UE keeps going back to Idle until TAU is initiated

Hi All.

I have the following issue in LTE: UE initiates service request, then RRC Connection Request, then RRC Connection Setup, then RRC Connection Setup Complete and then it goes back to idle.

This is repeated every 5 seconds.
For tens of time.
Only solved when a TAU is initiated.

What could it be?

Most probably Core issue causing disconnection.

Since UE finishes RRC phase and waits to receive the RAB assignment (RRC RECONFIG) but it did not come.

You need to know what happens above the eNB after the RRC succeeds.

Initial context setup might be failing.

Cause will be there in S1 logs, if you have.

Issue with only this UE or many UEs in the cell?

No idea, I have just the drive test log.

My concern is why UE is initiating service request just every 5 seconds?

Why not every 2 seconds?

Where is this 5 seconds coming from, what timer is it?

A Core one must be.

Anyone with more hints?

I am thinking it could be that MME is congested, time to time, and don’t accept initial UE message so RAB is not initiated and UE keeps trying for service.

Looks strange situation.

Less possibility of MME congestion related to this issue, as you mentioned that every-time issue gets resolved when TAU is initiated.
This cant be coincidence.

Are you able to check anything in intial context setp request?
Or is this message missing?

No mate, I have just UE logs, no access to RAN traces at all.


This is what comes in my mind for now.

Can we think of something related to TAU update procedures…

Information not stored and mismatch case is observed every-time and it gets resolved with TAU update…

What I am doing now I am manually grabbing all TAC with issue and the MME code (from old GUTI) at TAU request.

Maybe something comes out of this just some TACs having this pattern, who knows.

This might help to get better idea.