dary node. For a color version of this figure, see www.iste.co.uk/launay/5g.zip 1) 2) The procedure for changing the en-gNB entity starts with the X2-AP SgNB Addition Request and SgNB Addition Request Acknowledge message exchanges between the eNB and the target en-gNB entities. 3) 4) If the resource allocation from the target en-gNB entity is successful, the eNB entity triggers the resource release for the source en-gNB entity, with the exchange of the X2-AP SgNB Release Request and SgNB Release Request Acknowledge messages. 5) 6) The eNB entity triggers the new configuration at the mobile level in the RRC ConnectionReconfiguration message, containing the NR RRC configuration message generated by the target en-gNB entity. The mobile applies the new configuration and sends the RRC ConnectionReconfigurationComplete message. 7) The eNB entity informs the target en-gNB entity via the X2-AP SgNB Reconfiguration Complete message that the reconfiguration procedure is successful. The mobile synchronizes on the PCell of the target en-gNB entity and performs the random access procedure without contention. NG-RAN and 5G-NR 8) 9) If the bearer was configured in the acknowledgment mode, the source en-gNB entity sends the PDCP sequence number in the X2-AP SN Status Transfer message, that the eNB entity transfers to the target en-gNB entity. The eNB entity requests the MME entity to establish the S1-U bearer between the target en-gNB entities and the SGW. The MME entity acknowledges the request and the SGW entity notifies the target en-gNB entity about the establishment of the downlink tunnel. 10) Upon receipt of the X2-AP UE Context Release message, the source en-gNB entity releases the resources allocated to the mobile. 3.1.4. Removing a secondary node The procedure for removing a secondary node is either initialized by the MN (eNB entity, Figure 3.6) or by the SN (en-gNB entity). It is used to remove the mobile context at the SN (en-gNB entity). en-gNB X2-AP SgNB Release Request X2-AP SgNB Release Request ACK RRC Connection 