4.8.1 General

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 [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 [15]) or initial registration procedure (see clause 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 [15]) or registration procedure for mobility and periodic registration update (see clause 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 clause 4.8.2.3 or clause 4.8.3. Which clause the UE follows is chosen by the UE irrespective of the interworking without N26 interface indicator.