The X2AP protocol is used in X2 interface which is used to handle the UE mobility within E-UTRAN. X2 interface is inter eNB interface which provides the control & data plane for UEs performing inter eNB and intra-MME handovers. It helps in minimizing the handover related signaling going through the EPC and also by having Direct tunnels between the eNBs helps in buffering the downlink traffic until successful handover.
Protocol Architecture of X2 Interface
Function of X2AP Protocol
Mobility Management This function allows the eNB to move the responsibility of a certain UE to another eNB. Forwarding of user plane data, Status Transfer and UE Context Release function are parts of the mobility management.
Elementary Procedures
a) Handover Preparation
b) SN Status Transfer
c) UE Context Release
d) Handover Cancel
Load Management This function is used by eNBs to indicate resource status, overload and traffic load to each other.
Elementary Procedures
a) Load Indication
b) Resource Status Reporting Initiation
c) Resource Status Reporting
Reporting of General Error Situations This function allows reporting of general error situations, for which function specific error messages have not been defined.
Elementary Procedure
a) Error Indication
Resetting the X2 This function is used to reset the X2 interface.
Elementary Procedure
a) Reset
Setting up the X2 This function is used to exchange necessary data for the eNB for setup the X2 interface and implicitly perform an X2 Reset.
Elementary Procedure
a) X2 Setup
eNB Configuration Update This function allows updating of application level data needed for two eNBs to inter-operate correctly over the X2 interface.
Elementary Procedures
a) eNB Configuration Update
b) Cell Activation
Mobility Parameters Management This function allows the eNB to coordinate adaptation of mobility parameter settings with a peer eNB.
Elementary Procedure
a) Mobility Settings Change
Mobility Robustness Optimization This function allows reporting of information related to mobility failure events.
Elementary Procedures
a) Radio Link Failure Indication
b) Handover Report
Energy Saving This function allows decreasing energy consumption by enabling indication of cell activation/deactivation over the X2 interface.
Elementary Procedures
a) eNB Configuration Update
b) Cell Activation
