5.32.1 General

The ATSSS feature is an optional feature that may be supported by the UE and the 5GC network.

The ATSSS feature enables a multi-access PDU Connectivity Service, which can exchange PDUs between the UE and a data network by simultaneously using one 3GPP access network and one non-3GPP access network and two independent N3/N9 tunnels between the PSA and RAN/AN. The multi-access PDU Connectivity Service is realized by establishing a Multi-Access PDU (MA PDU) Session, i.e. a PDU Session that may have user-plane resources on two access networks. This assumes both 3GPP access and non-3GPP access are allowed for the S-NSSAI of the PDU Session.

The UE may request a MA PDU Session when the UE is registered via both 3GPP and non-3GPP accesses, or when the UE is registered via one access only.

After the establishment of a MA PDU Session and when there are user-plane resources on both access networks, the UE applies network-provided policy (i.e. ATSSS rules) and considers local conditions (such as network interface availability, signal loss conditions, user preferences, etc.) for deciding how to distribute the uplink traffic across the two access networks. Similarly, the UPF anchor of the MA PDU Session applies network-provided policy (i.e. N4 rules) and feedback information received from the UE via the user-plane (such as access network Unavailability or Availability) for deciding how to distribute the downlink traffic across the two N3/N9 tunnels and the two access networks. When there are user-plane resources on only one access network, the UE applies the ATSSS rules and considers local conditions for triggering the establishment or activation of the user plane resources over another access.

The type of a MA PDU Session may be one of the following types defined in clause 5.6.1: IPv4, IPv6, IPv4v6 and Ethernet. In this release of the specification, the Unstructured type is not supported. The clause 5.32.6.2.1, the clause 5.32.6.2.2 and the clause 5.32.6.3.1 below define what Steering Functionalities can be used for each supported type of a MA PDU Session.

The handling of 3GPP PS Data Off feature for MA PDU Session is specified in clause 5.24.

The ATSSS feature can be supported over any type of access network, including untrusted and trusted non-3GPP access networks (see clauses 4.2.8 and 5.5), wireline 5G access networks (see clause 4.2.8), etc. as long as a MA PDU Session can be established over this type of access network.

In this Release of the specification, a MA PDU Session using IPv6 multi-homing (see clause 5.6.4.3) or UL Classifier (see clause 5.6.4.2) is not specified.

In this Release of the specification, support for ATSSS assumes SMF Service Areas covering the whole PLMN or that a MA PDU Session is released over both accesses when the UE moves out of the SMF Service Area.

A MA PDU Session does not support LADN. If the AMF receives a request to establish a MA PDU Session for a LADN DNN, the AMF shall reject the request. If the AMF receives a request to establish a PDU Session for a LADN DNN with "MA PDU Network-Upgrade Allowed" indication, the AMF shall not forward "MA PDU Network-Upgrade Allowed" indication to the SMF.

If the UE, due to mobility, moves from being served by a source AMF supporting ATSSS to a target AMF not supporting ATSSS, the MA PDU Session is released as described in TS 23.502 [3].

NOTE 1: Deployment of ATSSS that is homogeneous per PLMN, or network slice enables consistent behaviour. In the case of non-homogenous support of ATSSS in a PLMN/slice (i.e. some NFs in a PLMN/slice may not support ATSSS), MA PDU Sessions can be released due to UE mobility.

A Multi-Access PDU Session may, for the 3GPP access and/or non-3GPP access, use user-plane resources of an associated PDN Connection in EPC (e.g. one 3GPP access path via EPC and one non-3GPP access path via 5GC or one 3GPP access path via 5GC and one non-3GPP access path via ePDG/EPC). Such use of ATSSS with EPS interworking may apply to Ethernet and IP-based PDU Session and PDN Connection types.

NOTE 2: Co-existence with NBIFOM is not defined. It is assumed that NBIFOM and the multi-access connectivity described in this clause are not deployed in the same network.

NOTE 3: To the MME and SGW this is a regular PDN Connection and the support for ATSSS is transparent to MME and SGW.

For a MA PDU Session established for the Ethernet PDU Session type, if the UE has not indicated support for Ethernet PDN connection type or if the network does not support Ethernet PDN connection type, when the 3GPP access use user-plane resources of an associated PDN Connection, the following takes place:

• The SMF+PGW-C considers that the Multi-Access PDU Session is still using the Ethernet PDU Session / PDN Connection type but in a restricted mode where EPS signalling can only refer to non-IP PDN Connection type.
• MAR rules in the UPF are still used for distributing DL traffic between 3GPP access and non-3GPP access.
• For traffic on 3GPP access, the SMF may update N4 rules and QoS rules/EPS bearer contexts on the UE to take into account that no QoS differentiation is possible over 3GPP access.

Support of Multi-Access PDU Sessions using one leg associated with PDN Connection in EPC and one leg associated with PDU Session in 5GC is further defined in TS 23.502 [3].

The following clauses specify the functionality that enables ATSSS.