5GS to Non-3GPP Untrusted Wi-Fi Handover Call Flow
This section describes the 5GS to non-3GPP untrusted Wi-Fi handover call flow.

Step |
Description | ||
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1 |
The UE and the SMF or the UPF communicate through the NG-RAN to establish one or more PDU sessions. | ||
2 |
The UE connects to an untrusted non-3GPP access and selects an ePDG. Then, the UE initiates the handover attach procedure, as defined in 3GPP TS 23.402, section 8.6.2.1. After establishing the IKE tunnel between the UE and the ePDG, and authenticating the UE over SWm interface with the AAA server, the UE initiates IKE_AUH. The IKE_AUH includes cfg_params of the earlier assigned IPv4 or IPv6 addresses in 5GS and P-CSCF and DNS options. | ||
3 |
The ePDG sends a Create Session request to the PGW-C. This request includes the following details:
The RAT type indicates the non-3GPP access technology type. If the UE supports the IP address preservation and includes it in the port analyzer adapter (PAA), then the ePDG configures the handover indication in the Create Session request. This configuration allows the PGW-C to reallocate the same IP address or the prefix assigned to the UE. The IP address or prefix assignment occurs while the UE is connected to the 3GPP IP access. The policy modification procedure begins with the PCF. | ||
4 |
The SMF does not perform the UDM registration as it has already been registered with UDM during the 5GS session establishment. | ||
4a |
The SMF detects the charging triggers with the information available in Step 3 against the charging triggers that are received during the Wi-Fi session. | ||
4b |
The SMF detects the policy triggers with the information available in Step 3 against the requested policy control triggers that are received while communicating with PCF during the Wi-Fi session establishment. | ||
5 |
Based on the detected armed Policy Control Triggers that are received in Step 4b, the SMF sends the SM Policy Control Update request with the detected access parameters to the PCF. | ||
6 |
The PCF sends the SM policy decision in the SM Policy Control Update response by including new or updated PCC rules. | ||
7 |
Based on the information received in Step 6 and the existing policy data of 5GS session, the SMF prepares the “ModPolData” information. | ||
8 |
If the SMF receives new PCC rules in Step 6, the SMF sends the Charging Update request to the CHF with new rating-group for quota information. This request includes the PDU session information with the new access parameters. | ||
9 |
The CHF sends the multi-unit information as Charging Update response to the SMF. The multi-unit information includes quota information for the rating-groups received in Step 8 and for the existing rating-group of 5GS session. | ||
10 |
The SMF processes the ModChargingData in the Charging Update response that SMF receives from the CHF. | ||
11 |
The SMF sends the N4 session modification request to the UPF for Wi-Fi tunnels. This request includes details on creation of uplink FAR, creation of QER, creation of URR for the received new rating-group quota information, and update on URR for the modified quota information. | ||
12 |
The UPF sends the N4 session modification response to the SMF with the UL tunnel information in the created PDR. | ||
13 |
The SMF sends the GTPv2 Create Session response to the S-GW. The response includes details on accepted request or partially accepted request, PGW-C S2b F-TEID, PAA, APN-AMBR, creation of bearer context, charging gateway address, and APCO. | ||
14 |
The SMF sends the GTPv2 Create Bearer request to the S-GW. This request includes information on bearer context list, which contains UL tunnel information for each dedicated bearer to end-user. | ||
15 |
The S-GW sends the GTPv2 Create Bearer response to the SMF. The response includes details on accepted request or partially accepted request and bearer contexts. | ||
16 |
The SMF processes the Create Bearer response and derives the DL tunnel information for the established bearer and the failed EBI list, if any. The SMF sends the N4 session modification request to the UPF for Wi-Fi tunnel. This request is to create DL PDR and DL FAR with the DL tunnel information or list of charging description IDs for the detected charging triggers. The SMF deletes the gnb tunnel resources and the N4 resources of the Wi-Fi tunnel for the failed bearer context list. | ||
17 |
The UPF sends the usage report in the N4 Session Modification response to the SMF. | ||
18 |
The SMF initiates the NAMF communication N1 N2 message transfer, to the S-GW. This transfer message includes the PDU Session Resource Release Request N2 message. |
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19 |
The AMF sends N1 N2 Transfer Acknowledgement to the SMF. |
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20 |
The AMF sends the SM Context Update request to the SMF. This request includes the SM Resource Release Acknowledgement N2 message. |
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21 |
The SMF sends the 200/204 OK as SM Context Update response to the AMF. |
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22 |
The AMF sends the SM Context Update request to the SMF. This request includes the PDU Session Release Complete N1 message. |
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23 |
The SMF sends the 200/204 OK as SM Context Update response to the AMF. |
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24 |
If the SMF supports the June 2019 compliance version of 3GPP specification 23.502, the SMF indicates the release details to the AMF. The SMF achieves this functionality by sending the SM Context Status Notification message (statusInfo {Cause: PDU_SESSION_HANDED_OVER, resourceStatus: RELEASED). The SMF sends this notification after a successful handover of 5GS to Non-3GPP Untrusted WiFi session. The SMF processes the message as per the compliance profile configured for the corresponding service. For information on the compliance profile configuration, see the Configuring Compliance Profile section.
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25 |
The AMF sends the 204 OK as SM Context Status Notify response to the SMF.
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26 |
The SMF sends the Charging Update request to the CHF. This request includes the PDU session information with the new access parameters and multi-usage report containing details on the old access parameters and the usage report that is received in Step 17. | ||
27 |
The CHF sends the multi-unit information as Charging Update response to SMF. The multi-unit information includes new quota information for the existing rating-groups. | ||
28 |
The SMF sends the SM Policy Control Update to PCF. This update includes the new access parameters and rule report for failed QFI list that are received from the AMF as part of Create Bearer response. | ||
29 |
The PCF sends the SM policy decision through the SM Policy Control Update response to the SMF. | ||
30 |
The SMF processes the SM policy decision and handles it as PCF-initiated modification procedure as defined in 3GPP TS 23.502, section 4.3.3.2. |