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Why S1 handover prepapration fails with cause unknown target?

+3 votes

This is observed in intra MME handover

MME sends handover preparation failure to source eNB with cause unknown target ID
Both eNBs are under same TAC

What could be reason for this failure with this cause?

posted Feb 17, 2015 by Alkesh Mehta

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2 Answers

+1 vote

As you have mentioned that both the eNodeBs are under same TAC but it is not necessary that both the eNodeBs have connection with same MME. Please verify , Is s1 setup done for both the eNodeBs using wireshark or tcpdump ? I am suspecting that target eNodeB is not registered with MME.

answer Feb 17, 2015 by Harshita
Thank you.

Target nodeB is registered with MME.

Have tried handover in reverse order where target eNB is source and source is target eNB. in this case also  same failure - unknown target ID

In  handover required message - target eNB is detected correctly but failure happens in preparation phase
0 votes

Are you adding neighbor information in NRT using ANR or manually. If you are adding neighbor information in NRT manually then i think need to check the neighbor cell id in the NRT. It may be possible that neighbor cell id in NRT is different from the actual neighbor cell id. Due to this mismatch MME can not find the target eNB so that MME can send MME preparation failure to source eNB.

answer Feb 18, 2015 by Veer Pal Singh Yadav
yes neighbor was added manually.

issue resolved. TAC conflict was causing the issue
Hi Alkesh, I am facing same issue but in my case "LTE intra frequency HO execution failure rate" this KPi is very Poor, Neighbor are created using ANR.
NOKIA RL60 . Could you please suggest some solution. 40-50 Cells are having >50 Failure. Thanks
I think you need to synchronise the a2/a3 event threshold so that HO should trigger at correct time to the target.
@Alkesh - can you tell me what do you mean by TAC conflict...Is it that the TAC reported from the enodeb to the mme in the S1setup request not matching in the mme...can you provide more information on this..
This was the case where two MMEs were using the same TAC

So when MME get HO prep message it was not able to route correctly to target eNB

There should be unique TAC
If MMEs are in pooling then there might be possiblilities of same TAC in both MME , if we have two MMEs in NW . Then what need to do
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+1 vote

Newly integrated 3rd carrier is facing IRAT handover failure with reason "unknown-targetID" and "Failure In Target EPC/eNB Or Target System". Other carriers of same enodeB are doing successful handover with the same WCDMA cell. Sometimes for the affected cell, handover is successful.

Thanks in advance,

+1 vote

I found two MME UE S1-AP IDs i.e. MME UE S1AP ID (Mandatory) and MME UE S1AP ID 2 (optional).
I got why mandatory one is present but what's use of optional one.

Can someone please explain it ?

+2 votes

My friend was talking about ANR feature of LTE. By using, ANR feature neighboring eNodeBs update their NRT table. One of my friend was saying that it is not always X2 handover is allowed in X2 link. But he could not tell about cause/reason. I know when X2 handover can not perform, S1-handover shall be executed.