VoNR call flow

In the previous section, we saw how an IMS PDU session is established. In this section, we will go over the VoNR call flow, which is used when a user places a voice call.

Figure 7.25 – VoNR call flow – part 1

Step 1: The UE initiates SIP registration toward the called party via UPF, and P-CSCF through the IMS core network in the backend.

Step 2: P-CSCF sends Npcf_PolicyAuthorization_Update to the PCF to enforce policies, modify service information, apply gate control for traffic shaping and policing, modify subscription to SDF notification/deactivation, and so on.

Step 3: The PCF sends an HTTP POST request (/callbacks/v1/smPoliciesUpdate
Notification/imsi-123xxxxxx:1:1591309591702656285/update) to the SMF to update PCC rules (SIP VoNR audio), policy control request triggers, SDF, and charging-related information.

Step 4: The SMF processes the received PCC rules and sends a 204 No Content (200 OK) message for...