Call Flow 1 - Precondition

Owned by Kelli Higdon
Last updated: Mar 06, 2024
2 min read

The Call flow below requires that the following be configured in the Incoming SIP Profile:

  • PRACK Support: Required

  • Precondition Support: Required

  • P-Early-Media: Enabled 

Early Media with Answer

 

In the call flow above, the following happens:

  • The IMG 2020 receives an INVITE message from Network A. Within the INVITE message the IMG 2020 looks at the following parameters:

    • Value given to the Require Header

    • Value given to the Supported Header

    • Precondition parameter given to the a=des: line.

  • The IMG 2020 receives the INVITE and depending on the values received, will either reject or proceed with the call. If the call proceeds, none of the preconditions or parameters in the Require or Supported Header are passed to the outgoing INVITE. The incoming values are used to determine whether the call proceeds to the outgoing leg or not.

  • If the correct values are received, an INVITE is generated to the outbound leg.

  • We receive the 183 Session Progress message from Network B informing the IMG 2020 whether the qos resources required are available or not. If they are, a 183 Session Progress message is then generated on the incoming Network A leg. Within this Session Progress message is the following:

    • The Require Header with its status displayed.

    • The P-Early-Media Header set to send/recv (Configured in the SIP Headers object under the SIP Profile object).

    • The returned SDP will include the (a=) lines as displayed in the call flow above. The (a=) values are copied from the incoming INVITE with the exception of the a=curr:qos remote none line. The value none will always be replaced with a value of sendrecv since we want as a precondition that the remote end have the resources available. As displayed in the call flow diagram, the current status for both local and remote sides are also send/recv since that is what was received.

  • The SIP Entity in Network A then sends back a Provisional Ack (PRACK) back to the IMG 2020 and a voice path is opened to be able to play media or tones.

  • After Early Media has been played, the IMG 2020 then sends a 200 OK to the initial INVITE back to the incoming network. Again, the message returned displays the Supported and Require headers along with the (a=) line and the status of the precondition values.

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