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Friday, 28 September 2012

H.225.0 Call Signaling

Once the address of the remote endpoint is resolved, the endpoint will use H.225.0 Call Signaling in order to establish communication with the remote entity. H.225.0 messages are:
  • Setup and Setup acknowledge
  • Call Proceeding
  • Connect
  • Alerting
  • Information
  • Release Complete
  • Facility
  • Progress
  • Status and Status Inquiry
  • Notify
Figure 3 - Establishment of an H.323 call
In the simplest form, an H.323 call may be established as follows (figure 3):
In this example, the endpoint (EP) on the left initiated communication with the gateway on the right and the gateway connected the call with the called party. In reality, call flows are often more complex than the one shown, but most calls that utilize the Fast Connect procedures defined within H.323 can be established with as few as 2 or 3 messages. Endpoints must notify their gatekeeper (if gatekeepers are used) that they are in a call.
Once a call has concluded, a device will send a Release Complete message. Endpoints are then required to notify their gatekeeper (if gatekeepers are used) that the call has ended.



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