5.17.2.3.3 Mobility for UEs in dual-registration mode

To support mobility in dual-registration mode, the support of N26 interface between AMF in 5GC and MME in EPC is not required. A UE that supports dual registration mode may operate in this mode when it receives an indication from the network that interworking without N26 is supported.

For UE operating in dual-registration mode the following principles apply for PDU Session transfer from 5GC to EPC:

• UE operating in Dual Registration mode may register in EPC ahead of any PDU Session transfer using the Attach procedure indicating that the UE is moving from 5GC without establishing a PDN Connection in EPC if the EPC supports EPS Attach without PDN Connectivity as defined in TS 23.401 [26]. Support for EPS Attach without PDN Connectivity is mandatory for UE supporting dual-registration procedures.

NOTE 1: Before attempting early registration in EPC the UE needs to check whether EPC supports EPS Attach without PDN Connectivity by reading the related SIB in the target cell.

• UE performs PDU Session transfer from 5GC to EPC using the UE initiated PDN connection establishment procedure with "handover" indication in the PDN Connection Request message (clause 5.10.2 of TS 23.401 [26]).
• If the UE has not registered with EPC ahead of the PDU Session transfer, the UE can perform Attach in EPC with "handover" indication in the PDN Connection Request message (clause 5.3.2.1 of TS 23.401 [26]).
• UE may selectively transfer certain PDU Sessions to EPC, while keeping other PDU Sessions in 5GC.
• UE may maintain the registration up to date in both 5GC and EPC by re-registering periodically in both systems. If the registration in either 5GC or EPC times out (e.g. upon mobile reachable timer expiry), the corresponding network starts an implicit detach timer.

NOTE 2: Whether UE transfers some or all PDU Sessions on the EPC side and whether it maintains the registration up to date in both EPC and 5GC can depend on UE capabilities that are implementation dependent. The information for determining which PDU Sessions are transferred on EPC side and the triggers can be pre-configured in the UE and are not specified in this Release of the specification. The UE does not know before-hand, i.e. before trying to move a given PDU session to EPC, whether that PDU session can be transferred to EPC.

NOTE 3: The Start of Unavailability Period and/or Unavailability Period Duration that the UE determines for NR satellite access with discontinuous network coverage in 5GS (see clause 5.4.1.4) and determines for satellite access with discontinuous coverage in EPS (see clause 4.13.8.2 of TS 23.401 [26]) can be different between 5GS and EPS.

For UE operating in dual-registration mode the following principles apply for PDN connection transfer from EPC to 5GC:

• UE operating in Dual Registration mode may register in 5GC ahead of any PDN connection transfer using the Registration procedure indicating that the UE is moving from EPC (clause 4.2.2.2.2 of TS 23.502 [3]).
• UE performs PDN connection transfer from EPC to 5GC using the UE initiated PDU Session Establishment procedure with "Existing PDU Session" indication (clause 4.3.2.2.1 of TS 23.502 [3]).
• UE may selectively transfer certain PDN connections to 5GC, while keeping other PDN Connections in EPC.
• UE may maintain the registration up to date in both EPC and 5GC by re-registering periodically in both systems. If the registration in either EPC or 5GC times out (e.g. upon mobile reachable timer expiry), the corresponding network starts an implicit detach timer.

NOTE 4: Whether UE transfers some or all PDN connections on the 5GC side and whether it maintains the registration up to date in both 5GC and EPC can depend on UE capabilities that are implementation dependent. The information for determining which PDN connections are transferred on 5GC side and the triggers can be pre-configured in the UE and are not specified in this Release of the specification. The UE does not know before-hand, i.e. before trying to move a given PDN connection to 5GC, whether that PDN connection can be transferred to 5GC.

NOTE 5: If EPC does not support EPS Attach without PDN Connectivity the MME detaches the UE when the last PDN connection is released by the PGW as described in clause 5.4.4.1 of TS 23.401 [26] (in relation to transfer of the last PDN connection to non-3GPP access).

When sending a control plane request for MT services (e.g. MT SMS) the network routes it via either the EPC or the 5GC. In absence of UE response, the network should attempt routing the control plane request via the other system.

NOTE 6: The choice of the system through which the network attempts to deliver the control plane request first is left to network configuration.