Section: 4.22 ATSSS Procedures 4.22.1 General
Content: This clause specifies the procedures that enable the support of Access Traffic Steering, Switching and Splitting (ATSSS), as defined in clause 5.32 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. These procedures can be applied only by ATSSS-capable UEs and 5GC networks. The key enabler of ATSSS is the Multi Access-PDU (MA PDU) Session. As specified in clause 5.32.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], a MA PDU Session is a PDU Session associated with two independent N3/N9 tunnels between the PSA and RAN/AN and with multiple access types, i.e. with one 3GPP access and one non-3GPP access both connected to 5GC. A MA PDU Session may also be a PDU Session associated with one 3GPP access connected to EPC and one non-3GPP access connected to 5GC, or a PDU Session associated with one non-3GPP access connected to EPC and one 3GPP access connected to 5GC. The traffic of a MA PDU Session can be transferred over 3GPP access, or over non-3GPP access, or over both accesses. How the traffic is transferred over the available accesses of a MA PDU Session is governed by the applicable policy created by the 5GC network. The UE determines whether ATSSS is supported by the network based on the MA PDU Session Support indicator provided by the AMF during the Registration procedures, as specified in clause 4.22.9.1. If the network does not support ATSSS, the UE shall not initiate the following procedures in this network: - establishment of a MA PDU Session (clause 4.22.2); - establishment of a PDU Session with "MA PDU Network-Upgrade Allowed" indication (clause 4.22.3); - addition of user-plane resources over one access for an existing MA PDU Session, which has been established over the other access in a different network (clause 4.22.7); or - PDU Session Modification with Request Type of "MA PDU request" or with "MA PDU Network-Upgrade Allowed" indication after moving from EPC to 5GC (clause 4.22.6.3).