ESFB stands for Emergency Services Fallback in 5G deployments.
EPS emergency services fallback is an early handover of an emergency connection to EUTRAN. This solution is new in 5GS, i.e. there is no equivalent in 4G. Similar to the fallback procedures for normal voice and depending on the core network and RAT support, the fallback can either be a RAT fallback so only the RAT changes from NR to EUTRA, or it can be a system fallback where the fallback goes from 5GC to EPC. During the registration accept procedure, the network indicates the support for emergency services fallback to the UE. The UE indicates the service request “emergency” towards the AMF if support for emergency services has been indicated by the AMF in the registration accept message to the UE. Upon receiving the service request for emergency, the AMF interacts with gNB to perform EPS fallback. In this solution, there is no need for NR and 5GC to support emergency features other than what is needed for the service request for emergency handling. However, voice over NR (possibly including EPS fallback) has to be supported on NR and 5GC; otherwise, the UE would not even camp on NR.
If the 5GC has indicated emergency services fallback support for the routing area and RAT where the UE is currently camping and if the UE supports emergency services fallback, the UE should initiate the signaling message flow shown in the architectural concept of EPS emergency fallback below:
