LTE introduces inter eNodeB interface i.e X2 in order to decongest the EPC from handover related signaling. The decongestion is significant as most of the handovers occur between enodeBs who are connected to the same MME/SGWs. X2 is comprised of controlplane(X2-AP) and data plane(X2-U).
It is also useful in,
-Direct forwarding in order to minimize data loss during the S1 handover process
-Configuration info and updates such as Served cell information, Number of Antenna Ports
-Load info such as Inter Cell Interference Coordination (ICIC)
etc
When the user moves to the target-enb, it sends Path switch request to the MME to indicate the UEs movement. The MME sends Modify bearer req to SGW which sends End marker request to the source-enb making sure that the dataplane switch has occurred and the source-enb can release the radio resources(source-enb sends Resource release to the UE thus completing the handover.
