Emergency Services Fallback to LTE Call Flow

Emergency Services Fallback to LTE


StepDescription
1

UE camps on E-UTRA or NR cell in the 5GS (in either CM_IDLE or CM_CONNECTED state).

2

UE has a pending IMS emergency session request (example, voice) from the upper layers.

3

If the AMF has indicated support for emergency services using fallback via the “Registration Accept” message for the current RAT, the UE sends a “Service Request” message indicating that it requires an emergency services fallback.

4

The 5GC executes an NG-AP procedure in which it indicates to the NG-RAN that this is a fallback for emergency services. This procedure triggers the “Emergency Services Fallback” request. Currently the Cisco SMF and PGW-C supports Emergency Services in the EPC core Network (LTE). The AMF includes the EPC as a target CN to trigger inter-RAT fallback. When the AMF initiates the redirection for UEs that are successfully authenticated, AMF includes the security context in the request to trigger fallback towards the NG-RAN.

5

The NG-RAN initiates the handover or redirection to the E-UTRAN connected to the EPS (N26 interface based handover or redirection procedure). The NG-RAN uses the security context that the AMF to secure the redirection procedure.

If the redirection procedure is used, the target CN is also conveyed to the UE to enable it to perform the S1 mode NAS procedures. The UE uses the emergency indication in the RRC message and E-UTRAN provides the emergency indication to the MME during the “Tracking Area Update”.
6

After handover to the target cell, the UE establishes a PDU session or PDN connection for IMS emergency services and performs the IMS procedures for establishment of an IMS emergency session (example, voice).