RATFallBack for Voice in 5G SA

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).

Simple Architecture

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.

Control and User Plane for ng-eNB


RatFallback Stack (Courtesy Huawei WP)

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.

5GS-WiFi(ePDG) Calling/Offloading Aspects

1       EPDG <–>5GS IMS Media (Voice/Video) Offloading

1.1     Introduction

We already have seen about IMS Media Voice / Video etc offload to ePDG from EPS to seek better Quality coverage and throughput and Seamless experience for Video / Voice calls in a closed environment like In-house, office, etc where User can access good WiFi coverage than EPS. To get same kind of good quality coverage experience in epDG <–> 5GS HO deployment, I have covered possible call flows with required changes in IKE_AUTH and 5GS message to support these scenarios. We will not cover N3IWF <–> 5GS Handover scenarios in this document now and will cover it in the future.

1.2     Technical Aspect and Interfaces

1.2.1     General

WLAN 5 standard and above (WLAN 6 and WLAN 6E) are preferred to do better seamless HO between eDPG ßà 5GS.  802.11ax is highly desirable as a WLAN standard.

1.2.2     Interworking Architecture between ePDG <–> 5GS

1.2.3     Pre-Requisite

To Simplify and for better understanding, I have taken case as UE is 3GPP REGISTERED with 5GS in upcoming sections. Furthermore,  There is N26 interface between AMF and MME present and UE support only Single Registration mode only.

  1. If UE is Registered with 3GPP EPS than during ePDG -> 5GS HO, UE will initiate RegistrationRequest on 5G with “requesttype” as “Mobility Registration” in Single Registration mode.
  2. If UE support Dual Registration mode than UE will initiate RegistrationRequest on 5G with “requesttype” as “initial Registration

1.3     5GS –> epDG IMS Media Offloading

1.3.1     3GPP defined Call Flow as per 23.502

1.3.2     General Call Flow

  1. UE is registered with 5GS and IMS and Internet PDU session established. Voice  / Video / RTT call is successfully established on 5GS.
  2. NW measure WLAN / ePDG power and find good WLAN power, which is appropriate to do Voice / Video offload on ePDG.
  3. ePDG DNS message exchanged and UE gets ePDG Tunnel information.
  4. IKE_SA_INIT Procedure on epDG start to get Encryption algos and other epDG policies exchange.
  5. In IKE_AUTH_Req , UE inform NW about N1_Mode_Capabilty with PLMN info which are equivalent to 5GS and ongoing PDU_Session_ID . Same PCO information ( Port, UE IP, etc) as used in 5GS will be reused.
  6. In IKE_AUTH_RESP, NW will provide N1_Mode_Information with S-NSSAI information and equivalent PLMN ID and other default parameters ( same as used in EPS).
  7. IMS PDU session on 5GS released.
  8. Once EAP-AKA IKEV2 procedure completed. UE will initiate IMS Re-Registration on epDG
  9. IMS Voice/ Video/RTT call offload from 5GS to epDG.

1.4     ePDG–> 5GS IMS Media HO

1.4.1     3GPP defined Call Flow as per 23.502

1.4.2     General Call Flow

  1. UE is 5GS 3GPP Registered and Internet PDU session established.
  2. IMS PDU and Media (Voice, Video, RTT) are established on ePDG and RTP date for Video / Audio are flowing on ePDG.
  3. In IKE_AUTH message, UE is already provided N1 Mode capability / information to NW about N1 Mode support .
  4. ePDG power reduced below to threshold and 5G RAN power increased.
  5. UE will send PDU Session Establishment Req for IMS DNN with requesttype as “existing PDU Session” and same PCO information (UE IP, port etc, ) to 5GS.
  6. NW will respond with PDUSessionAccept with same PCO information and 5QI as 5 and other QoS Flow information as required  for IMS PDU Session .
  7. IMS Re-Registration procedure happened on 5GS with PANI as “NR-FDD”.
  8. PDU Modification command from NW with 5QI =1 and other QoS Rules to established Voice completed.
  9. Voice Media RTP packets started on 5GS between UE and 5GS.
  10. IMS PDU ePDG tunnel release between UE-epDG.

1.5     ePDG –> 5GS –> EPS Fallback case

1.5.1     3GPP defined Call Flow as per 23.502

1.5.2     General Call Flow

  1. Please follow till step 7 of section 1.4.2
  2. UE may send Re-INVITE to establish and request Dedicated Voice QoS Flow on 5GS and set PANI as “3GPP-NR-FDD” .
  3. NW will decide to do Fallback Voice / video on EPS during Voice QoS Flow establishment.
  4. Voice Media RTP may be paused or go on default IMS PDU Session for some time until dedicated bearer / QoS flow not established.
  5. Please follow EPS Fallback procedure for Voice establishment on EPS.
  6. After Voice call ends on EPS , UE will do either Redirection or Reselection to move back to NR.

1.6     References

  • 24.501
  • 23.502
  • 23.402
  • 24.302
  • 23.501