When a user initiates a VoNR call at the edge of a 5G cell, poor signal coverage may lead to call difficulties, poor call quality, or even dropped calls. To mitigate these issues, EPS Fallback (EPS FB) is often employed, forcing the UE to switch to LTE/EPS for the call during the call setup. EPS FB is also applicable when the UE is not configured for VoNR calls, the 5G cell does not provide voice services, or the UE lacks the necessary VoNR capabilities.
There are two primary options for EPS FB:
· Option A: After a successful initial call attempt, the 5G radio connection is released, and the UE is redirected to a 4G cell via a 5G RRC Release command. A new radio connection is then established for the VoLTE call within the 4G cell. In this scenario, the UE context is transferred from the AMF to the MME via the N26 interface.
· Option B: A 5G-to-4G cross-RAT handover is performed, and the session management and user plane tunnels in the core network are also handed over from SMF/UPF to MME/S-GW. This is achieved through a GTPv2 forward relocation procedure over the N26 interface. *It is estimated that EPS FB introduces an additional call delay of approximately 2 seconds.
