5.16.4.11 Emergency Services Fallback

In order to support various deployment scenarios for obtaining Emergency Services, the UE and 5GC may support the mechanism to direct or redirect the UE either towards E-UTRA connected to 5GC (RAT fallback) when only NR does not support Emergency Services or towards EPS (E-UTRAN connected to EPC System fallback) when the 5GC does not support Emergency Services. Emergency Services fallback may be used when the 5GS does not indicate support for Emergency Services (see clause 5.16.4.1) and indicates support for Emergency Services fallback.

Following principles apply for Emergency Services Fallback:

• If the AMF indicates support for Emergency Services fallback in the Registration Accept message, then in order to initiate Emergency Service, normally registered UE supporting Emergency Services fallback shall initiate a Service Request with Service Type set to Emergency Services fallback as defined in clause 4.13.4.1 of TS 23.502 [3].
• AMF uses the Service Type Indication within the Service Request to redirect the UE towards the appropriate RAT/System. The 5GS may, for Emergency Services, trigger one of the following procedures:
• Handover or redirection to EPS.
• Handover or redirection to E-UTRA connected to 5GC.
• After receiving the Service Request for Emergency Fallback, the AMF triggers N2 procedure resulting in either CONNECTED state mobility (Handover procedure) or IDLE state mobility (redirection) to either E-UTRA/5GC or to E-UTRAN/EPC depending on factors such as N26 availability, network configuration and radio conditions. In the N2 procedure, the AMF based on support for Emergency Services in 5GC or EPC may indicate the target CN for the RAN node to know whether inter-RAT fallback or inter-system fallback is to be performed. The target CN indicated in the N2 procedure is also conveyed to the UE in order to be able to perform the appropriate NAS procedures (S1 or N1 Mode).
• When the AS re-keying procedure and the Emergency Fallback procedure collides, the AMF gives up the AS re-keying procedure and only initiates the Emergency Fallback procedure.

NOTE 1: Emergency Services Fallback to EPS can be followed by an onward movement to GERAN or UTRAN via CSFB procedures if the PLMN does not support IMS emergency services.

NOTE 2: If the UE has signalled that S1 mode is disabled for a network that only supports IMS voice via EPS Fallback, the AMF will not indicate that Emergency Services Fallback is supported over 3GPP access.

Emergency Services fallback is supported only in case of PLMN. Emergency Services Fallback is not supported for SNPN.