The purpose of this document is to study how the 5G System (5GS) can be extended in order to support Access Traffic Steering, Switching and Splitting (ATSSS) between 3GPP and non-3GPP access networks. In particular, this document considers solutions that specify the following:

• How the 5GC and the 5G UE can support multi-access traffic steering (as defined in clause 3.1) between 3GPP and non-3GPP accesses.
• How the 5G Core network and the 5G UE can support multi-access traffic switching (as defined in clause 3.1) between 3GPP and non-3GPP accesses. This includes the conditions that can trigger the switching of data traffic to a new access type.
• How the 5G Core network and the 5G UE can support multi-access traffic splitting (as defined in clause 3.1) between 3GPP and non-3GPP accesses (multi access PDU session). This includes the conditions that can trigger the splitting of data traffic across multiple accesses.
• How the multi-access traffic steering, switching and splitting (ATSSS) can be taken into account by the charging framework in order e.g. to enable the network operator to differentiate charging for data traffic that is switched and/or split between 3GPP and non-3GPP accesses.
• How the policy framework can be extended in order to support the requirements for ATSSS.
• How ATSSS can be supported by using multi-access PDU sessions (as defined in clause 3.1). ATSSS solutions not requiring multi-access PDU sessions can also be considered.
• How to handle issues related to ATSSS that may occur when the UE moves between EPC and 5GC.

The scope of this document excludes the following aspects:

• Changes to the charging framework are not considered. However, it may be considered what information needs to be provided to the charging framework in order to charge traffic that is switched and/or split between 3GPP and non-3GPP accesses.
• ATSSS procedures that may be applied in the NG-RAN are not considered. The study is restricted only to ATSSS procedures applied in the 5G core network.
• 5GS enhancements to support trusted non-3GPP access networks are not considered.
• 5GS enhancements to support wireline access networks are not considered.

All solutions in this document should be based on and aligned with the 5GS Phase-1 normative work including work on policy management, as documented in TS 23.501, TS 23.502 and TS 23.503. The study in this document is organized into two phases:

• Initially, the study considers ATSSS solutions that enable traffic selection, switching and splitting between NG-RAN and untrusted non-3GPP access networks.
• Subsequently, after the 5GS architecture is enhanced to support trusted non-3GPP access networks, the study will also consider ATSSS solutions that enable traffic selection, switching and splitting between NG-RAN and trusted non-3GPP access networks.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.