Possible reasons of high TAU rejection due with implicit detach cause

RAN
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1

Hello Experts.

Can you please explain what could be the possible reasons of high TAU rejection due with implicit detach cause ?

2

One could be UE not doing periodic TAU with the nw configured t3412.

3

Is there any possibility such scenario can happen more in the area of bad coverage and UE moving a lot between different RAT?

4
  1. Please check the exact cause of the rejection?
  2. Can you please to cross verify Validity of the both Tracking area, PLMN’s and integrity keys at MME side and counters are correctly added or not etc.?
5

Exact release cause for TAU reject is 10 (implicit detach) which means UE reachable timer has expired.

Will check more.

6

The reason MME couldn’t get the context of the UE.

So did the UE reselect to new MME (previously before reselection it could have been on W or LTE).

7

It seems for the moment it was on GSM.

Just before TAU request, there seems to have routing area update not successful, then UE send TAU update request with native GUTI which is outrightly rejected with release cause as mentioned above.

8

Yes agree with @Sean_sean.

Either MME / AMF / other legacy couldn’t get the context of the UE.

UE is in de-registered state with the Core N/W.

9

Yes.

But trying to correlate this part in trace…

Will check further.

10

Check may be issue with co-exist of RAI in both SGSN.

RAI = LAC + RAC

This RAI is exist in both SGSN which connect to different BSC, thus may give Routing area problem.

If issue casue bcz of RAI then change the RAC in one of the SGSN.

Maybe it’s Core issue and above method may will definitely will help you.

Please let us know the update after checking please.

11

Sure will share the findings.

Thanks for the suggestion.