Scenario 2: INVITE without SDP

For an offerless call flow, the system creates a media session when the offer comes in a reliable provisional or final response. The Access Session Border Controller (A-SBC) applies the codec policy and sends the egress offer to the calling UE.

The INVITE Without SDP call flow is described in the paragraphs above and below the image.

When the A-SBC receives an answer in either a PRACK or ACK message, the A-SBC compares the offer and answer. An answer containing the codec added in the egress offer causes the A-SBC to invoke the Transcoding Session Border Controller (T-SBC) and create a standalone UAC dialog, just as in Scenario 1.

Because the A-SBC advertised its media address in the egress offer to the calling UE, the UAC dialog uses the original media session.

The INVITE Without SDP diagram is described above.