LTE: On what basis source MME forwards S10 handover message to target MME ?

Question

As part of S1-AP procedure with MME relocation, Handover Required message sent to source MME. Target eNodeBId is present within the handover required. How source MME figure out target cell is being served by XYZ MME and sends "Forward relocation Request" message to XYZ MME.

Please provide reference also if it is there ?

Answer 1

There may be following two scenarios:

1. Target Cell is served by source MME
   MME shall send handover request message to target eNodeB based on the global enodeb id which was present in the handover required message.

2. Target cell is not served by source MME.
   Based on the tracking area code, source MME sends forward relocation request to target MME over the S10 interface.

Answer 2

S1-AP handover required message carries "Target ID". This IE contains sub-elements like target "global enodeb Id" and "tracking area code". When source MME receives handover required message for an UE, it check whether it serves target eNodeB or not.

If MME serves target eNodeB then it shall prepare and send handover request message to target eNodeB. If it doesn't then based on tracking area code of target cell, source MME prepares and sends Forward Relocation Request to target MME.