5.32.7.2 Interworking with N26 Interface

Interworking with N26 interface is based on clause 5.17.2.2, with the following differences and clarifications:

• When the UE is registered to the same PLMN over 3GPP and non-3GPP accesses and the UE request a new MA PDU Session via non-3GPP access, the AMF also includes the indication of interworking with N26 to SMF.
• The SMF does not request EBI allocation when MA PDU Session is established only over non-3GPP access. If MA PDU Session is released over 3GPP access, the allocated EBI(s) for the MA PDU Session is revoked by the SMF as described in clause 4.11.1.4.3 of TS 23.502 [3].
• The SMF does not request EBI allocation for GBR QoS Flow if the GBR QoS Flow is only allowed over non-3GPP access.
• If the UE and the network support MA PDU Sessions with 3GPP access connected to EPC, the MA PDU Session may be simultaneously associated with user-plane resources on 3GPP access network connected to EPC and with non-3GPP access network connected to 5GC. This case is further described in clause 5.32.1 and in clause 4.22.2.3 of TS 23.502 [3].
• If the UE or the network does not support MA PDU Session with 3GPP access connected to EPC, the MA PDU Session is handled as follows:
• When UE moves from 5GS to EPS, for both idle mode and connected mode mobility, if the MA PDU Session is moved to EPS as a PDN connection, the SMF triggers PDU Session Release procedure to release the MA PDU Session over Non-3GPP access in 5GS. UE and SMF remove ATSSS related contexts e.g. ATSSS rules, Measurement Assistance Information.
• When UE moves from 5GS to EPS, for both idle mode and connected mode mobility, if the MA PDU Session is not moved to EPS as a PDN connection, the 3GPP access of this MA PDU session becomes unavailable and the AMF notifies the SMF. In turn, the SMF may decide to move the traffic to Non-3GPP access of the MA PDU session, if it is available. When UE moves back from EPS to 5GS, after the UE is registered over the 3GPP, the UE may add user-plane resources over the 3GPP access to the MA PDU session by triggering PDU Session Establishment procedure as specified in clause 5.32.2.
• After UE moves from EPS to 5GS, for both idle mode and connected mode mobility, if the UE requires MA PDU session, or if no policy in the UE (e.g. no URSP rule) and no local restrictions mandate a single access for the PDU Session, UE triggers the PDU Session Modification procedure as described in clause 4.22.6.3 of TS 23.502 [3] to provide the ATSSS Capability to SMF+PGW-C. The SMF+PGW-C may determine whether to modify this PDU Session to a MA PDU Session in 5GS, e.g. based on SMF+PGW-C and UE's ATSSS Capability, subscription data and local policy. If dynamic PCC is to be used for the MA PDU Session, the PCF decides whether the MA PDU session is allowed or not based on operator policy and subscription data. If the MA PDU Session is allowed, the SMF provides ATSSS rule(s) and Measurement Assistance Information to the UE. If the UE receives ATSSS rules and is not registered to non-3GPP access, the UE establishes the second user-plane over non-3GPP access after the UE is registered to non-3GPP access. If UE was registered to non-3GPP access in 5GS, the UP resources over non-3GPP access are also established by the SMF using the PDU Session Modification procedure.
• If the MA PDU Session is established using one 3GPP access path via 5GC and one non-3GPP access path via ePDG/EPC and the UE moves from NG-RAN/5GC to E-UTRAN/EPC, the SMF+PGW-C may keep the MA PDU Session if MA PDU Sessions with 3GPP access and non-3GPP access user plane resources both connected to EPC is allowed based on operator policy.
• If the MA PDU Session is established using one 3GPP access path via 5GC and one non-3GPP access path via ePDG/EPC and the UE moves from NG-RAN/5GC to E-UTRAN/EPC, the SMF+PGW-C may release the user plane resources either over 3GPP access or non-3GPP access if MA PDU Sessions with 3GPP access and non-3GPP access user plane resources both connected to EPC is not allowed based on operator policy. In this case, while the UE is connected to EPC via both 3GPP access and non-3GPP access, the UE shall not trigger PDN Connection establishment to add an additional EPC access leg to the MA PDU Session.