 of the load of the cells that they serve; - X2 interface management: this function is used for the activation of the X2 interface, the reconfiguration and re-initialization of the X2 interface. 46 VoLTE and ViLTE Table 2.5 summarizes the X2-AP messages exchanged for mobility management, load management and X2 interface management. Function Request Response HANDOVER REQUEST ACKNOWLEDGE or HANDOVER REQUEST HANDOVER PREPARATION FAILURE Mobility management SN STATUS TRANSFER UE CONTEXT RELEASE HANDOVER CANCEL Function Request Response LOAD INFORMATION RESOURCE STATUS RESOURCE STATUS RESPONSE or Load management REQUEST RESOURCE STATUS FAILURE RESOURCE STATUS UPDATE Function Request Response X2 SETUP RESPONSE or X2 SETUP REQUEST X2 SETUP FAILURE ENB CONFIGURATION UPDATE X2 interface management ENB CONFIGURATION ACKNOWLEDGE or UPDATE ENB CONFIGURATION UPDATE FAILURE RESET REQUEST RESET RESPONSE Table 2.5. X2-AP messages 2.4.1. Mobility management The function of mobility management contains the following elementary procedures: - handover preparation; - transfer of the state of the SN field of the PDCP protocol; Signaling Protocols - deactivation of the context of the mobile; - handover cancellation. The procedure of handover preparation is initiated by the source eNB entity by transmission of the HANDOVER REQUEST message to the target eNB entity. The target eNB reserves the resources and responds with the HANDOVER REQUEST ACKNOWLEDGE message. This message contains the value of the tunnel endpoint identifier (TEID) used by the GTP-U protocol for the traffic transferred by the source eNB entity to the target eNB entity. If the resources are unavailable, the target eNB entity sends back the HANDOVER PREPARATION FAILURE message. The procedure of transfer of the state of the SN field consists of transferring to the eNB entity the value of the SN of the PDCP protocol with the SN STATUS TRANSFER message. At the source eNB entity, this message stops the attribution of the SN of the PDCP protocol for the downstream direction. T