UE detach from EPC immediately after it attach to EPC successfully?

Question

When doing IOT test, I encounter an issue. One sony mobile phone initiate detach request just 2 seconds after it attach to EPC successfully.

The attach request is a combined EPS/IMSI attach and the attach accept result is EPS only with EMM cause 'CS domain not available'.

Some one suggest it maybe due to the mismatch of the parameter PTI (procedure translation identifier) in the attach request and attach accept. I checked it and find PTI are the same in the two message.

I also tried another old type sony mobile phone, this old phone does not have such strange behavior.

After compare the attach request and attach accept messages of these two mobile phone, I did not find anything wired.

So does any one knows what is the reason behind?

Comments

Can you please tell the "Detach Type"?

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Its IMSI detach (normal only)

Answer 1

As per 24.301 section 5.1.3.2.4.9 EMM-REGISTERED.IMSI-DETACH-INITIATED

The UE performs a combined detach procedure for non-EPS services only (detach type "IMSI detach"). This substate is entered if the UE is attached for EPS and non-EPS services and wants to detach for non-EPS services only. User data and signalling information may be sent and received.

It look like after as UE has initiated combined attach but receive attach accept for EPS only, it is trying do initiate detach for non-EPS services.

Please refer 24.301 section 5.5.1.3.4.3 Combined attach successful for EPS services only for more detail.

Comments

But why it is not happening with old sony phone ????