erring of context, installation, and teardown of VM, etc. are out of the scope of the book, however general aspects related to NG-RAN virtualizing are explained in Section 6.2. The E1 (and F1) procedures required to support VM migration are illustrated in Figure 4.4.5. 1. The procedure is triggered by a gNB-CU-UP sending the E1-AP Bearer Context Modifi- cation Required message when VM migration requires changes in the transport network addresses used. The message may carry new uplink TNL address information of F1-U, and downlink TNL address information of S1-U or NG-U. The TNL information received by the gNB-CU-CP in step 1 needs to be forwarded to the the gNB-DU, which is done using the F1AP UE Context Modification Request message. 3. If the gNB-DU accepts the request, it replies with the UE Context Modification Response message. 4. If this is successful, the procedure is completed with the E1AP Bearer Context Modifi- cation Confirm message sent by the gNB-CU-CP to the gNB-CU-UP. Notes on Select IEs In this section we described in detail certain important Information Elements present in E1AP messages mentioned above. CHOICE system {E-UTRAN, NG-RAN}. In almost every single bearer context management message defined in (3GPP TS 38.463), there is a "system" IE of type choice, which can be set either to E-UTRAN or NG-RAN. The CHOICE type indicates that a gNB can either interface an EPC or a 5GC. The latter case is the so-called "Standalone" NR deployment, with a gNB connected directly to the 5GC. In the former case, gNB may indirectly connect to EPC by EN-DC (refer to Section 4.3) gNB-DU gNB-CU-CP gNB-CU-UP 1. Bearer Context Modification Required 2. UE Context Modification Request 3. UE Context Modification Response 4. Bearer Context Modification Confirm Figure 4.4.5 NG-RAN procedure to support VM migration. 4.4 Control-User Plane Separation operation. The EN-DC is one of the MR-DC options, in which a UE may be connected to one eNB that acts as an MN and one en-gNB that as acts as an SN, both connecting to the EPC vi