Section: 4.11.1.3 Idle Mode Mobility procedures 4.11.1.3.1 General
Content: When a UE moves from EPC to 5GC, the UE always performs Registration procedure. When a UE moves from 5GC to EPC, the UE performs either Tracking Area Update or Initial Attach. The UE performs Tracking Area Update procedure if - Both the UE and the EPC support "attach without PDN connectivity", or - The UE has at least one PDU Session for which Session Continuity is supported during interworking, i.e. the UE has EPS Bearer ID and mapped EPS QoS parameters received as described in clause 4.11.1.1. The UE performs an initial attach procedure if - The UE is registered without PDU Session in 5GC or the UE is registered only with PDU Session for which Session Continuity is not supported during interworking to EPC; and - Either the UE or the EPC does not support attach without PDN connectivity.