LTE eNodeBs can directly communicate with other eNodeBs on the X2 interface. The X2 interface is used to perform a handover between eNodeB.
The messaging in X2 handover is detailed in LTE X2 Handover Presentation. A few excerpts from the presentation as shown below.
X2 Handover Sequence Diagram
The X2 handover flow is shown in the following sequence diagram:
Figure 1 X2 Handover Sequence Diagram
The handover procedure is triggered by the X2AP Handover Request message. The RABs to be handover over are sent from the source eNodeB to the target eNodeB.
Figure 2X2AP Handover Request
The target eNodeB then admits the user and responds with X2AP Handover Request Acknowledge message. This message contains a transparent container that carries the Handover Command message that needs to be sent to the UE.
Figure 3 X2AP Handover Request Acknowledge
The source eNodeB sends the handover command to the UE. It then sends sequence number information to the target eNodeB.
The target eNodeB then requests the MME to switch the path from the source eNodeB to target eNodeB.
Figure 4 S1AP Path Switch Request
This was an overview of the messaging involved in the X2 handover. For details refer to the LTE X2 Handover Presentation.