Normal Release vs Unspecified CAUSE code in NAS/LTE?

Question

Following is the list of NAS cause codes

• Normal Release
• Authentication Failure
• Detach
• Unspecified
• CSG Subscription Expiry

Can someone provide some reference to when exactly a "Normal Release" cause is used vs a "Unspecified" cause.

Answer 1

Those NAS cause codes are provided by the S1-AP protocol, that carries among others, the NAS protocol. between the UE and the MME.

"Normal Release" in this case, indicates that the release of the UE-associated logical S1 connection is due to a normal NAS EMM or EMS successful completion.

"Unspecified" means that a particular event on the NAS protocol triggered the S1AP protocol to stop the UE -associated S1 connection from reasons NOT in the following group A:

{ Normal Release after a NAS successful completion, NAS Authentication Failure, NAS detach, NAS CSG Subscription Expiry}

Therefore, we conclude that a NAS abnormal event triggered S1AP with a cause other than one of those in group A. That cause is called Unspecified