Question

What MME shall perform when receiving S1 path switch request (changing tac and lac for the UE) after a NAS tau request. Shall MME respond to UE with TAU reject (which cause code ) or not ?

Comments

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Answer 1

This is the case of handover procedure followed by TAU. Tracking are update procedure is always triggered by UE when TAU list maintained at UE doesn't match with tracking area of current cell. Now coming to handover procedure. LTE supports two types of handovers X2 based handover and S1 based handover.

In case of S1 based handover, MME is already aware about target eNodeB of UE. MME takes decision on SGW change. MME chooses suitable SGW if required based on the target eNodeB. So there is no requirement to execute 3GPP PATH SWITCH procedure.

But in case of X2 handover, MME doesn't aware about what's happening at eNodeB for an UE in terms of handover. Once handover successfully gets executed, target eNodeB sends PATH SWITCH REQUEST message to MME for that UE. PATH SWITCH REQUEST message contains IEs (ECGI, TAI etc) which helps MME to select appropriate SGW. If there is no change is SGW then MME executes Modify Bearer Procedure else MME creates bearer with new SGW for UE by executing the Create Session Procedure and responds to eNodeB with PATH SWITCH REQUEST ACKNOWLEDGE message.

UE, after sending the RRC Connection Reconfiguration Complete message to target eNodeB, it can perform Tracking Area Update Procedure if the target eNodeB belongs to Tracking are area which is not present in the TAI list. MME can reject TAU request sent by UE due to various reasons. One of reason might be Tracking Area not allowed.

Comments

I meant the following scenario :
   1) MME receives a NAS-TAU request from UE
   2) MME sends SGS-Loc-Update-Request to MSS for the new LAC linked to the new TAC
   3) Then before MME receives any answer from MSS, MME receives a X2 HO Path Switch Request for this UE from target eNB linked to a TAC different from the previous one (that has triggered a Loc-Update). And this TAC belongs to a different LAC.
   4) In this case, what the 3GPP recommended behaviour concerning the current in progress TAU procedure ? Does MME has to send immediately a TAU Reject to UE or to do anything on S1 interface ?

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I went through 3GPP TS 24.301 and found following statement.
"If a cell change into a new tracking area occurs before the tracking area updating procedure is completed, the tracking area updating procedure shall be aborted and re-initiated immediately. The UE shall set the EPS update status to EU2 NOT UPDATED".

Now I concluded from the above statement is "MME shall abort the on going TAU procedure".

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Hi Harshita ,

In the mentioned query which TAU procedure is rejected ? since if I am able to understand question correctly , there are two TAU requests happening - 1st for the NAS TAU and then followed by a TAU request due to Handover (X2 HO Path switch request from a eNB that belongs to a different TAC i.e. 3rd TAC in this scenario ) ?
And what is finally going to happen overall in this scenario ?

Thanks & Regards