Scenario 1: INVITE with SDP
When the Access Session Border Controller (A-SBC) receives an INVITE with SDP, the A-SBC creates a SIP session and an associated media session with two flows for audio. The A-SBC applies the appropriate codec policy (with add-on-egress configured), so that the egress INVITE contains the necessary codec.
![The INVITE with SDP call flow is described in the paragraphs above and below the image. The INVITE with SDP call flow is described in the paragraphs above and below the image.](img/GUID-60DA9CDD-5C7A-44F0-81D8-09282DDA36A0-default.gif)
The A-SBC receives an answer to the INVITE, and when the answer contains the added codec the A-SBC invokes the Transcoding Session Border Controller (T-SBC) using an INVITE with the same SDP as the INVITE received on ingress. The communication between the A-SBC and the T-SBC is a separate dialog associated with a new media session.
The following code block shows and example of the INVITE the A-SBC sends to the T-SBC.
INVITE sip:192.168.101.78:5060 SIP/2.0 Via: SIP/2.0/UDP 192.168.101.18:5060;branch=z9hG4bK10dspa3058b1io4rk721 Max-Forwards: 70 Call-ID: 9fc71d79b43b4b95de41acefd71a8bc4@192.168.101.78 To: sip:192.168.101.78:5060 Contact: sip:172.16.101.18 From: sip:172.16.101.18;tag=bcf80756721880b7996ccb488b6a1b2f CSeq: 1 INVITE Target-Dialog: 1-16881@172.16.18.5;local-tag=1;remote-tag=16880SIPpTag011;realm=net192 Acme-Codec-Policy: ;ingress;name="in-2833";allow-codecs="* ";add-codecs-on-egress="";force-ptime="disabled";packetization-time="20";dtmf-in-audio="disabled";order-codecs="" Acme-Codec-Policy: ;egress;name="out-2833";allow-codecs="* ";add-codecs-on-egress="AMR ";force-ptime="disabled";packetization-time="20";dtmf-in-audio="disabled";order-codecs="" Content-Type: application/sdp Content-Length: 196^M P-Visited-Network-ID: open-ims.test Route: <sip:192.168.101.18:5060;lr;transport=UDP> v=0 o=user1 53655765 2353687637 IN IP4 192.168.101.18 s=- c=IN IP4 192.168.101.18 t=0 0 m=audio 20002 RTP/AVP 96 0 a=rtpmap: 96 AMR/8000 a=rtpmap:0 PCMU/8000
Note that the Target Dialog and Acme-Codec-Policy headers communicate operational parameters for pooled transcoding. Using such information, the T-SBC applies two codec policies and returns the INVITE to the A-SBC. The A-SBC moves the media session to itself, but the SIP session does not have a media session at this time.
![The INVITE with SDP diagram is described above. The INVITE with SDP diagram is described above.](img/GUID-56BE9D5C-2FC9-4583-9AB7-1271C04FE8A6-default.gif)