## 4 Simultaneous Utilization of Multiple RANs within a 3 GPP Framework
### Simultaneous Utilization of Two 3 GPP Access Networks and Future Enhancements  
The ATSSS feature was initially designed as a transport-layer solution with a specific focus on concurrently utilizing user-plane resources across both a 3 GPP access network (such as NG-RAN or E-UTRAN) and a non-3 GPP access network (like WLAN), all without causing any disruptions to the underlying radio access networks. This concept of optimizing user-plane resources across multiple RANs with different RATs can be extended to encompass access networks specified exclusively within the framework of 3 GPP standardization. The demand for this expansion has become more pressing due to the limitations of non-3 GPP access networks such as WLAN in enhancing communication coverage with secure reliability and Quality of Experience (QoE) in certain use cases (e.g., maritime usage). Moreover, as the market evolves, there is an expectation of the coexistence of multiple 3 GPP access networks, including NG-RAN, E-UTRAN, and 5G LEO/GEO satellite access networks, within the framework of 5G-Advanced and future systems.  
From the perspective of the maritime sector and considering the diverse wireless communication environments that ships encounter during global navigation, the imperative of simultaneously utilizing radio resources of multiple RANs, including LEO/GEO satellites, E-UTRAN, and NG-RAN, becomes even more pronounced, as illustrated in Figure 8. This is a fundamental requirement for the digitalization of the maritime sector, including autonomous ships, to ensure the secure provision of seamless ubiquitous connectivity within a single subscription provided by a mobile operator with roaming services, facilitated through mutual agreements with other mobile or satellite operators.  
Figure 8: Example of available multiple 3 GPP access networks including 5G satellite access ((Adapted from Figure 5.10-1 of 3 GPP TR 22.841 V2.0.0 [90]).  
Figure 7: Protocol stack of the user plane with MP-QUIC steering functionality within the Release 18 framework of 3 GPP standardization (Adapted from Figure 5.32.6.2.2-1 of 3 GPP TS 23.501 V18.1.0 [89]).  
Lately, 3 GPP has been investigating pertinent use cases that bring about new service requirements, requiring corresponding transport-layer solutions to support the utilization of user-plane resources across two 3 GPP access networks [91]. With a single subscription, it is considered that two 3 GPP access networks are connected to the same Public Land Mobile Network (PLMN) [92, 93, 94], two different PLMNs, or a PLMN and a Non-Public Network (NPN) [95] called a private network on the market. In addition, two 3 GPP access networks may involve combinations such as NG-RAN and 5G LEO satellite access networks, E-UTRAN and 5G LEO satellite access networks, or 5G LEO and GEO satellite access networks.  
In the traditional approach, data-traffic distribution within the 3 GPP standardization framework relies on the utilization of user-plane resources from a single-access network. This implies that existing procedures may need to be reconsidered to enable the simultaneous utilization of user-plane resources from two separate 3 GPP access networks without disrupting RAN entities. Looking ahead to the progress of 3 GPP standardization in Release 19, there is an expectation of further enhancements. These enhancements are expected to continue into Release 20, which is considered the inaugural version of the 6G communication system within the 3 GPP framework.