EPS Fallback Voice in 5G – 5G Resource Center Blogs

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1.1 Introduction

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
Inter-system handover

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 4.11.1.1 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 4.13.6.1 of 3GPP 23.502

1.4 General CallFlow in Details

1.4.1 End to End Call Flow (Redirection and N26 Supported)

End to End EPS Fallback with N26 and Redirection 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.