VoLTE PDU Session Creation Call Flow

To enable the connectivity through a 5G core, the initial attach on the E-UTRAN or EPS deviates from the defined 3GPP procedures in the following ways:

  • An SMF+P-GW replaces the P-GW in the procedure.

  • The SM Policy Association Establishment procedure replaces the IP-CAN Session Establishment and modification.

  • The integrated charging over the NCHF interface with CHF replaces the online and offline charging functionality by using the Gy and Gz interfaces.

  • Communication with the User Plane node happens over the N4 interface instead of the Sxb interface.

Note

Depending on the mapped PCC rules, the SMF+PGW-C can initiate the dedicated bearer creation.

The following call flow depicts the creation of a VoLTE PDU session.

VoLTE PDU Session Creation Call Flow
VoLTE PDU Session Creation Call Flow Description
StepDescription
1UE sends the attach request to MME through eNodeB.
2MME determines if the UE is active and subscribed for the handoff to NR. Then, MME selects a SMF+PGW-C node as the P-GW for the PDU session.
3MME sends the create session request to the selected S-GW and includes the selected SMF+PGW-C address in the request.
4

S-GW initiates the create session request toward SMF+PGW-C by including the “P-CSCF IPv4 or IPv6 request” container identifier in the extended PCO IE options.

SMF+PGW-C extracts and saves the PDU session ID that UE sends in the PCO option. Then, SMF+PGW-C performs a UDM registration and sends both the N11 and S5 or S8 interface ID to UDM. Based on the local configuration or the session management subscription data, which is received from UDM for respective DNN, SMF+PGW-C determines to support “IMS Voice over PS”.

5

SMF+PGW-C sends the NPCF SM policy control creation request to PCF to initiate the SM policy association establishment procedure. In this procedure, 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:

  • SUPI contains the IMSI.

  • DNN contains the APN.

  • PEI contains the IMEI-SV.

  • Session AMBR contains the APN-AMBR.

  • Default QoS information that contains the default EPS bearer QoS. The QCI values are mapped into 5QI values.

6PGW-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. Then, SMF+PGW-C associates them with the corresponding default and dedicated bearers.
7Based on the charging policies received from the PCF, the SMF+PGW-C initiates the NCHF converged charging creation procedure toward CHF. This procedure is based on the charging rules that are received from the PCF.
8The 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).
9The SMF+PGW-C sends create session response to the S-GW. This response includes the bearer information and the TEID for the default bearer. The SMF+PGW-C also includes the 5G QoS parameters in PCO options 001CH (QoS rules), 001DH (Session-AMBR), 001EH (PDU session address lifetime), and 001FH (QoS flow descriptions) to the UE.
10Based on the charging policies received from PCF, the SMF+PGW-C initiates NCHF converged charging creation procedure toward CHF. This procedure is based on the charging rules that are received from PCF.
11S-GW sends create session response to MME.
12MME sends the Initial Context Setup Request to eNodeB with the N1 Attach Accept message.
13eNodeB and UE perform the RRC configuration.
14UE sends the direct transfer message to eNodeB.
15eNodeB sends the attach completion message in the Initial Context Setup Response and the TEID of eNodeB to MME.
16MME sends a modify bearer request to S-GW with eNodeB TEID.
17S-GW sends the modify bearer request to SMF+PGW-C with eNodeB TEID.
18SMF+PGW-C performs the N4 session modification to update the eNodeB TEID on the data path to the UPF+PGW-U.
19SMF+PGW-C sends the modify bearer response to the S-GW.
20S-GW sends the modify bearer response to MME.