The article discusses the technical aspects of Voice over NR with EPS Fallback, a feature that allows a user equipment (UE) to switch from NG-RAN to LTE during call establishment. This fallback mechanism is triggered when all features for voice over NR are not implemented in the UE or when there is a temporary lack of radio resources in NR.

EPS fallback is a mobility trigger that improves voice KPIs. It enables phones to use the 5GC with NR. However, RAN may trigger the phone to move from LTE connected to EPC during call establishment. The reasons for this transition can be a temporary lack of radio resources in NR for voice, the UE being in an area where NR in 5GS is not dimensioned and tuned for voice, or the absence of all necessary voice features in the phone.

When a smartphone connected to NR tries to establish a voice connection, it may perform an EPS fallback at call setup. This is triggered by the attempt to establish the Quality-of-Service (QoS) flow for voice media in NR. During this process, the NG-RAN rejects the QoS flow setup towards the SMF with an indication that mobility is in progress. The NG-RAN then initiates the transfer of all PDU sessions from 5GS to EPS, using either the “Release with redirect” or “Inter-system handover” procedures.

When a UE moves from EPC to 5GC, it always performs a Registration procedure. Conversely, when a UE moves from 5GC to EPC, it performs either a Tracking Area Update or Initial Attach. The specific procedure depends on whether both the UE and the EPC support “attach without PDN connectivity”, or if the UE has at least one PDU Session for which Session Continuity is supported during interworking.

The article also includes detailed diagrams and call flows to illustrate these processes. It concludes with a list of references for further reading.