## 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
#### 4.1.3 ATSSS Phase 3  
ATSSS Phase 3 feature is currently undergoing standardization within the 5G-Advanced framework in 3 GPP, with a target completion date set for March 2024. The architectural aspects have already been fully specified in Stage 2 specifications, which were finalized in June 2023. Building upon the achievements in architectural specification, this section provides an in-depth exploration of the four noteworthy enhancements introduced in ATSSS Phase 3 [75,76]. First, ATSSS Phase 3 introduces a new steering mode known as the redundant steering mode integrated into both the UE and the UPF. This steering mode enables the distribution of duplicated data traffic between both a 3 GPP access network and a non-3 GPP access network. It offers the flexibility to seamlessly suspend or resume this distribution through a message transmitted via the user plane of any available access network within the MA PDU session. Additionally, this steering mode operates efficiently, whether configured with or without Threshold Values for RTT and PLR. It is specifically designed for non-GBR service data flow and is not configured with ATSSS-LL steering functionality. The roaming scenario involving network entities (i.e., Non-3 GPP InterWorking Function (N3 IWF) or Trusted Non-3 GPP Gateway Function (TNGF)) among non-3 GPP access networks is designed to facilitate the transition of data-traffic distribution within the MA PDU session. This transition shifts the data flow from the user-plane resource of an existing non-3 GPP access network to that of a new non-3 GPP access network, while keeping the data-traffic distribution over the 3 GPP access network connected for the MA PDU session. When both the UE and the 5G network components (i.e., AMF, SMF) have the capability to switch the non-3 GPP access path, such path redirection becomes achievable through the procedure of the mobility registration update initiated by the UE over the user-plane resource of the new non-3 GPP access network. Furthermore, this capability for non-3 GPPaccess-path switching extends to the user-plane resources of non-3 GPP access networks, even in the context of a single-access PDU session. In pursuit of enhancing the configuration of connectivity to the core network, ATSSS Phase 3 introduced additional improvements aimed at facilitating the MA PDU session over a 3 GPP access network linked to the 5 GC and non-3 GPP access network connected to the evolved Packet Data Gateway (ePDG) [77; 78] via the EPC. In addition to two steering functionalities introduced in ATSSS Phase 1, namely MPTCP functionality and ATSSS-LL functionality, a new steering functionality referred to as Multi-Path Quick UDP Internet Connection (MP-QUIC) has been defined to steer, switch and split UDP traffic flows according to ATSSS/N4 rules based on policies decided by the Policy Control Function (PCF). The MP-QUIC steering functionality consists of three components as follows:  
* QoS flow and steering mode selection: Following the establishment of an MA PDU session, this component within the UE initiates the setup of one or more multipath QUIC connections. The UE and UPF components then determine the QoS flow, steering mode, and transport mode based on ATSSS/N4 rules, respectively;
* HTTP/3 layer: It supports the HyperText Transfer Protocol version 3 (HTTP/3) HTTP/3 protocol [79], along with extensions for UDP proxying over HTTP [80], HTTP datagrams [81], and extended CONNECT [82]. This layer selects a multipath QUIC connection to be used for each UDP flow and allocates a new QUIC stream on this connection linked with UDP flow;
* QUIC layer: It supports the QUIC protocol [83; 84; 85] and the extensions [86; 87] defined by the Internet Engineering Task Force (IETF) [88]. As depicted in Figure 7, the MP-QUIC steering functionality within the UE communicates with a corresponding MP-QUIC proxy steering functionality in the UPF. This communication employs the QUIC protocol and its multipath extensions over the HTTP/3.