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"Homogeneous Support of IMS Voice Over PS Sessions" in S6a ULR

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Referring to the 3GPP 29.272 the "Homogeneous Support of IMS Voice Over PS Sessions " AVP is set to TRUE in the ULR means that the serving MME can provide the IMS voice support homogeneously in all TA assigned to it.
Does the MME insert this AVP in the ULR message only if the UE required voice service over IMS during the attach procedure.
In other word, if this AVP is present in the ULR, can we conclude that the UE support Voice call over IMS (VOLTE).

In the same context, if this AVP is absent ("Homogeneous Support of IMS Voice Over PS Sessions ")m can we conclude that either the MME does not support VOLTE at all or the UE does not support it (but only CSFB?

posted Apr 23, 2018 by Michael Semama

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1 Answer

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Homogeneous-Support-of-IMS-Voice-Over-PS-Sessions AVP comes into picture when a UE perform attach or tracking area update along with requesting IMS Voice service from network. As part of both the procedures, MME provides TAI list to UE and if each entry in TAI list supports IMS Voice through PS sessions then MME informs HSS by including Homogeneous-Support-of-IMS-Voice-Over-PS-Sessions AVP with value = "Supported" in ULR.
Another case, when none of entry in TAI list supports IMS Voice Over PS-Session, then MME informs HSS by including Homogeneous-Support-of-IMS-Voice-Over-PS-Sessions AVP with value = "Not Supported" in ULR.
Only two possible values have been defined for this AVP. In case, few entries in TAI list support IMS voice Over PS session and few don't then MME does not include the Homogeneous-Support-of-IMS-Voice-Over-PS-Sessions AVP itself in ULR.

If UE registers with network without asking IMS service, MME does not even consider this AVP. This AVP is considered only when UE attach to network for VoLTE service.

answer Apr 24, 2018 by Harshita
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Way of use Homogeneous Support of IMS Voice Over PS Sessions

There is an AVP 1493 (Homogeneous Support of IMS Voice Over PS Sessions) that might be sent or not within ULR message. According to 29.272, this AVP is presented in ULR if either all TA/RA of the MME homogeneously support VoLTE (AVP 1493=Supported) or non of RA/TA of the MME supports VoLTE (AVP 1493=Not_Supported). According to 29.272, the AVP 1493 value =“NOT SUPPORTED” may also indicate the lack of VoLTE roaming agreement between HPMN and VPMN. AVP 1493 is not presented in ULR if some of RA/TA support VoLTE and some not or MME has no info about VoLTE support.

I have several questions:
1. I assume MME will not include the AVP 1493 in ULR if UE has not requested voice over PS. Is this correct?
2. Whether MME inserts the AVP 1493=Not_Supported in ULR towards HPMN when there is no VoLTE roaming agreement irrespectively to UE request for voice over PS or only when UE has requested for voice over PS?
3. How MME decides about VoLTE support on TA/RA level(configuration, dynamic conditions etc.)?
4. What could be scenario when MME has no info about VoLTE support?
5. Whether the case when AVP 1493 is not presented in ULR due to reasons defined in 29.272 is common or it is just intermediate state while VoLTE deployment?

+1 vote
How to decide which attach type shall use when UE set “IMS PS Voice preferred and CS Voice as secondary”?

In TS 23.221 7.2a, there is a note describe that "Whether to perform combined EPS/IMSI attach/combined TA/LA Update or EPS attach/TAU for the case of "IMS PS Voice preferred" is to be evaluated in each release of the specifications.", but I can not find any information about that.

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