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150 TAU-72.IP/TAU-36.IP Universal Network Terminal
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. 15—H.323 call algorithm
Algorithm description:
Connection establishment (via ITU-Q.931/H.225 protocol)
1
Subscriber A gateway rings up Subscriber B (sends 'setup' message).
2
Subscriber B gateway sends a message, stating the possibility of process continuation.
3
Subscriber B gateway sends 'Alerting' notification message.
Subscriber B is free. In this moment,
'ringing' tone is sent to the Subscriber B phone, and 'ringback' tone to Subscriber A phone.
4
Subscriber B gateway answers the call.
Logic channel establishment (via H.245 protocol)