5.17.2.3.2 Mobility for UEs in single-registration mode

When the UE supports single-registration mode and network supports interworking procedure without N26 interface:

• For mobility from 5GC to EPC, the UE with at least one PDU Session established in 5GC may either:
• if supported and if it has received the network indication that interworking without N26 is supported, perform Attach in EPC with a native EPS GUTI, if available, otherwise with IMSI with Request type "Handover" in PDN CONNECTIVITY Request message (clause 5.3.2.1 of TS 23.401 [26]) and indicating that the UE is moving from 5GC and subsequently moves all its other PDU Session using the UE requested PDN connectivity establishment procedure with Request Type "handover" flag (clause 5.10.2 of TS 23.401 [26]), or.
• perform TAU with 4G-GUTI mapped from 5G-GUTI sent as old Native GUTI (clause 5.3.3 of TS 23.401 [26]) indicating that it is moving from 5GC, in which case the MME instructs the UE to re-attach. IP address preservation is not provided in this case.
• for the first TAU after 5GC initial Registration, the UE and MME for the handling of UE Radio Capabilities follow the procedures as defined in clause 5.11.2 TS 23.401 [26] for first TAU after GERAN/UTRAN Attach.

NOTE 1: The first PDN connection may be established during the E-UTRAN Initial Attach procedure (see TS 23.401 [26]).

NOTE 2: At inter-PLMN mobility to a PLMN that is not an equivalent PLMN the UE always uses the TAU procedure.

• For mobility from 5GC to EPC, the UE with no PDU Session established in 5GC
• performs Attach in EPC (clause 5.3.2.1 of TS 23.401 [26]) indicating that the UE is moving from 5GC.
• For mobility from EPC to 5GC, the UE performs Mobility Registration Update in 5GC with 5G-GUTI mapped from EPS GUTI and a native 5G-GUTI, if available, as Additional GUTI and indicating that the UE is moving from EPC. In this case, the AMF determines that old node is an MME, but proceeds as if the Registration is of type "initial registration". The UE may either:
• if supported and if it has received the network indication "interworking without N26 supported", move all its PDN connections from EPC using the UE initiated PDU Session Establishment procedure with "Existing PDU Sessions" flag (clause 4.3.2.2.1 of TS 23.502 [3]), or
• re-establish PDU Sessions corresponding to the PDN connections that it had in EPS. IP address preservation is not provided in this case.

NOTE 3: The additional native 5G-GUTI enables the AMF to find the UE's 5G security context (if available).

NOTE 4: When single-registration mode UE uses interworking procedures without N26, the registration states during the transition period (e.g. while UE is transferring all PDU Sessions / PDN Connections on the target side) are defined in Stage 3 specifications.

• If the network determines that the UE is changing RAT type, if the UE requests to relocate the PDU session from EPC to 5GC or 5GC to EPC, the SMF/MME uses the "PDU session continuity at inter RAT mobility" or "PDN continuity at inter-RAT mobility" information, respectively, in the subscription to determine whether to maintain the PDU session/PDN connection (if being handed over) or reject the PDU session request, with the relevant cause.
• If the UE requested to move the PDU session and the "PDN continuity at inter RAT mobility" information indicated "disconnect the PDN connection with a reactivation request" the network should provide a suitable cause code to the UE so that it can request a new PDU session.