## 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
#### 4.1.2 ATSSS Phase 2  
In ATSSS Phase 2, two significant advancements have been achieved: the refinement of steering modes and the integration of MA PDU session support with a 3 GPP access network connected to the EPC, which serves as the LTE system's core network. In the realm of steering-mode enhancement, a new indicator, referred to as the Steering Mode Indicator, has been incorporated into the ATSSS/N4 rules [73], specifically designed for the load-balancing steering mode. This indicator signifies that the UE is empowered to modify the default steering parameters and autonomously adjust traffic steering based on its own decision making. In situations where ATSSS/N4 rules do not provide predefined split ratios for the load-balancing steering mode, this innovation allows both the UE and the UPF to independently and autonomously determine the data-traffic split percentage between two access networks. To indicate the absence of predefined or fixed percentages, the parameter called the autonomous load-balance indicator [74] is included within the Steering Mode Indicator. The primary goal of this enhancement is to optimize data flow throughput in both the uplink and downlink directions. It operates under the assumption that the UE and UPF typically select percentages that maximize aggregate throughput, considering RTT measurements that can vary depending on the radio environment in which the UE is situated. In ATSSS Phase 1, data-traffic distribution policies across two access networks were primarily controlled by the network infrastructure, irrespective of the specific circumstances encountered by the UE, regardless of their criticality. ATSSS Phase 2 has effectively addressed this limitation by including the parameter called the UE-assistance indicator [74] within the _Steering Mode Indicator_. This indication empowers the UE to make informed decisions regarding data-traffic distribution based on its internal status, such as battery consumption. Furthermore, the UE can request the UPF to consider its preferences when determining data-traffic distribution in the downlink direction, thereby providing increased control and flexibility to the UE in optimizing its network experience. It is important to note that the _Steering Mode Indicator_ from ATSSS/N4 rules includes only one parameter, either the autonomous load-balance indicator or the _UE-assistance indicator_. Furthermore, ATSSS Phase 2 introduces a new element called _Threshold Values_ within the ATSSS/N4 rules. This information establishes specific conditions for adjusting the data-traffic distribution for the load-balancing steering mode with fixed split percentages and the priority-based steering mode. A threshold value can represent either an RTT value or a Packet Loss Rate (PLR), and it applies to both access networks. Both the UE and the UPF utilize these threshold values. For instance, in the load-balancing steering mode with fixed split percentages, if at least one measured parameter (either RTT or PLR) on one access network surpasses the designated threshold value, the UE and UPF may cease sending data traffic on that access network or reduce the data traffic on that access network in a manner specific to the implementation. Consequently, the reduced data-traffic amount is redistributed to the other access network. Importantly, it should be noted that the Steering Mode Indicator and the Threshold Values are not concurrently included within the ATSSS/N4 rules. To enhance the precision of QoS flow control, ATSSS Phase 2 introduces the capability to conduct performance measurements, such as RTT and PLR measurements, over the same QoS flow used to carry the data flow. This improvement addresses the challenges associated with less-optimized control that stem from relying on rough estimates of RTT/PLR, which are obtained through measurements over the default QoS flow in ATSSS Phase 1. The network determines whether measurements are conducted 'over the default QoS flow' or 'over a certain QoS flow' during MA PDU session establishment.