processing.
7. Subscriber B is free. Subscriber B is free. In this moment, 'ringing' tone is sent to the Subscriber B phone,
and 'ringback' tone to Subscriber A phone.
8. Subscriber B answers the call.
9. Subscriber A gateway confirms session establishment.
10. Subscriber A clears back, 'busy' audio tone is sent to the Subscriber B.
11. Subscriber B gateway confirms received clearback command.
8.4 Algorithm of a Successful Call via H.323 Protocol
H.323 is ITU-T standard that describes specifications for audio and video data transmission via packet
switching networks and includes standards for video and voice codecs, public domain applications, call and system
management. H.323 protocol family includes three basic protocols: terminal equipment and zone controller
interaction protocol—RAS, connection management protocol—Н.225, and logic channel management protocol—
H.245.
This section describes an example of a basic connection initiation scenario via H.323 protocol between two
gateways without a gatekeeper.
Fig. 18—H.323 call algorithm
Algorithm description:
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