What is the difference between iRAT Handover and SRVCC for 4G & 5G?

Hi Experts.

In VoLTE Case, what is SRVCC for 4G & 5G?

  1. In EN-DC(NSA), When UE moves/Switched from LTE to 3G/2G or 5G?
  2. In SA, When UE moves/Switched from LTE to 5G?

Or: UE switched from 5G to 4G or 4G to 5G?

Is SRVCC when UE moves from Higher to Lower or Lower to Higher Technology?

What is the difference between iRAT Handover and SRVCC?

Admin note: this post was updated with image below.
iRAT Handover and SRVCC

Yes, but in case of call.

SRVCC is capable of performing this handover for both CS and PS domain or CS Domain only :point_down:

iRAT Handover and SRVCC

If UE is moving from VoLTE TO VoNR is it called SRVCC?

In case of VoLTE call, if coverage gets weak, we prefer to transfer calls from VoLTE to 3G/2G.

SRVCC performs / supports the PS handover or PS + CS Handover from VoLTE to lower RAT.

So VoLTE to VoNR, we cannot say SRVCC?

I don’t have an idea about it.

Maybe someone with 5G expertise can comment better.

No, it’s handover between 4G and 5G using N26 interface both for voice (PS) and data (PS).

CSFB / SRVCC is from 4G to legacy networks only.

EPS-FB is from 5G to LTE when vonr is not deployed in 5G or it’s congested/other reasons.

And recently an eSRVCC to UTRAN functionality in 5G is introduced in 3gpp Rel 16.

2 solutions exist here:

  1. EPS fallback
  2. 5G to 4G HO

Hmmm…what do you say about VoLTE to VoNR transition?

Or about mobility for data session from 4G to 5G?

I heated about the opposite way, not 4G to 5G transition…

I dunno if it is applied in any operator or defined by the 3GPP. This Need to be checked.

Chat GPT says: " …SRVCC comes into play if user moves out of the LTE coverage area and enters an area covered by NR"

That’s why I don’t believe in ChatGpt theory! :wink:

Don’t use chatgpt, please.

Sure, I don’t use. :wink: :point_down:

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While learning 3gpp functionalities or vendor specific implementation, at least.

Chatgpt may be good for surface level knowledge or things which are frequent on internet.

If it works out a state-machine with possibility from A to N only in reality…it will respond you that N to A is also possible which may not be the case in reality.

The best way to get answers is this forum here, with real experts. :slight_smile:

Yes. And you can also use sharetechnote, 3g4gforum, in addition many websites that are very useful while googling on any topic.


ShareTechNote is gem! Thanks @Jaeku_Ryu :clap:

Totally agree!

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Usually in 3gpp there is no smooth handover process defined from lower technology to higher technology.

It’s different and vendor tries different method to improvise.

These procedures are discussed in 3GPP TS 23.502 : Procedures for the 5G System (5GS) :point_down:

Very good info, but real use case would have no use because IMS is the same in VoLTE and VoNR.

After VoLTE call is finished, we can push handset to 5G.

If this transition is seemless, no interription in RTP , we can test this, if interription it is not worthy.