EPS Fallback and eNB action

Can the 4G BS/eNB understand that the IMS voice bearer (QCI-1) is due to the EPS Fallback?


Not sure if I understood the question - do you mean fall back from 5G?

Because if fallback is happening in 4G, call will go to 3G.

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Is it the case of EPSFB with release with redirection or with IRAT HO?

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Referring to 36.413:

If the IMS voice EPS fallback from 5G IE is included in the Source eNB to Target eNB Transparent Container IE within the HANDOVER REQUEST message, the target eNB shall, if supported, store the information in the UE context and consider that the UE is handed over from NG-RAN to E-UTRAN due to an IMS voice fallback.

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Does eNB understand that IMS is due to EPS?

I would say Yes since AMF or MME will inform eNB to set resource for incoming voice call coming from 5G.

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But here it only transfer the context related to QCI-5 over N26 interface, not related to QCI-1.

This is the case of EPSFB with IRAT Handover from 5G to 4G right?

Because in case of EPSFB with RwR servig gNB will reject PDUsessionmodify request coming from AMF for 5QI-1, UE will trigger TAU request in LTE where it contains context related to eps bearer associated with QCI-5 and Then MME will send context request to AMF over N26.

There is no information transferred for QCI-1 over N26…

I think eNB will not understand whether QCI-1 is being created later is for EPS fallback or it is for VoLTE.

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According to 36.413, For EPSFB with IRAT Ho, it’s clear that eNB knows that IMS voice is due to EPS Fallback.

I’m looking for RwR case.

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Does RwR imply the target system knows or aware of incoming fallback?

Looking at this call flow, eNB is already informed of incoming request. UE is released from 5G with target 4G cell information.

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Yes, in RwR case it looks like eNB won’t know whether QCI-1 is being created is due EPSFB or for VoLTE.

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Right, according to 36.413, in case of EPS Fallback with Ho, eNB is informed.

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Do you mean to say Release without Redirect?

In Release with Redirect, it seem eNB is already made aware, and hence cell configuration is sent to UE in RRC Release message.

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In Release with Redirect (RwR) it gets only information related to Eutra carrier where UE needs to select a best cell to make RRC connections and then to send TAU request.

Here till this time eNB is not aware about any dedicated bearer information so that it can establish DRB.

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3GPP TS 38.331

  1. If the RRCRelease message includes redirectedCarrierInfo indicating redirection to eutra:
  2. If voiceFallbackIndication is included:
  3. Consider the RRC connection release was for EPS fallback for IMS voice (see TS 23.502);
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Please add subclause also.

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Yes, i checked it.

But this message is transmitted from gNB to UE, later when UE come in LTE…

eNB is still not aware about any dedicated bearer or any indication from core network.

After receiving TAU request message, MME will ask for context to AMF for that UE.

Since there was no dedicated bearer in 5G, AMF won’t give any context for dedicated bearer-QCI-1.

Dedicated bearer will establish post TAU accept message transfer.

Here eNB won’t know whether drb need to add for dedicated bearer is for EPSFB or VoLTE.

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I also came to the same conclusion that for Release with Redirect(RwR), eNB will not be aware of EPSFB in case of QCI-1.I created this question for more clarification.
We can also wait for the response of other experts.