## 4 Simultaneous Utilization of Multiple RANs within a 3 GPP Framework
### Simultaneous Utilization of 3 GPP-Based and Wi-Fi Access Networks over 5 GC or EPC
#### 4.1.1 ATSSS Phase 1
* ATSSS traffic steering--This procedure is employed to select an access network for a new data flow to transfer the data traffic of this data flow over the selected access network;
* ATSSS traffic switching--This procedure is employed to seamlessly transition all data traffic of an ongoing data flow from one access network to another access network;
* ATSSS traffic splitting--This procedure is employed to split the data traffic of a data flow across two access networks. This splitting allows some data traffic of the data flow to be routed through one access network, while simultaneously directing other data traffic of the same data flow through another access network. The network-provided policy, which includes ATSSS rules for UE and N4 rules for UPF, is derived by the UE's serving Session Management Function (SMF) within the 5 GC. Concurrently, the UE reports the availability of the access network. In light of the network-provided policy and the access network availability reported from the UE, the UPF determines the appropriate procedure for distributing downlink data traffic across the two access networks. Simultaneously, the UE governs uplink data-traffic distribution, selecting the appropriate procedure based on the network-provided policy, access network availability, and local factors, including signal strength, user preferences, and signal loss. The ATSSS Phase 1 feature encompasses two steering functionalities: Multi-Path Transmission Control Protocol (MPTCP) functionality for TCP traffic, facilitated by an MPTCP proxy in the UPF, and ATSSS Low Layer (ATSSS-LL) functionality, which extends to all types of data traffic, including TCP, User Datagram Protocol (UDP), Ethernet, and more. Furthermore, four distinct steering modes, included in ATSSS rules, have been defined to guide UE in routing data traffic for MA PDU sessions, and three steering modes, except for active standby, are applicable only to non-Guaranteed Bit Rate (non-GBR) [71; 72] data flow:  
* Active-standby steering mode: When an access network, indicated as 'active', is available, all data traffic for the MA PDU session is routed through this active access network. If the active access network becomes unavailable while another access network, indicated as'standby', remains accessible, the data traffic for the MA PDU  
Figure 6: Multi-Access PDU (MA PDU) session with user-plane resources across both 3 GPP access and non-3 GPP access networks (The figure does not show entities related to the control plane in a 5G system). session is then directed through the standby access network. The determination of the active or standby status of the 3 GPP access network and non-3 GPP access network is conveyed through the ATSSS/N4 rules;
* Smallest delay steering mode: The data traffic for the MA PDU session is routed through the access network with the smallest RTT if two access networks are available;
* Load-balancing steering mode: Data traffic within the MA PDU session is intelligently distributed across both 3 GPP and non-3 GPP access networks, with a specified percentage allocation when both access networks are concurrently accessible. This distribution of data-traffic percentages between 3 GPP and non-3 GPP access networks is communicated via the ATSSS/N4 rules;
* Priority-based steering mode: Data traffic within the MA PDU session is primarily directed over a high-priority access network, unless the high-priority access network experiences congestion or becomes unavailable. In instances of congestion on the high-priority access network, new data traffic is redirected to the low-priority access network. In the event of the high-priority access network being unavailable, all data traffic for the MA PDU session is seamlessly rerouted to an alternative low-priority access network if available. The designation of which access network holds the high-priority status is specified within the ATSSS/N4 rules.