1       RAT FallBack  in 5G SA

1.1     Introduction

Similar to EPS FB, the 5G NR does not provide voice/video communication services at the initial stage. When the gNB establishes an IMS voice/video communication channel on the 5G network, a handover is triggered. In this case, the gNB sends a redirection or inter-RAT handover request to the 5GC to fall back to the eLTE network and use the VoeLTE (use ng-eNB) service.
User experience and introduction causes of RAT FB are the same as those of EPS FB. The deployment of RAT FB requires overlapping coverage of eLTE and NR networks. It is used for NR and LTE networks without overlapping coverage, and the gNB does not deploy an IMS voice/video communication channel (RTP or RTCP over NR).

1.2     Technical Aspect and Stack

1.2.1     Ng-eNB Control Plane and User Plane

Here, eNB evolved as ng-enB to support / connect with 5GC instead EPC. PDCP Layer upgrade as NR-PDCP. In User Plane, SDAP layer is used to map PDU Session, QFIs with Radio Bearers.

 

1.3     3GPP defined CallFlow (3GPP 23.502)

1.4     General Call Flow for RATFallBack for Voice

1. During “Registration Request” from UE, it will indicate UE Usage Settings as “Voice Centric”.
2. AMF request NGRAN to enquire about IMS Voice Support on NR from UE. NGRAN send UE Capability Enquiry –NR to UE.
3. UE send UECapabiltyInformtion-NR Capability with VoiceOverNR flag set as FALSE and provide “VoiceOverEUTRA-5GC” as TRUE so that NW knows UE support Voice on ng-eNB . That means , UE does not support Voice over NR and eventually Fallback Voice call session on ng-ENB .NGRAN inform AMF about IMSVoiceSupportIndicator to FALSE in UERadioCapabiltyCheckResponse message.
4. AMF inform NGRAN that RedirectionEPSFallbackIndicator as TRUE in IntialContextSetupReq message.
5. 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.
6. Internet PDU Session Established.
7. IMS PDU Session Established .
8. 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 ng-ENB as InterRAT HO or Redirection
• NGRAN initiate RRCRelease or HandoverRequest  with ReDirectionInfo with EUTRAN Cell info to Fallback on LTE / EPS and CnType as “5GC”
• UE will connect on LTE Cell with RRCConnection and send ServiceRequest with cause as “MO-Voice Call” and PDUSessionStatus IE with “IMS” and “Internet” PDU status as ACTIVE (Please Refer next section for Layer 3 procedures).
• UE will send IMS Re-Register with PANI as “3GPP-EUTRA-FDD” ( But TAC is 6 digit as EUTRA connected with 5GC) for RAT change.
• Other SIP messages transferred between UE and IMS Server and Dedicated QoS Flow for Voice with 5QI  = 1 established on UE and ng-eNB / 5GC.

1.5     RRC / NAS CallFlow for RATFallBack

1. Please check previous section for IMS and Internet PDU Session establishment and SIP Flows.
2. Once gNB initiate RRCRelease with CnType as “5GC” , RedirectCarrierInfo as EUTRA,-Freq and VoiceFallbackIndication should be FALSE.
3. Ng-eNB send SystemInformationBlock1 with PLMNInfoList-R15 with “CellAccessRelatedInfoList-5GC” , TAC, “CellIdentity-5GC”.
4. UE will use these PLMN info and Send RRCConnectionRequest with establishment cause as “MO-Voice” and other parameters. Ng-eNB provide required information in RRCConnectionSetup
5. In RRCConnectionSetupComplete UE will provide Connectedto5GC-r15 Flag as TRUE and provided registered AMF Info, ng-S-TMSI, GUMMI information.
6. UE will send Service Request as NAS message to 5GC. This Service Request ServiceType as “MO-Voice” and active PDU Session IEs for IMS and Internet PDU .
7. Ng-eNB will configured and send RRCConnectionReconfiguration to activate these PDUs and mapped with Radio Bearer. Sdap-config is used to map QFIs to Radio Bearers.
8. Once IMS and Internet PDU are activated on ng-eNB side, NW will initiate PDUSessionModificationCommand to create and activate Voice dedicated QoS Flow 5QI=1.

Mapped PDU Session ID will be IMS PDU.

• After receiving PDUModificationSession Command Complete , Audio Bearer established for Voice on ng-eNB.