Terminal over WLAN - non3GPP network PDU establishment process

After accessing 5GC via non 3GPP, the terminal (UE) will start PDU session establishment after completing registration, authentication and authorization, and the specific processes are as follows;

I. PDU Session Establishment After the terminal (UE) accesses the 5GC via WLAN, the PDU session establishment involves N31WF, AMF, SMF, and UPFF, etc., and the flow is shown in Figure (1) below;

Fig. 1.PDU session establishment of 5GCN terminal (UE) accessed via WLAN

II. PDU session establishment steps

1. The UE sends a PDU session establishment request using NAS signaling IPsec SA to the N3IWF, which transparently forwards it to the AMF in a NAS UL message.
2. A process similar to the PDU session establishment in 3GPP access is performed in the 5GCN (shown in Figure 1 above).
3. The AMF sends an N2 PDU Session Resource Setup Request message to the N3IWF to establish the WLAN resources for this PDU session. This message includes the QoS profile and associated QFI, PDU session ID, UL GTPU tunnel information, and NAS PDU session establishment acceptance.
4. The N3IWF determines the number of IPsec sub-SAs to be established and the QoS profile associated with each IPsec sub-SA based on its own policy, configuration, and QoS profile received.
5. The N3IWF sends an IKE Create Sub-SA request to establish the first IPsec sub-SA of the PDU session. which includes the QFI, PDU session ID, and UP IP address associated with the sub-SA, as well as an optional DSCP value and default sub-SA indication.
6. The UE sends an IKE Create Sub-SA response when it accepts an IKE Create Sub-SA request.
7. The N3IWF establishes determines other IPsec sub-SAs, each associated with one or more QFIs and a UP IP address.
8. After all IP sub-SAs are established, the N3IWF forwards a PDU Session Establishment Acceptance message to the UE via the signaling IPsec SA to initiate UL data.
9. The N3IWF also sends an N2 PDU Session Resource Setup Response to the AMF that includes the DL GTPU Tunnel information, which further performs a process similar to the PDU Session Establishment process in 3GPP Access (as shown in Figure 1) and enables the initiation of D Data.

The PDU session for 3GPP access may be serviced by a different SMF than the one that serves the PDU session for non3GPP access.

III. PDU session deactivation The deactivation of an existing PDU session UP connection results in the deactivation of the corresponding NWu connection (i.e., IPsec sub-SA and N3 tunnel). When the UE is in the CM-CONNECTED state, it can independently deactivate the UP connections of different PDU sessions. If the PDU session is an always-on PDU session, the SMF shall not deactivate the UP connection for this PDU session due to inactivity. Release of a PDU session via non3GPP access does not imply release of the N2 connection.

IV. Paging Issues The non-granting WLAN does not support paging; therefore, when the AMF receives a message corresponding to the PDU session of the UE in CM-IDLE state in non3GPP access, it may perform the network-triggered service request procedure over 3GPP access regardless of the 3GPP access UE state. The network-triggered service request procedure for non3GPP access can also be executed in the AMF for the UE in CM-IDLE state in 3GPP access and for the UE in CM-CONNECTED state in non 3GPP access when 3GPP access paging is not performed.

V. 3GPP and non 3GPP Access Multiple PDU Sessions A UE registered over both 3GPP access and non-granted WLAN may have multiple PDU sessions on both accesses, with each PDU session being active in only one of the accesses. When the UE switches to CM-IDLE in either access, the UE may move the PDU session in the corresponding access to the target access according to the UE policy.The UE may need to initiate the registration procedure for the switchover in the target access, and then initiate the PDU session to establish and move the PDU session ID of the session; the core network maintains the PDU session but deactivates the N3 user-plane connection for such PDU session; Depending on the implementation the UE may initiate the logout procedure in the absence of PDU session access.

VI. Multiple Access PDU Sessions 3GPP Release16 supports Access Traffic Control, Switching and Splitting (ATSSS), which allows PDU sessions with multiple packet flows in a multiple access PDU session to be able to select either a 3GPP access or an untrusted WLAN for each of the packet flows or the packet flows to be able to switch between a 3GPP access and an ungranted WLAN or the packet flows to be able to split between 3GPP access and untrusted WLAN; the PDU session establishment process contains additional information and user plane establishment for the same purpose.