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The message flow diagram in figure below shows the flow of messages from application startup (Microsoft Office Communicator) to a successful MakeCall operation and associated Originated event (AE Services and Communication Manager).
Steps 1 and 2 show some initial provisioning between the application, Microsoft Active Directory Services, and Live Communications Server.
Steps 3 through 10 show the establishment of the SIP dialog.
The Communicator client sends a SIP INVITE containing a Request System Status message.
When Live Communications Server receives this message, it opens a TLS connection to AE Services. AE Services will be provisioned with the certificate of the Live Communications Server server so that Live Communications Server will be a trusted server on the TR/87 port.
After the TLS connection has been established, Live Communications Server forwards the INVITE message on to AE Services. At this point, the AE Services Device, Media, and Call Control service will extract the user identity from the SIP message and query Microsoft Active Directory Services to find the extension(s) that the given user can control. This will be used for authorization of all subsequent requests.
It will then send a SIP OK message with a System Status message indicating everything is operational.
Steps 11 through 14 show the Communicator client requesting the set of supported CSTA features. This is because not all telephony systems support all of the services that Communicator uses.
Steps 15 through 18 show the Communicator client establishing a monitor for Pat's station.
Steps 19 through 22 show a Make Call request from Communicator being sent through to AE Services. For more details on what happens when this request is received, refer to the figure below.
Steps 23 through 26 show an Originated event coming from AE Services and being delivered to the Communicator client. This would have started with a CSTA 1 event from TSAPI to Call Control Services. CCS would then map this to the appropriate Call Control Listener, convert the event to CSTA 3, and invoke the proper method on the listener. This would result in the event being sent to the SIP UA and out to the Live Communications Server and eventually to the Communicator Client.
The below figure illustrates the Message flow for the Make Call operation in terms of a basic connectivity diagram. Other requests would follow a similar pattern.