Some queries on VoNR

Hi Experts.

Some queries on VoNR:

  1. Does every NR SA UE support VoNR? (I don’t think so)
  2. If NR SA UE do not support VoNR, then is it like it wont even support 5QI-5 for IMS signalling?
    In LTE UE, if it does not support VoLTE, then it wont establish QCI-5, same in NR SA?
    And last an important question:
  3. If UE so not support VoNR/5QI-5-> How EPS FB is triggered?
    Because EPS FB is triggered after gNB gets request from AMF about 5QI-1 establishment, which wont be there in basense of 5QI-5.
    Is any gap in my understanding?

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  1. No, from UE capability we can see whether VoNR support or not.
  2. If NR SA UE does not support VONR, but it will support VOLTE, hence during registration time UE will send 5QI -5 PDU session establishment for IMS signalling.which is mapped with EPS bearer I’d. Using that ue will perform voicefb.

Voicefb also not support (volte not support) in that case ue will not send pdu session establishment request for IMS signalling


Core will have mapping of VONR enabled or not.

Means VONR profiled or not and based on that SRVCC will be triggered from Core end and call be routed to 4G VoLTE.

Or say CSFB, to be correct

  1. Not necessarily.
  2. It would, for EPSFB.
    LTE: It has CSFB as backup, VoNR’s backup is EPSFB which needs 5QI.
  3. If No EPSFB support, UE can stay on 5G only as Data Centric or needs to move to other RAT

My main question is 3) :point_up_2:
What if 5QI-5 is not supported by UE. How EPS FB will be triggered?

So if UE do not support 5QI-5, then EPSFB is not possible, right?

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5Qi-5 is not supported by UE means IMS is not supporting.

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During the Initial Registration Request (in that Usage setting will voice centric or Data centric) will be conveyed & GMM capability S1 mode is true / false.

So, In S1 mode capability indicates whether UE support EPS Fallback or not.
So it may not be set to false - when the voice call is expected purely through VoNR.

Even if in case that UE was in 5G and now some abrupt things happened and VoNR is not there.

In such case PDU Session Establishment Request will send target CSCF etc details or in worst scenario PDU Session Establishment Accept will be rejected and SIP registration will be initiated to check New RAT.


Yes, right.


UE will not get 5G voice services.


So 5QI-5 support and QCI-5 support are same you mean?

For NR and LTE respectively?

Is there any case where UE support QCI-5 in LTE but do not support 5QI-5 in NR?

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I guess with NSA.
For SA I guess no.
I mean not possible.


Thats fine, we are only talking about SA here.

So conclusion is every NR SA UE support 5QI-5. It answers my all questions.

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In case of EPS FB, the UE move from 5G to 4G during the signalling stage only i.e. QCI1 is established after the UE move from 5G to 4G.

EPS FB can happen by HO/ Redirection/ Blind HO or Redirection. If there is is N26 between AMF and MME, no Attach is required as the UE context can retrieved from the MME in the Forward Relocation Response message.