bile changes cell. This message is acknowledged in return by the UE CONTEXT RELEASE COMPLETE response. 2.3.2. Bearer management Establishment and modification of the E-RAB dedicated bearer are initiated by the MME entity by sending the E-RAB SETUP/MODIFY REQUEST messages. The eNB entity responds positively or negatively by sending the E-RAB SETUP/MODIFY RESPONSE messages. Release of the dedicated bearer is initiated by the MME entity by sending the E-RAB RELEASE COMMAND message or by the eNB entity by sending an E-RAB RELEASE INDICATION message. Upon receiving this message, the MME entity begins the procedure of release of the dedicated bearer. 2.3.3. Mobility management The decision regarding handover based on the S1 interface is made by the source eNB entity. The phase of handover preparation begins with the sending of the HANDOVER REQUIRED message to the MME entity. When the reservation of resources by the target eNB entity is effective, the MME entity responds with the HANDOVER COMMAND message. VoLTE and ViLTE The MME entity directs the target eNB entity to reserve the radio resources using the HANDOVER REQUEST message. If the operation is successful, the target eNB entity responds with the HANDOVER REQUEST ACKNOWLEDGE message. This message can contain the elements for construction of a GTP-U tunnel to transfer the received data from the source eNB entity to the target eNB entity SO they can be transmitted to the mobile. If not, the target eNB entity responds with the HANDOVER FAILURE message. The source eNB entity has to transfer the value of the field sequence number (SN) of the packet data convergence protocol (PDCP) to the target eNB entity to preserve the continuity of the PDCP frame numbering. This operation is done by the transmission of the following messages: - eNB STATUS TRANSFER from the source eNB entity to the MME entity; - MME STATUS TRANSFER from the MME entity to the target eNB entity. This procedure applies only to bearers who use the acknowledgment mode (AM) of the RLC protocol, which is not