 (SDP Offer) Negotiation of IP addresses, ports, codecs 183 Session Progress (SDP Answer) etc. for media PRACK 183 Session Progress reception 200 OK (PRACK) confirmation UPDATE (SDP Offer) Exchanged each QoS flow establishment 200 OK (SDP Answer) or modification 180 Ringing Called party is ringing 200 OK (INVITE) Call is answered and the INVITE procedure is acknowledged by 200 OK Figure 8.7 - SIP signaling exchange This is the high-level SIP signaling message exchange in a typical VoNR call establishment. It can be more complicated, and several iterations of the UPDATE message can be transferred to manipulate the codecs or other information. QoS flow establishment Remember that the default QoS flow, which has the 5QI value 5, carries the SIP signaling and it was in place in the SIP signaling exchange for both calling and called parties. We also observed that during the SDP Offer and SDP Answer messages in the SIP signaling exchange, additional QoS flows are created to carry the actual voice packets. The following diagram outlines the steps for the establishment of those additional QoS flows for both parties: Voice over New Radio (VoNR) Policy rule sent to SMF PDU Session Modification Diameter P-CSCF 1 SIP/SDP Figure 8.8 QoS flow establishment In step 1, the called party passes the SIP invite with SDP to the network and in response, it gets the 183 Session Progress message coming back from the called party. At this stage, P-CSCF in the IMS analyzes the SPD Offer and SDP Answer, and it knows the full description of the media bearer, IP address, ports, codecs, and bandwidth requirements. In step 2, the information from the SDP is extracted and placed into Diameter to describe the media bearer and be passed up to the PCF. In step 3, the PCF generates a policy rule and sends it to the SMF. A policy rule describes the required information, such as the following: Guaranteed Flow Bit Rate (GFBR) QoS flow at 40 kbps From 24 . 2 4 6 : 23554 To 24.1.2.43:11224 In step 4, the SMF conducts a relevant message exchange with bot