5G System Architecture Supporting ATSSS

I. ATSSS is an abbreviation for Access Traffic Steering, Switching, Splitting; this is a function introduced by 3GPP for 5G (NR) that allows mobile devices (UEs) to simultaneously use 3GPP and non-3GPP access, manage user data traffic, control new data flows, select (new) access networks, switch all ongoing data to different access networks to maintain data continuity, and split individual data flows, allocating them to multiple access networks to improve performance or achieve redundancy. Specifically:

• Control:The network determines which access method (e.g., 5G and Wi-Fi) a new data flow should use based on operator-defined rules and real-time conditions.
• Switching:The network transfers an ongoing data session from one access network to another. For example, a video call can be switched from Wi-Fi to 5G without interruption.
• Splitting:The network can simultaneously allocate a single data flow to two or more access networks. This can be used to increase bandwidth (link aggregation) or ensure reliability (redundancy).

II. Working Principle ATSSS can operate at the IP layer (using protocols such as MPTCP) or below the IP layer (using underlying routing functions). Control is handled by the 5G core network's PCF (Policy Control Function), based on operator-defined rules and performance measurement data from the User Equipment (UE) and the network itself.

III. ATSSS Modes The main ATSSS modes are as follows:

• Primary/Backup Mode:Traffic is sent through the active link. If the active link fails, it switches to the backup link.
• Load Balancing Mode:Traffic is distributed among available access networks, typically based on a percentage to balance the load.
• Minimum Latency Mode:Traffic is routed to the access network with the lowest latency (round-trip time).
• Priority Mode:Traffic is initially sent through a high-priority link. If that link becomes congested, traffic is split or diverted to a lower-priority link.

IV. Architecture Expansion and Functionality The 5G system architecture has been expanded to support ATSSS functionality (see Figures 4.2.10-1, 4.2.10-2, and 4.2.10-3); the 5G terminal (UE) supports one or more flow control functions, namely MPTCP, MPQUIC, and ATSSS-LL. Each flow control function in the UE can perform flow control, handover, and splitting between 3GPP and non-3GPP access networks according to the ATSSS rules provided by the network. For Ethernet-type MA PDU sessions, the UE must have ATSSS-LL functionality, with the following specific requirements for the UPF:

• - The UPF can support MPTCP proxy functionality, which communicates with the MPTCP function in the UE using the MPTCP protocol (IETF RFC 8684 [81]).
• - UPF can support MPQUIC proxy functionality, which communicates with the MPQUIC function in the UE using the QUIC protocol (RFC9000 [166], RFC9001 [167], RFC9002 [168]) and its multipath extension (draft-ietf-quic-multipath [174]).
• - UPF can support ATSSS-LL functionality, which is similar to the ATSSS-LL functionality defined for the UE.

IV. ATSSS Application Characteristics

4.1 Ethernet type MA PDU sessions require the ATSSS-LL functionality (conversion) in 5GC. In addition:

• - UPF supports Performance Measurement Function (PMF), which the UE can use to obtain access performance measurements on the 3GPP access user plane and/or non-3GPP access user plane.
• - AMF, SMF, and PCF extend new functionality, which is discussed further in Section 5.32.

4.2 ATSSS control may require interaction between the UE and the PCF (as specified in TS 23.503[45]).

4.3 The UPF shown in Figure 4.2.10-1 can be connected via the N9 reference point instead of the N3 reference point.

V. Roaming Scenarios

5.1 Figure 4.2.10-2 shows ATSSS support in a roaming scenario for the 5G system architecture; this scenario includes home-roaming traffic, and the UE is registered to the same VPLMN via 3GPP and non-3GPP access. In this case, the MPTCP proxy function, MPQUIC proxy function, ATSSS-LL function, and PMF are located in the H-UPF.

5.2 Figure 4.2.10-3 shows ATSSS support in a roaming scenario for the 5G system architecture, this scenario includes home-roaming traffic, and the UE is registered to the VPLMN via 3GPP access and to the HPLMN via non-3GPP access (i.e., the UE is registered to different PLMNs). In this case, the MPTCP proxy function, MPQUIC proxy function, ATSSS-LL function, and PMF are all located in H-UPF.