IP address continuity during inter-system mobility to UEs that support 5GC NAS and EPC NAS. Networks that support interworking procedures with- out N26 must support procedures to provide IP address continuity during inter-system mobility to the UEs that operate in both single-registration and dual-registration modes [3]. Interworking procedures using N26 interface enable the exchange of MM and SM states between the source and target networks. Handover procedures are supported through N26 interface. When interworking procedures with N26 is used, the UE operates in single- registration mode. The network retains only one valid MM state for the UE, either in the AMF or MME. Either the AMF or the MME is registered in the HSS + UDM. The support for N26 interface between AMF in 5GC and MME in EPC is required to enable seamless session continuity (e.g., for voice services) for inter-system handover. When the UE moves ((<(H))) (foy) (<(H)) Control plane User plane 4G/5G NSA UE 4G/5G NSA UE 4G/5G SAUE 4G/5G RAN-level interworking 4G/5G SAUE 4G/5G RAN-level interworking 4G/5G CN-level interworking with connected to EPC 4G/5G CN-level interworking with connected to 5GC single registration dual registration Figure 1.79 RAN-level and core-network-level interworking models [82]. 5G Network Architecture 161 from 5GC to EPC, the SMF determines which PDU sessions can be relocated to the target EPS. The SMF can release the PDU sessions that cannot be transferred as part of the hand- over. However, the target EPS determines if the PDU session can be successfully moved to the target network. The dual registration approach requires the UE to register separately with the EPC and the 5GC. Thus it does not need to forward the UE context between MME and AMF, and the interface between MME and AMF is not required. The handover between LTE and 5G systems is decided by the UE. The UE performs normal access procedures after moving to the other network. Therefore, the solution can be supported by LTE eNBs with minimal changes. Furthermore, the 