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Call Flow 2 - Precondition

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 / No 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 have been allocated. 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-MediaHeader. (The P-Early-Media Header is configured in the SIP Headers field 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.

  • In this case, the 200 OK message to the initial INVITE is not received (call not answered), a 183 Session Progress is generated in Network B which causes the IMG 2020 to send back the 183 Session Progress back to network A. The 183 with SDP will continue to be sent until either a 200 OK is received or the PRACK Timer configured in the SIP Profile object has elapsed.

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