Section: 4.22.2.3 MA PDU Session establishment with 3GPP access connected to EPC and non-3GPP access connected to 5GC 4.22.2.3.1 General
Content: This clause applies to the case where, for a PDU Session, multi-access connectivity via both EPC (over 3GPP access) and 5GC (over non-3GPP access) is supported and allowed in the UE and network. In this case, multi-access connectivity using ATSSS via both 3GPP access to EPC and non-3GPP access to 5GC may be provided as described in this clause. For this scenario, the general principles for ATSSS as described in clause 5.32 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] apply, with the additions provided in this clause 4.22.2.3. A Multi-Access PDU Session may be extended with user-plane resources via an associated PDN Connection on 3GPP access in EPC. This enables a scenario where a MA PDU Session can simultaneously be associated with user-plane resources on 3GPP access network connected to EPC and non-3GPP access connected to 5GC. Such a PDN Connection in EPS would thus be associated with multi-access capability in the UE and PGW-C+SMF. NOTE: To the MME and SGW this is a regular PDN Connection and the support for ATSSS is transparent to MME and SGW. The UE may operate in either single-registration mode or dual-registration mode in 3GPP access. Irrespective of whether the UE operates in single-registration mode or dual-registration mode in 3GPP access, it is assumed that the UE supports simultaneous registrations for non-3GPP access in 5GC and 3GPP access in EPC. The ATSSS rules are provided from the PGW-C+SMF to the UE via SM NAS signalling over 5GC, as described in clause 5.32.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. ATSSS rules are not provided via the EPC. When a UE establishes a MA PDU Session in 5GS, the UE indicates whether it supports 3GPP access leg over EPC. Based on the UE capability, the SMF determines whether the non-3GPP access should be released or not when the MA PDU Session is moved to EPS as described in clause 4.22.6.2. After the establishment of a MA PDU Session and setting up user-plane resources in 3GPP access in EPC and non-3GPP access in 5GC, the UE distributes the uplink traffic across the two access networks as described in clause 5.32.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. Similarly, the PDU Session Anchor UPF performs distribution of downlink traffic across the two access networks as described in clause 5.32.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The PMF protocol may be used via any user plane connection, i.e. via 3GPP access in EPC or non-3GPP access in 5GC. The PCF functionality to support ATSSS, as described in clause 5.32.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] and TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [20] applies also in the case of interworking with EPC. When the 3GPP access leg of a MA PDU Session using both 3GPP and non-3GPP access connected to 5GC is transferred to EPC, the PDU Session continues to work as a MA PDU Session using E-UTRAN/EPC and non-3GPP access connected to 5GC, as described in clause 4.22.6.