Inter-RAT handover: 5G ↔ 4G

Moving Between Radio Technologies

As 5G coverage is not yet universal, UEs frequently need to switch between 5G NR and 4G LTE. This inter-RAT handover involves transferring the UE context between the 5G Core (AMF/SMF) and the EPC (MME/SGW). The N26 interface between AMF and MME enables this context transfer. When the UE moves from 5G to 4G, the AMF transfers the NAS security context and PDU session information to the MME, and PDU sessions are mapped to EPS bearers with equivalent QoS parameters.