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 126.96.36.199
1.1.2 Registration Procedure (Message / IEs related with Emergency Support)
1.1.3 Emergency Service FallBack
188.8.131.52 23.502 Call Flow for Emergency Service FallBack (ES-FB)
184.108.40.206 General Call Flow for Emergency Service FallBack (ES-FB) . Redirection Case
UE camps on E-UTRA or NR cell in the 5GS (in either CM_IDLE or CM_CONNECTED state).
In SIB1 (optional) , NW indicate support for Emergency.
Registration Request from UE set required parameters like “Voice Centric” and “S1 Mode “Flag.
In Registration Accept message, NW indicates VoPS flag as TRUE and EMF flag as Non Zero (01 – 11) and EMC Flag as 00.
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.
For Redirection case, NW trigger Intersystem Redirection by sending RRCRelease message with RedirectInfo as E-ARFCN and VoiceFallBackIndication as TRUE.
RRCConnectionRequest on LTE with establishmentcause as “Emergency” triggered.
TAU procedure triggered with active flag to indicate that the UE has “user data pending”.
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.
220.127.116.11 Emergency Service FallBack (ES-FB) during QoS Flow Establishment
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:
In Registration Accept message, NW indicates that EMF as 01 and EMC as 00 .
INTERNET and IMS PDU Session established.
SoS / Emergency PDU session established and P-CSCF and IP assigned to the UE.
During E911 call initiation, Call FallBack to EPS due to lack of QoS Flow establishment for Emergency call on 5GS.
Redirection procedure triggered with RRCRelease with E-UTRA ARFCN info and voicefallbackindication flag as TRUE.
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.
Once All three PDN become active, Emergency call established on LTE.
Voice over NR with EPS Fallback includes an additional mobility trigger by which the UE falls back from NG-RAN to LTE during call establishment. This may be needed, e.g., in case not all feature for voice over NR are implemented in the UE or in case of temporary lack of radio resources in NR.
EPS fallback is an additional mobility trigger for improving voice KPIs. EPS Fallback enables phones to use the 5GC with NR, but RAN may trigger moving the phone LTE connected to EPC during call establishment. The reason for moving the UE to LTE can be :
temporary lack of radio resources in NR for voice
the UE is in area where NR in 5GS is not dimensioned and tuned for voice
Prior to all needed voice features are in place in the phone.
A smartphone (UE), connected to NR that tries to establish a voice connection, may perform an EPS fallback at call setup, triggered by the attempt to establish the Quality-of-Service (QoS) flow for voice media in NR.
At the attempt to establish the QoS flow for the voice media over NR during call set-up, the NG-RAN rejects the QoS flow setup towards the SMF with an indication that mobility is in progress. The NG-RAN initiates transfer of all PDU sessions from 5GS to EPS, using one of the two standardized procedures:
Release with redirect
1.2 Technical Aspect
When a UE moves from EPC to 5GC, the UE always performs Registration procedure.
When a UE moves from 5GC to EPC, the UE performs either Tracking Area Update or Initial Attach.
The UE performs Tracking Area Update procedure if :
– Both the UE and the EPC support “attach without PDN connectivity”, or
– The UE has at least one PDU Session for which Session Continuity is supported during interworking, i.e. the UE has EPS Bearer ID and mapped EPS QoS parameters received as described in clause 18.104.22.168 of 23.502.
The UE performs an initial attach procedure if :
– The UE is registered without PDU Session in 5GC or the UE is registered only with PDU Session for which Session Continuity is not supported during interworking to EPC, and
– Either the UE or the EPC does not support attach without PDN connectivity
1.3 3GPP CallFlow from 3GPP 23.502
Please refer Section 22.214.171.124 of 3GPP 23.502
1.4 General CallFlow in Details
1.4.1 End to End Call Flow (Redirection and N26 Supported)
During “Registration Request” from UE, it will indicate UE Usage Settings as “Voice Centric” and “S1 Mode “ is TRUE.
AMF request NGRAN to enquire about IMS Voice Support on NR from UE. NGRAN send UE Capability Enquiry –NR to UE.
UE send UECapabiltyInformtion-NR Capability with VoiceOverNR flag set as FALSE. That means , UE does not support Voice over NR and eventually Fallback Voice call session on EPS.NGRAN inform AMF about IMSVoiceSupportIndicator to FALSE in UERadioCapabiltyCheckResponse message.
AMF inform NGRAN that RedirectionEPSFallbackIndicator as TRUE in IntialContextSetupReq message.
In RegistrationAccept message from NW, IWKN26 bit is set to FALSE (we are assuming that UE support N26 Interface and support Single Registration Mode) and IMS VoPs 3gpp Flag set as TRUE in 5GS Network Feature Support IE.
PDU Session for INTERNET DNN is established.
PDU Session for IMS DNN is established (Please check other post for IMS PDU Session Establishment).
IMS Registration Completed with P-CSCF and UE IP assigned in IMS PDU Session. In REGISTER , SUBSCRIBE message P-Access-Network-Info should be use wit value of NR cell like “3GPP-NR-FDD” and MCC+MNC+TAC+NCI like :
For MO Voice call, UE initiate INVITE with P-ANI as “3GPP-NR-FDD” and SDP content with EVS audio Codec. NW sends 183SessionInProgress message and FallBack Procedure triggered during reservation of dedicated resources for Voice Session.
During Dedicated QoS Flow Establishment, NGRAN and 5GC take decision to FallBack Voice Call on EPS as Voice over NR cannot be established due to VoiceOverNR Flag is FALSE.
NGRAN initiate RRCRelease or HandoverRequest with ReDirectionInfo with EUTRAN Cell info to Fallback on LTE / EPS.
As NW indicate N26 interface supported than UE will send TAU Request message with ”request type “ as “handover” with IMS PDN and Internet PDN as Active in EBI information. NW mapped these PDN information with INTERNET / IMS PDU session (MME communicate with AMF over N26 interface to fetch UE Context information). Please check next section for other important IEs of TAU.
UE will send IMS Re-Register with PANI as “3GPP-EUTRA-FDD” for RAT change.
Other SIP messages transferred between UE and IMS Server and Dedicated Bearer for Voice with QCI =1 established on LTE.
1.4.2 Layer 3 Call Flow ( NR – LTE) for EPSFallBack (N26 Supported)
Please refer previous section Except TAU procedure.
After receiving RRCRelease (Redirection Case RRC_IDLE) UE will camp on LTE Cell as per ARFCN provided.
In TAU Request message (5G Integrity Protected):
UE will provide Request Type as “Handover”, UE Status as “5GMM REGISTERED”, Old GUTI (mapped with 5G-GUTI), EPS Bearer status set for INTERNET and IMS PDN as “Active”, PCO – 0001AH as PDU Session ID (mapped for IMS and INTERNET PDN)
For TAU Accept, MME interact with AMF on N26 interface and fetch UE context and mapped PDU session (IMS and INTERNET) and assign Bearer ID for IMS and INTERENT PDN. Same UE IP and P-CSCF is used as assigned by 5GC.
Once both Default PDN established, Dedicated bearer request procedure for Voice Call QCI = 1 started and Voice call established.