feature can be used to separate 5G services from the LTE ser- vices. In this case, all LTE eNBs will have to be upgraded to ng-eNBs SO that they can be connected to 5GC [16]. In core-network-level interworking, a direct interface between the LTE eNB and the NR gNB is not required and the EPC SGW is connected to the 5GC UPF. The UE manages LTE and NR interface connection independently, and can be connected to a single network, either LTE or 5G. When the UE is located in 5G coverage, it can only connect to the 5G network and receive 5G service. When the UE moves out of 5G coverage, it releases NR interface connection and establishes LTE radio interface connection. Although the network to which the UE connects changes, the IP address assigned to the UE stays the same and seamless service can be provided to the user. The core-network-level interworking is neces- sary in stand-alone architecture models, where NR can be used without relying on LTE net- work. In this case, single registration or dual registration is possible, as shown in Fig. 1.79. With the single registration, the UE registers with either LTE EPC or 5G networks at any time, and the UE context can be transferred via the control interface between the EPC MME and 5GC AMF when the UE's network association changes. In order to support the single registration, the MME will have to be upgraded in order to support the MME-AMF interface and the SGW needs to be connected to UPF in 5GC. LTE eNB must also be upgraded to support the mobility between the LTE and the 5G networks. N26 interface is an inter-CN interface between the MME and AMF in order to enable interworking between EPC and the 5G core. Support of N26 interface in the network is optional for interworking. N26 supports a subset of the functionalities that are essential for interworking. Networks that support interworking with EPC may support interworking procedures that use the N26 interface or interworking procedures that do not use the N26 interface. Interworking proce- dures with N26 support provide 