Emergency Services(E911) FallBack procedures in 5G

1.1     Emergency Services FallBack ES-FB

1.1.1     Introduction

Service Request for emergency: This solution is new in 5GS, i.e., there is no equivalent in 4G. The UE uses service-request for emergency towards AMF if support for this feature has been indicated by the AMF. If 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 needed for 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 TA and RAT where the UE is currently camping, and if the UE supports emergency services fallback, the UE shall initiate the Emergency Services Fallback procedure described below in section 1.1.3.2

1.1.2     Registration Procedure (Message / IEs related with Emergency Support)

1.1.3     Emergency Service FallBack

1.1.3.1     23.502 Call Flow for Emergency Service FallBack (ES-FB)

1.1.3.2     General Call Flow for Emergency Service FallBack (ES-FB) . Redirection Case

1. UE camps on E-UTRA or NR cell in the 5GS (in either CM_IDLE or CM_CONNECTED state).
2. In SIB1 (optional) , NW indicate support for Emergency.
3. Registration Request from UE set required parameters like “Voice Centric” and “S1 Mode “Flag.
4. In Registration Accept message, NW indicates  VoPS flag as TRUE and EMF flag as Non Zero (01 – 11) and EMC Flag as 00.
5. UE sends a Service Request message as requesttype as emergency services fallback. The UE is not required to include the PDU Sessions that are not relevant for the emergency service in the List Of PDU Sessions to be Activated in the Service Request for the emergency service.
6. For Redirection case, NW trigger Intersystem Redirection by sending RRCRelease message with RedirectInfo as E-ARFCN and VoiceFallBackIndication as TRUE.
7. RRCConnectionRequest on LTE with establishmentcause as “Emergency” triggered.
8. TAU procedure triggered with active flag to indicate that the UE has “user data pending”.
9. After redirection to the target cell the UE establishes a PDN connection for IMS emergency services and performs the IMS procedures for establishment of an IMS emergency session.

1.1.3.3     Emergency Service FallBack (ES-FB) during QoS Flow Establishment

1. If Emergency Services Fallback occurs during QoS Flow establishment than this is the same procedure as defined during EPS Fallback ( check older post for EPS – FallBack call flow). Only additional/different procedures for Emergency Services FallBack are mentioned below:
2. In Registration Accept message, NW indicates that EMF as 01 and EMC as 00 .
3. INTERNET and IMS PDU Session established.
4. SoS / Emergency PDU session established and P-CSCF  and IP assigned to the UE.
5. During E911 call initiation, Call FallBack to EPS due to lack of QoS Flow establishment for Emergency call on 5GS.
6. Redirection procedure triggered with RRCRelease with E-UTRA ARFCN info and voicefallbackindication flag as TRUE.
7. As N26 interface supported, UE trigger TAU Request with three (Emergency, Internet, IMS PDN) as Active. RRCCoonectionRequest with establishmentcause as “Emergency” used on EUTRA / EPS.
8. Once All three PDN become active, Emergency call established on LTE.

1.2     References

• 24.229
• 23.502
• 24.501
• 24.301
• 23.501
• GSMA NG.114
• 38.331
• 38.413