Section: 4.8 Interworking with E-UTRAN connected to EPC 4.8.1 General
Content: In order to interwork with E-UTRAN connected to EPC, the UE supporting both S1 mode and N1 mode can operate in single-registration mode or dual-registration mode (see 3GPP TS 23.501[ System architecture for the 5G System (5GS) ]  [8]). Support of single-registration mode is mandatory for UEs supporting both S1 mode and N1 mode. During the EPS attach procedure (see 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15]) or initial registration procedure (see subclause 5.5.1.2), the mode for interworking is selected if the UE supports both S1 mode and N1 mode, and the network supports interworking. The mode for interworking may also be selected during the EPS tracking area updating procedure (see 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [15]) or registration procedure for mobility and periodic registration update (see subclause 5.5.1.3). For interworking between E-UTRAN connected to EPC and TNGF or N3IWF connected to 5GCN, the UE shall operate as specified in either subclause 4.8.2.3 or subclause 4.8.3. Which subclause the UE follows is chosen by the UE irrespective of the interworking without N26 interface indicator.