5G: Voice over NR (VoNR) and Inter-RAT Handover from NR SA to LTE

:heavy_check_mark: When a VoNR capable SA UE registers with the 5G core and is notified via the registration accept message that IMS voice over Packet Switch (PS) Session is supported by the 5G network, it can therefor request an IMS PDU Session including a QoS Flow with default QoS rule and then if the 5G RAN supports the VoNR, a dedicated QoS Flow within that PDU session is established due to mo-call /mt-call.

:heavy_check_mark: When a 5G device with voice traffic(VoNR) is moved to the NR poor coverage (getting the Event A2 report by the gNB), there can be inter system handover from NR to LTE.

:heavy_check_mark: To support VoNR to VoLTE Inter System Handover for a single-registration mode UE, N26 interface between the AMF (Access and Mobility Management Function) within the 5G core and he MME (Mobility Management Entity) within the 4G core is vital due to voice interruption time.

:heavy_check_mark: Here I try to depict the IRAT handover procedures from NR to LTE for a single- registration mode UE that makes a VoNR call and then moves to the NR poor coverage.

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VoNR is still not matured , getting lot of issues

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Hello @RahimNavaei it’s been awhile…
I only saw ENDC 5G VoNR and when Serving gNB / Serving eNB and CA eNBs need HO. And when A5 HO event is triggered in case of low RSRP during ENDC VoNR call then PCell in LTE HO as well as with 5G gNB. But, looking at your article, 5G SA RAN is connected to AMF and MME is connected through N26 interface…IP re-allocation will be done and IMS will detach in this scenario. Thus, Call will likely drop.

So Please if this call flow is in 3GPP Spec, Can you please refer us into the spec numbering?

Don’t you think if you are referring that there will be smooth handover between SA to LTE during VoNR call then wouldn’t be interfaces linking between eNB and SA gNB?? like X2 interface in ENDC or CA scenarios?

I honestly haven’t spent much time looking for this but I got somehow these questions just looking at your slides…
I might be missing out on some new technical stuff but can you elaborate on this little bit more?


Jung Hyun Jeong


You can refer to 3GPP 23. 502 (Rel.17,16,)

4.11.1 N26 based Interworking Procedures General
N26 interface is used to provide seamless session continuity for single registration mode UE.
Interworking between EPS and 5GS is supported with IP address preservation by assuming SSC mode 1.

Based on your feedback on VoNR, how are most carriers supporting voice? Are most calls falling back to 4G? I am very interested in the real world deployments, since I am not involved in network deployment and support. Any info you would be willing to share would be greatly appreciated!