An operator was having difficulties with inter PLMN Handover. The handover was X2 with no change in MME, MME Group or SGW. The TAC number was same for both eNB’s but the TAI’s were different. After successful handoff TAU event was triggered. In the TAU response, the MME sometimes sent the source eNB PLMN as the serving PLMN and sometimes the MME sent the target PLMN as the serving PLMN.
Which behaviour is correct? Which spec outlines the correct behaviour?
Additional info: When the serving PLMN changed to the target eNB PLMN then subsequent handoffs succeeded otherwise they were rejected by the MME.
Explanation / 3dB consult recommendation
In X2-based HO the TAU process is launched after HO is completed, if the TA of the target eNB is not in the list of TAIs that the UE registered with the network. Moreover, in the HO process, if the serving PLMN changes during X2-based handover, the source eNodeB shall indicate to the target eNodeB (in the Handover Restriction List) the PLMN selected to be the new Serving PLMN. So, taking all that into account, once the HO was completed, the MME should always send the target PLMN as the serving PLMN.
For more information about X2-based HO and TAU triggers please refer to TS-23.401 sections 126.96.36.199.2 and 188.8.131.52 or contact 3dB consult
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