VoWiFi PDU Session Creation Call Flow
To enable connectivity through a 5G core, the initial attach on the ePDG or EPS deviates from the defined 3GPP procedures in the following ways:
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An SMF+P-GW replaces the P-GW in the procedure.
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The SM Policy Association Establishment procedure replaces the IP-CAN Session Establishment and modification.
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The integrated charging over the NCHF interface with CHF replaces the online and offline charging functionality by using the Gy and Gz interfaces.
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Communication with the User Plane node happens over the N4 interface instead of the Sxb interface.
The following call flow depicts the creation of a VoWifi PDU session.

Step | Description |
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1 |
The UE initiates the IKE_SA_INIT and IKE_SA_AUTH exchange. The UE then sends the IKE_SA_AUTH exchange message to ePDG to create the IPSec tunnel. |
2 | The UE sends the IKE_SA_AUTH exchange message to the SMF+PGW-C as a GTP Create Session Request by including the “P-CSCF IPv4 or IPv6 request and DNS IPv4 or IPv6” container identifier in APCO IE Options. |
3 | The SMF+PGW-C extracts and saves the PDU Session ID that the UE sent in the APCO IE option. The SMF+PGW-C then performs a UDM registration and sends both the N11 and S2b interface IDs to UDM. Based on the local configuration or session management subscription data that is received from UDM for respective DNN, SMF+PGW-C determines to support “IMS Voice over PS”. |
4 |
The SMF+PGW-C sends the NPCF SM Policy Control Creation Request to the PCF to initiate the SM Policy Association Establishment procedure. In this procedure, the PGW-C+SMF includes the information elements that are received in the Create Session Request message into the Npcf_SMPolicyControl_Create service. These elements comprise the following information:
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5 | The PGW-C+SMF receives the PCC rules, PDU session policy information, and 5G QoS information. The PCC rules are mapped into EPS QoS information. The SMF+PGW-C creates TFT from the SDF filters that are received in the PCC rules. The SMF+PGW-C then associates them with the corresponding default and dedicated bearers. |
6 | Based on the charging policies received from the PCF, the SMF+PGW-C initiates Nchf_ConvergedCharging_Create procedure toward CHF. This procedure is based on the charging rules that are received from the PCF. |
7 | The SMF+PGW-C starts the UPF+PGW-U selection and N4 Session Establishment procedure. As this session is a 4G session that connects to the SMF+PGW-C, a separate CN tunnel is created for each bearer. Also, the QoS Flow Identifier (QFI) is not sent in the QoS Enforcement Rule (QER) and Packet Detection Rule (PDR). |
8 | Th eSMF+PGW-C sends Create Session Response to the ePDG. This response includes the bearer information and the TEID for the default bearer. The SMF+PGW-C also includes the 5G QoS parameters in APCO options 001CH (QoS rules), 001DH (Session-AMBR), 001EH (PDU session address lifetime), and 001FH (QoS flow descriptions) to the UE. |
9 |
The ePDG sends IKE_SA_AUTH Response to the UE. Then, depending on the mapped PCC rules, the SMF+PGW-C initiates the dedicated bearer creation. |