ave gone over the details of the process to establish a 4G session using the 5G interworking function. This may turn out as a popular choice for the telecom operators to deploy, as it encourages the use of common infrastructure and network functions for both 4G and 5G, and hence the need to maintain two large 4G and 5G networks simultaneously is reduced to a large extent. This saves operational expenses (OPEX) to a large extent. In the upcoming section, we will explore how voice calls are supported in a 5G network. IMS 5G PDU session establishment Voice and video calls are use cases that need to be supported by every service provider network. The IP Multimedia Subsystem (IMS) enables these services in a telecom network. In this section, we will see how voice, video, and rich communication services are provided with the help of the IMS in 5G, which is very similar to the support of voice in 4G (VoLTE). Figure 7.12 shows the establishment of an IMS PDU session in 5G: gNB/RAN PCSCF PDU Session Establishment Request Nsmf_PDUSession Create SMContext Request UDM Registration Request 201 Created UDM Subscription Fetch Request 200 OK UDM Subscription Notification Request 201 Created Nsmf PDUSession CreateSMContext Response Figure 7.12 - IMS 5G PDU attach in 5G - part 1 Packet Core Procedures Let's look at these steps in detail: Step 1: The UE initiates the N1 message with the PDU session establishment request by including a container identifier, P -CSCF IPv4/v6 Request contained within the ePCO (extended Protocol Configuration Options) IE, and DNN set to IMS. Step 2: The AMF sends the Jsmf_PDUSession_CreateSM context request to the SMF by including N1 and N2 messages as multipart, along with the ePCO IE if it is received from the UE in the PDU session establishment request. Step 3: The SMF sends an HTTP PUT request for UDM registration. Step 4: After a successful transaction, the UDM reports with a 201 created message. Step 5: The SMF fetches the session management subscription data for the corresponding SUPI, DNN, and S