## 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  
Incorporating the ATSSS Phase 1 feature into the 5G system involves the introduction of a Multi-Access PDU (MA PDU) session into the existing 5G network functions. This integration is designed to ensure alignment with the foundational structure of the 5G system architecture, as depicted in Figure 6. The MA PDU session enables the seamless exchange of PDUs between the UE (application client) and the internet (remote application server host) for a wide range of applications. This exchange is made possible by simultaneously \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Function** & & **ATSSS Phase 1** & **ATSSS Phase 2** & **ATSSS Phase 3** \\  & & **(Release 16)** & **(Release 17)** & **(Release 18)** \\ \hline \multirow{4}{*}{\begin{tabular}{c} Steering functionality \\ \end{tabular} } & \(\bullet\) & \begin{tabular}{c} MPTCP functionality for \\ TCP traffic \\ ATSSS-LL functionality for all \\ types of traffic \\ \end{tabular} & \(\bullet\) & Same as ATSSS Phase 1 & \(\bullet\) & Same as ATSSS Phase 1 \\  & & \(\bullet\) & \begin{tabular}{c} ATSSS-LL functionality for all \\ types of traffic \\ \end{tabular} & \(\bullet\) & Same as ATSSS Phase 1 & \(\bullet\) & MQPUC steering \\ \hline \multirow{4}{*}{\begin{tabular}{c} Steering Mode (SM) \\ \end{tabular} } & \(\bullet\) & Active-standby SM & \multirow{4}{*}{\begin{tabular}{c} Same as ATSSS Phase 1 \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} Same as ATSSS Phase 1 \\ \end{tabular} } \\  & \(\bullet\) & \begin{tabular}{c} Smallest delay SM \\ Load-balancing SM with \\ pre-defined or fixed split \\ percentages \\ \end{tabular} & \(\bullet\) & Same as ATSSS Phase 1 & \(\bullet\) & Same as ATSSS Phase 1 \\  & \(\bullet\) & Priority-based SM & & & \(\bullet\) & Redundant SM \\ \hline \multirow{4}{*}{\begin{tabular}{c} Performance measurement \\ function (PMF) to assist \\ access selection \\ \end{tabular} } & \(\bullet\) & \begin{tabular}{c} Round Trip Time (RTT) \\ measurement in case of smallest \\ delay mode in use \\ UE’s reporting on access \\ to UPF \\ \end{tabular} & \(\bullet\) & \begin{tabular}{c} RTT measurement same as \\ ATSSS Phase 1 \\ Packet Loss Rate (PLR) \\ measurement for a service \\ data flow (SDF) over \\ both accesses \\ \end{tabular} & \(\bullet\) & Same as ATSSS Phase 2 \\ \cline{2-6}  & \(\bullet\) & \begin{tabular}{c} N/A \\ \end{tabular} & \(\bullet\) & \begin{tabular}{c} Access performance \\ measurement over a certain \\ QoS flow \\ \end{tabular} & \(\bullet\) & Same as ATSSS Phase 2 \\ \hline \multirow{4}{*}{\begin{tabular}{c} Combination of two access \\ networks and core networks \\ \end{tabular} } & \(\bullet\) & a 3 GPP access path over 5 GC/a & \(\bullet\) & Same as ATSSS Phase 1 \\  & non-3 GPP access path over 5 GC & \(\bullet\) & \
\begin{tabular}{c} a 3 GPP access path (e.g., E-UTRAN) over EPC/a \\ non-3 GPP access path \\ over 5 GC \\ \end{tabular} & \(\bullet\) &
\begin{tabular}{c} Same as ATSSS Phase 1 \\ a 3 GPP access path over 5 GC/a non-3 GPP access path over \\ path over ePDG/EPC \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of ATSSS functions in the context of 3 GPP standardization. utilizing both a 3 GPP access network and a non-3 GPP access network, each operating within two independent N3/N9 interfaces. Regarding the MA PDU session type, ATSSS Phase 1 exclusively accommodates structured types, including IPv4, IPv6, IPv4v6, and Ethernet, while it does not provide the support for unstructured types of a MA PDU session. The three following procedures are defined in the context of an ATSSS Phase 1 feature, which is applicable between one 3 GPP access and one non-3 GPP access.