MF in the PDU session update context response message. In this section, we have gone through the details of the IMS PDU session establishment. We have studied the contents of the various HTTP messages exchanged between the network functions, and seen how the whole IMS PDU session is set up and the various network functions used when setting it up. In the upcoming section, we will study the details of Voice over New Radio (VoNR). 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. gNB/RAN PCSCF 5G PDU Session Established and PCSCF IP Address provided SIP Register Npcf_Policy Authorization_Update Npcf_SMPolicy Control_Update Notify request 204 No Content NIN2 TransferRequest NIN2 TransferReqAck N2PDUSession Request AN Specific resource modification N2 PDU Session Ack Figure 7.25 - VoNR call flow - part 1 Packet Core Procedures 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 sNpcf_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/vl/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 a successful transaction. Step 5: The SMF sends an HTTP POST request to AMF (http://x.x.x.x 8080/namf-comm/ v1/uecontexts/imsi- - 123xxxxxxxxx/nl-n2-messages/NIN2) with details (PDU session modification command ID, QFIS, QoS profile(s), and Session - AMBR), N1SM container (PDU session modification command (PDU Session ID, QoS rule(s), QoS flow level parameters