The source eNB issues a HANDOVER REQUEST message to the target eNB passing the necessary information to prepare the HO on target side (UE X2 signaling context reference at source eNB, UE S1 EPC signaling context reference, target cell ID, KeNB*, RRC context including the C-RNTI of the UE in the source eNB, ASconfiguration, E-RAB context and physical layer ID of the source cell + MAC for possible RLF recovery). 
UE X2 / UE S1 signaling references enable the target eNB to address the source eNB and the EPC. The E-RAB context includes necessary RNL and TNL addressing information, and QoS profiles of the E-RABs.
Admission Control may be performed by the target eNB dependent on the received E-RAB QoS information to increase the likelihood of a successful HO, if the resources can be granted by the target eNB. The target eNB configures the required resources according to the received E-RAB QoS information and reserves a C-RNTI and optionally an RACH preamble.

The target eNB prepares HO with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to the source eNB.

The HANDOVER REQUEST ACKNOWLEDGE message includes a transparent container to be sent to the UE as an RRC message to perform the handover. The container:
  • includes a new C-RNTI, target eNB security algorithm identifiers for the selected security algorithms,
  • may include a dedicated RACH preamble, and possibly some other parameters i.e. access parameters, SIBs, etc.