Unknow TAC during VoLTE call

Guys, I have a question…
During VoLTE call, if I go from TAC to new TAC, but the neighbor TAC is not identified to the source TAC…
What will happen to the VoLTE call?
Will it drop or will continue with srvcc or will continue with VoLTE call normally?
Thanks in advance :blush:

I think TAC update is not required , And In case the TAC is in same mme normal VOLTE call will continue.

Not sure what you mean by “TAC is not identified to the source TAC”, maybe you could elaborate a little bit better about it, but…

  1. Every time you change TACs which are NOT part of TAI List, you must perform tracking area update.
  2. Even if the new TAC has IMSVoPS=0, the call should proceed and once the UE ends the call (human hits end), the UE should perform IMS deregistration.
    2.1) It was very common to see UE with non-3GPP behavior sending SIP BYE under these circumstances.
    2.2) Depending on the RAN configuration, you may have a partial handover (PS GBR goes on, QCI1 is dropped) resulting in a abnormal release, thus, call drop.
  3. SRVCC is out of the question.

What if the MMEs are different?
Is that makes any difference?
I mean, different TACs & also different MMEs.
What do you think?

By an unknown you mean it is part of TAL or not?
Unknown TAC?

Will remain the same.

TAC update after call end, or you mean not at all?

Each MME has its own TACs.

But there must be TAL (Tracking Area List).

Yes. You mean TAL should contains all TACs even if the TACs belongs to different MMEs.

Yes, we are using likewise and for that.

This is more logic.

And MMEs are in pool.
Also there is one thing:
Even we consider altogether a New Random TAC.Then also your call will continue beacuse TAC update failed proces is a repetitive process depending on T3411 and T3420.
And during this process your call will continue.

Both timers in CORE.

RAN timers at UE side… periodic TAU update timers.
Also if suppose during all these your VoLTE coverage ends, then based on your preference during your attach you will have SRVCC or CSfallback.

Appreciate your support guys, many thanks :handshake: :blush:.