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 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.