After accessing the 5GC via non3GPP WALN, the terminal (UE) starts PDU session establishment after completing registration, authentication and authorization, during which user data, uplink and downlink traffic and QoS are defined as follows;
I. User plane After completing the PDU session establishment and establishing the user plane IPsec sub-SA between the UE and the N3IWF, the UE can use the established IPsec sub-SA and the associated GTPU tunnels between the N3IWF and the UPF to send upstream and downstream traffic with various QoS flows for the session over the untrusted WLAN network.
II.When the UE has to transmit a UL PDU, it shall determine the QFI associated with the PDU using the QoS rules of the corresponding PDU session and encapsulate the PDU in a GRE packet, with the QFI value located in the header of the GRE packet.The UE shall forward the GRE packet to the N3IWF via the IPsec sub-SA associated with the QFI by encapsulated into an IPsec packet in tunnel mode, with the source address being the UE IP address and the destination address being the UP IP address associated with the sub-SA.
When the N3IWF receives a UL PDU, it shall decapsulate the IPsec header and the GRE header and determine the GTPU tunnel ID corresponding to the PDU session.The N3IWF shall encapsulate the UL PDU in a GTPU packet and place the QFI value in the header of the GTPY packet and forward the GTPU packet to the UPF via the N3.
III.Downstream Traffic When the N3IWF receives a DL PDU from the UPF via the N3, the N3IWF shall decapsulate the GTPU header and use the QFI and the PDU session identifier in the GTPU header to determine the IPsec Child SA to be used to send the DL PDU to the UE via the NWu.
The N3IWF shall encapsulate the DL PDU within a GRE packet and place the QFI value in the header of the GRE packet.The N3IWF may also include a Reflected QoS Indicator (RQI) in the GRE header, which shall be used by the UE to enable Reflected QoS.The N3IWF shall forward the GRE packet, along with the DL PDU, through the IPsec Child SA associated with the QFI to the UE by encapsulating the GRE packet into an IP packet in tunnel mode, where the source address is the UP IP address associated with the sub-SA and the destination address is the address of the UE.
IV.QoS For UEs accessing the 5GCN over untrusted WLANs, the N3IWF supports QoS differentiation and mapping of QoS flows to non 3GPP access resources.The QoS flows are controlled by the SMF and can be pre-configured or established through the PDU session establishment or modification process requested by the UE.The N3IWF shall determine the user plane to be established based on the local policy, configuration, and QoS profile received from the network. profile to determine the number of user plane IPsec sub-SAs to be established and the QoS profile associated with each sub-SA. The N3IWF shall then initiate an IPsec SA creation process to the UE to establish the sub-SAs associated with the QoS flows of the PDU session.The QoS functions of the UE, the N3IWF, and the UPF are specified in figure (1) below.
Figure 1.QoS for ungranted WLAN access to 5GCNs
Non-granted non 3GPP access essentially corresponds to a WLAN interworking with 5GCN, which is served over N3IWF. However, unlike earlier architectures in which the WLAN pass-through network elements (PDG/ePDG) were part of the 3GPP core network, the N3IWF acts as an access network similar to the 3GPP access. This allows common procedures for registration, authentication and session handling in both 3GPP Access and non 3GPP Access. Paging, mobile registration, and periodic registration are not supported in non-granted WLANs. Multiple PDU sessions can be established on both 3GPP access and non-granted WLANs, and PDU sessions can be switched between them. It is also possible to establish multiple access PDU sessions on 3GPP Access and Ungranted WLANs that support ATSSS.
