EPC to Non-3GPP Untrusted Wi-Fi Handover Call Flow
This section describes the EPC to non-3GPP untrusted Wi-Fi handover call flow.

Step | Description |
---|---|
1 | UE is attached to the 3GPP access network. The 4G LTE PDU session is established over SMF+PGW-C by communicating with UPF, PCF, and CHF for IPv4, IPv6, or dual-stack. PCF sends the Policy Control Request trigger, which is the SM Policy Decision, in response to SM Policy Control Create. CHF provides session-level or rating-group-level triggers to SMF in Charging Data Create response. |
2 | UE connects to an untrusted non-3GPP access and an ePDG is selected through the ePDG selection process. Then, UE initiates the handover attach procedure, as defined in 3GPP TS 23.402, section 8.6.2.1. After the IKE tunnel is established between UE and ePDG and after UE is authenticated over SWM interface with AAA server, UE initiates IKE_AUH. The IKE_AUH includes cfg_params of the earlier assigned IPv4 or IPv6 addresses in EPC and P-CSCF and DNS options. |
3 | ePDG sends a Create Session Request to the PDN gateway. This request includes details, such as IMSI, APN, handover indication, RAT type, ePDG TEID of the control plane, ePDG address for the user plane, ePDG TEID of the user plane, EPS bearer identity, and user location. The RAT type indicates the non-3GPP access technology type. If the UE supports the IP address preservation and is included in the port analyzer adapter (PAA), then ePDG configures the handover indication in the Create Session Request to allow the PDN gateway to re-allocate the same IP address or the prefix assigned to the UE. This IP address or prefix is assigned while UE is connected to the 3GPP IP access and initiates the policy modification procedure with PCF. |
4 |
SMF performs UDM registration by updating the PGW-C FQDN with UDM. The UDM registration does not happen during the session establishment with EPC. |
4a | SMF detects the charging triggers with the information available in Step 3 against the charging triggers that are received during EPC session establishment. |
4b | SMF detects the PCF triggers with the information available in Step 3 against the Request Policy Control triggers that are received in the communication with PCF during EPC session establishment. |
5 | Based on the detected armed Policy Control Triggers that are received in Step 4b, SMF sends the SM Policy Control Update request with the detected access parameters in Step 3 to PCF. |
6 | PCF sends the SM Policy Control Update response, which is the SM policy decision, by including new or updated PCC rules. |
7 | Based on the information received in Step 6 and existing policy data of EPC session, SMF prepares the information for the new or updated PCC rules. |
8 | If new PCC rules are received in Step 6 with new Rating Group that requires quota information, SMF sends the Charging Update request to CHF. SMF also includes new access parameters for the PDU session information. |
9 | CHF sends the Charging Update Response with multi-unit information that contains quota information for the requested rating-group in Step 8 to SMF. CHF may also send the new quota information for the existing rating-group of EPC session. |
10 | SMF processes the information that is received as Charging Update response from CHF. |
11 | SMF sends the N4 session modification request to UPF for Wi-Fi tunnel. This request includes details on creation of uplink PDR, creation of QER, creation of URR for received new rating-group quota information, and update on URR for modified quota information. |
12 | UPF sends the UL tunnel information that is in created PDR as the N4 session modification response to SMF. |
13 | SMF sends the GTPv2 Create Session response to S-GW. This response details on request accepted or request accepted partially, P-GW S2b F-TEID, PAA, APN-AMBR, bearer context creation, charging gateway address, and APCO. |
14 | SMF sends the GTPv2 Create Bearer request to S-GW. This request includes information on bearer context list, which contains DL tunnel information to end-user, to be created. |
15 | S-GW sends the GTPv2 Create Bearer response to SMF. The response includes details on request accepted or request accepted partially and bearer contexts. |
16 | SMF processes the Create Bearer response and derives the DL tunnel Information for the established bearer and the the failed EBI list, if any. SMF sends the N4 session modification request to UPF for Wi-Fi tunnel. This request is to create the DL PDR and DL FAR with DL tunnel information for each bearer, RAT modification information, and to delete resources for the 4G tunnel. SMF also deletes the N4 resources of Wi-Fi tunnel for the received failed EBI list or the failed QFI list. |
17 | UPF sends the usage report as N4 Session Modification response to SMF. |
18 | SMF+PGW-C sends the GTPv2 DB request to S-GW. This request includes EBI or list of EBIs. |
19 | S-GW sends the GTPv2 DB response to SMF+PGW-C. |
20 | SMF sends the Charging Update request to CHF. This request includes the PDU session information with the new access params and multi-usage report containing details on the access params and usage report that is received in Step 8 |
21 | CHF sends the multi-unit information as Charging Update response to SMF. The multi-unit information may include new quota information for the existing rating-groups. |
22 |
SMF sends the SM Policy Control Update request to UPF. This request includes the new access params and rule report for failed QFI list that is received from AMF as part of Create Bearer response. PCF sends the SM policy decision as SM Policy Control Update response. SMF processes the SM policy decision and handles it as PCF Initiation Modify procedure as defined in 3GPP 23.502, section 4.3.3.2. |