Target MME selection in S1-handover?

Question

In case of S1-handover the source MME gets the TA and the target enodeB id in the Handover required message. How is the target-MME decided from this?

Answer 1

UE mobility is managed by MME in term of tracking area's. An MME manages a large geographical location in term of TAs (i.e. tracking area). So when source MME receives handover required message for an UE, which is moving towards a target cell that is belong to a different tracking area which is not managed by source MME. Then source mme sends S10 handover message towards the target MME by using the "Forward Relocation Request" message.

In short, Tracking area is a key for target MME selection. Source MME may support static and dynamic configurable. It is up to the vendor how they want to manager their network.

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Comments

The source enodeB passes all the information to the source MME in the Handover Required Message. Info like absence of X2 interface, no direct forwarding available & so the data need to be tunneled via SGW, bearers for data forwarding and also TAI (Tracking Area Identity). The source MME checks whether it is capable of serving this particular TAI or not. If yes, Forward Relocation procedure is skipped else a suitable target MME is selected using the TAI.