Inter-PLMN Reestablishment supports

Question

I want to know inter-plmn reestablishment supported if we have X2 links between 2 Enb having different MME and PLMN ID's.

If yes what all changes required to support. Will 3Gpp specify the same anywhere ?

Answer 1

I would like to share few points based on my knowledge, not sure all those will be fully correct.

• If eNodeBs with X2 link not connected to same MME, X2 handover can't be performed. However, both can exchange other information.

You have to go through S10 based handover and make sure first MME (source MME in case of handover) includes the list of equivalent PLMN to UE in Attach Accept. Equivalent PLMN list should contain PLMN of target MME.

In case handover fails at target cell, UE perform the cell re-selection and it may select the same target cell which was prepared for the UE as part of handover signaling. Since target cell is already prepared, re-establishment for UE at target cell will be successful.
UE will perform TAU with target MME because current TAI of target cell won't be matching with the TAI list it got from first MME.

Since current serving PLMN has been changed for UE and PLMN is used to derive Kasme at HSS, I think target MME should re-perform EPS AKA and Security Procedure. I am not sure, target MME NAS can continue with the same Kasme (inside MM Context) sent by source MME in Forward Relocation Request message, let others to respond on that part.

Comments

Thanks for the response but with the same conditions our handover is working fine. My concern is if there is no handover attempt due to target freq is less that threshold and due to poor coverage of source cell, radio link failed and UE scan other freq of other enodeb having plmn 2. Will it be allowed if we have x2 relation between them.

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Specification clearly says if RRC Connection Re-establishment Request comes to a prepared cell (source cell/target cell in case of handover), it would be handled and if goes to un-prepared cell, UE will be responded by RRC Connection Re-establishment Reject. And at the same time eNodeB belonging to un-prepared cell may send X2-AP RLF indication to eNodeB wherein UE was connected.

As far as, I worked in LTE handover procedure, I haven't seen source eNodeB sending multiple Handover Required messages (one for each target eNodeB) to prepare multiple target cells for UE handover.

Answer 2

Hello Vimal,

In 2 eNB if we have X2 connection but both of them not connected with same MME then X2 HO not possible..? I can see we have "Source MME GUMMEI" IE in Path Switch request message which is intended for Inter MME handover.

Please correct me if i am wrong.