7 Functions of X2AP

36.4233GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17TSX2 Application Protocol (X2AP)

The X2AP protocol provides the following functions:

– 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.

– Dual Connectivity. This function allows the eNB to request another eNB to provide radio resources for a certain UE while keeping responsibility for that UE.

– E-UTRA-NR Dual Connectivity. This function allows the eNB to request another en-gNB to provide radio resources for a certain UE while keeping responsibility for that UE.

– Load Management. This function is used by eNBs to indicate resource status, overload and traffic load to each other.

– Reporting of General Error Situations. This function allows reporting of general error situations, for which function specific error messages have not been defined.

– Resetting the X2. This function is used to reset the X2 interface.

– Setting up the X2. This function is used to exchange necessary data for the eNB or en-gNB for setup the X2 interface and implicitly perform an X2 Reset.

– eNB Configuration Update. This function allows updating of application level data needed for two eNBs to interoperate correctly over the X2 interface.

– Mobility Parameters Management. This function allows the eNB to coordinate adaptation of mobility parameter settings with a peer eNB.

– Mobility Robustness Optimisation. This function allows reporting of information related to mobility failure events.

– Energy Saving. This function allows decreasing energy consumption by enabling indication of cell activation/deactivation over the X2 interface.

– X2 Release. This function allows an eNB to be aware that the signalling connection to a peer eNB is unavailable.

– Message Transfer. This function allows indirect transport of X2AP messages to a peer eNB.

– Registration. This function allows registration of eNB in case indirect transport of X2AP messages is supported.

– Removing the X2. This function allows removing the signalling connection between two eNBs or between eNB and en-gNB in a controlled manner.

– Inter-eNB UE Context Retrieval. This function allows retrieval of a UE context in case of resumption or re-establishment of an RRC connection.

– Secondary RAT Data Usage Report. This function allows eNB to get the uplink and downlink data volumes for the Secondary RAT on a per E-RAB basis.

– E-UTRA – NR Spectrum Sharing. This function allows uplink and downlink spectrum sharing between a number of E – UTRA and a number of NR cells with overlapping coverage.

– EN-DC Configuration Transfer. This function supports en-gNB X2 TNL address discovery.

– EN-DC Load Management. This function is used by MeNB/en-gNB to indicate resource status, overload and traffic load to each other.

– UE Radio Capability ID Mapping.

The mapping between the above functions and X2 EPs is shown in the table below.

Table 7-1: Mapping between X2AP functions and X2AP EPs

Function	Elementary Procedure(s)
Mobility Management	a) Handover Preparation b) SN Status Transfer c) UE Context Release d) Handover Cancel e) Handover Success f) Conditional Handover Cancel
Dual Connectivity	a) SeNB Addition Preparation b) SeNB Reconfiguration Completion c) MeNB initiated SeNB Modification Preparation d) SeNB initiated SeNB Modification e) MeNB initiated SeNB Release f) SeNB initiated SeNB Release g) SeNB Counter Check
E-UTRA-NR Dual Connectivity	a) SgNB Addition Preparation b) SgNB Reconfiguration Completion c) MeNB initiated SgNB Modification Preparation d) SgNB initiated SgNB Modification e) SgNB change f) MeNB initiated SgNB Release g) SgNB initiated SgNB Release h) SgNB Counter Check i) RRC transfer j) EN-DC X2 Setup k) EN-DC Configuration Update l) EN-DC Cell Activation m) SgNB Activity Notification n) EN-DC X2 Removal o) gNB Status Indication p) EN-DC Resource Status Reporting Initiation q) EN-DC Resource Status Reporting r) F1-C Traffic Transfer
Load Management	a) Load Indication b) Resource Status Reporting Initiation c) Resource Status Reporting
Reporting of General Error Situations	Error Indication
Resetting the X2	Reset
Setting up the X2	X2 Setup
eNB Configuration Update	a) eNB Configuration Update b) Cell Activation
Mobility Parameters Management	Mobility Settings Change
Mobility Robustness Optimisation	a) Radio Link Failure Indication b) Handover Report
Energy Saving	a) eNB Configuration Update b) Cell Activation
X2 Release	X2 Release
Message Transfer Registration	X2AP Message Transfer
Removing the X2	X2 Removal
Inter-eNB UE Context Retrieval	a) Retrieve UE Context b) Data Forwarding Address Indication
Secondary RAT Data Usage Report	Secondary RAT Data Usage Report
E-UTRA – NR Spectrum Sharing	E-UTRA – NR Cell Resource Coordination
EN-DC Configuration Transfer	EN-DC Configuration Transfer
UE Radio Capability ID Mapping	UE Radio Capability ID Mapping