LTE: What does source MME execute "Relocation Cancel Procedure" ?

Question

In case of S1-handover with the MME relocation, source MME sends an information "Target Identification" to target MME.
What the target MME does by receiving TAC, when an enodeb can be identified uniquely by using combination of (PLMN + Macro/Home ENodeB) ?

Can someone tell me what the purpose behind to send TAC as part of Target Id IE ?

Answer 1

A relocation cancel request message should be sent from the source MME/SGSN to the target MME/SGSN on the S3/S10/S16 interface as part of the inter-RAT handover cancel procedure and on the S16 interface as part of the SRNS relocation cancel procedure. A relocation cancellation typically happens if :

--- a relocation times expires in the source MME/RNC- this means the relocation handover cannot be performed within an expected time.
--- the UE cannot handover on the radio interface either because of defects in the UE software (as seen with early releases of HSDPA data cards) or because of the wrong radio interface parameters assigned by the target system (e.g wrong BCCG ARFCN of the GSM cell).