In order to support the ATSSS feature, the 5G System Architecture is extended as shown in Figure 4.2.10-1, Figure 4.2.10-2 and Figure 4.2.10-3. The additional functionality that is supported by the UE and the network functions shown in these Figures is specified in clause 5.32 below. In summary:
The UE supports one or more of the steering functionalities specified in clause 5.32.6, e.g. MPTCP functionality and/or ATSSS-LL functionality. Each steering functionality in the UE enables traffic steering, switching and splitting across 3GPP access and non-3GPP access, in accordance with the ATSSS rules provided by the network. The ATSSS-LL functionality is mandatory in the UE for MA PDU Session of type Ethernet.
The UPF may support MPTCP Proxy functionality, which communicates with the MPTCP functionality in the UE by using the MPTCP protocol (RFC 8684).
The UPF may support ATSSS-LL functionality, which is similar to the ATSSS-LL functionality defined for the UE. There is no user plane protocol defined between the ATSSS-LL functionality in the UE and the ATSSS-LL functionality in the UPF. The ATSSS-LL functionality in the UPF is not shown in the following three Figures.
In addition, the UPF supports Performance Measurement Functionality (PMF), which may be used by the UE to obtain access performance measurements (see clause 5.32.5) over the user-plane of 3GPP access and/or over the user-plane of non-3GPP access.
The AMF, SMF and PCF are extended with new functionality that is further discussed in clause 5.32.
Figure 4.2.10-2 shows the 5G System Architecture for ATSSS support in a roaming case with home-routed traffic and when the UE is registered to the same VPLMN over 3GPP and non-3GPP accesses. In this case, the MPTCP Proxy functionality and the PMF are located in the H-UPF.
Figure 4.2.10-3 shows the 5G System Architecture for ATSSS support in a roaming case with home-routed traffic and when the UE is registered to a VPLMN over 3GPP access and to HPLMN over non-3GPP access (i.e. the UE is registered to different PLMNs). In this case, the MPTCP Proxy functionality and the PMF are located in the H-UPF.
The reference architecture shown with reference point representation in Figure 4.2.15-1 and with Service Based Interface (SBI)-representation in Figure 4.2.15-2, enables a UE to connect to a WLAN access network using its 5GS credentials without registration to 5GS. This architecture is based on the Non-Seamless WLAN Offload Function (NSWOF), which interfaces to the WLAN access network using the SWa interface as defined in TS 23.402, and interfaces to the AUSF using the Nausf SBI. The functionality of the NSWOF and the procedures applied for supporting a WLAN connection using 5GS credentials for Non-seamless WLAN offload are further defined in TS 33.501Annex S. The roaming architectures are shown with reference point representation in Figure 4.2.15-3 and with SBI representation in Figure 4.2.15-4. The architecture in Figure 4.2.15-1 and Figure 4.2.15-2 applies to UEs with PLMN or SNPN credentials.
The UE can also connect to a WLAN access network using 5GS credentials by performing the 5GS registration via Trusted non-3GPP access procedure defined in clause 4.12a.2.2 of TS 23.502. With this procedure, the UE connects to a WLAN access network using 5GS credentials and simultaneously registers in 5GS. However, the architecture defined in Figure 4.2.15-1, Figure 4.2.15-2, Figure 4.2.15-3 and in Figure 4.2.15-4, enables a UE to connect to a WLAN access network using 5GS credentials but without registration in 5GS.
If the WLAN is configured as Untrusted Non-3GPP access, in the case that the WLAN supports IEEE 802.1x, the UE may first use the 5G NSWO procedure to obtain a connection with and the local IP address from the WLAN, and any time after that, the UE may initiate the Untrusted Non-3GPP Access to obtain the access to 5GC.