The gNB-DU sends an F1 SETUP REQUEST message to the gNB-CU including a list of cells that are configured and ready to be activated. For each cell supporting NPN the gNB-DU includes NPN specific information.
The gNB-CU sends an F1 SETUP RESPONSE message to the gNB-DU that optionally includes a list of cells to be activated. The cells in the list of cells to be activated in F1 SETUP RESPONSE message become active, while the cells not in the list are inactive. The cells that are active are Out-of-Service until the gNB-DU indicates that they are In-Service . The gNB-DU will initiate the gNB-DU Configuration Update procedure towards the gNB-CU and includes the cell(s) that are In-Service and/or the cell(s) that are Out-Of-Service. The gNB-DU may also indicate cell(s) to be deleted, in which case the gNB-CU removes the corresponding cell(s) information.
The gNB-CU may send a GNB CU CONFIGURATION UPDATE message to the gNB-DU that optionally includes a list of cells to be activated, e.g., in case that these cells were not activated using the F1 SETUP RESPONSE message.
The gNB-DU replies with a GNB CU CONFIGURATION UPDATE ACKNOWLEDGE message that optionally includes a list of cells that failed to be activated. The gNB-CU regards all Active cells as Out-Of-Service until the gNB-DU indicates that they are In-Service.
The gNB-CU may initiate either the Xn Setup towards a neighbour NG-RAN node or the EN-DC X2 Setup procedure towards a neighbour eNB.
Over the F1 interface between a gNB-CU and a gNB-DU pair, the following two Cell States are possible:
Inactive: the cell is known by both the gNB-DU and the gNB-CU. The cell shall not serve UEs;
Active: the cell is known by both the gNB-DU and the gNB-CU. The cell should try to provide services to the UEs.
The gNB-CU decides whether the Cell State should be "Inactive" or "Active". The gNB-CU can request the gNB-DU to change the Cell State using the F1 SETUP RESPONSE, theGNB DU CONFIGURATION UPDATE ACKNOWLEDGE, or the GNB CU CONFIGURATION UPDATE messages.
The gNB-DU reports to the gNB-CU the Service Status. The Service Status is the state of the radio transmission over the air. The Service Status is reported by the gNB-DU for cells for which the Cell State is "Active". The following Service Status are defined:
In-Service: the cell is operational and able to serve UEs.
Out-Of-Service: the cell is not operational, and it is not able to serve UEs. The gNB-DU is trying to make the cell operational.
The gNB-DU reports the Service Status using the GNB DU CONFIGURATION UPDATE message.
For UE Inactive to UE Active transitions, excluding transitions due to signalling exchange only, the gNB-CU allocates gNB-CU UE F1AP ID and sends UE CONTEXT SETUP REQUEST message to gNB-DU, which may include SRB ID(s) and DRB ID(s) to be setup, CellGroupConfig stored in gNB-CU or retrieved from the old NG-RAN node may also be included. In case of NG-RAN sharing, the gNB-CU includes the serving PLMN ID (in case of SNPNs the serving NID).
The gNB-CU generates RRCResume/RRCSetup/RRCReject/RRCRelease message or receives RRCRelease message from the old NG-RAN node towards the UE. The RRC message is encapsulated in DL RRC MESSAGE TRANSFER message together with SRB ID.
The gNB-DU forwards RRC message to the UE either over SRB0 or SRB1 as indicated by the SRB ID.
If step 5 and 6 are not performed, the gNB-DU deduces the SRB on which to deliver the RRC message in step 7 from the SRB ID, i.e. SRB ID "0" corresponds to SRB0, SRB ID "1" corresponds to SRB1.
The gNB-DU includes the RRC message and, if the UE is admitted, the corresponding low layer configuration for the UE in the INITIAL UL RRC MESSAGE TRANSFER message and transfers to the gNB-CU. The INITIAL UL RRC MESSAGE TRANSFER message includes the new C-RNTI.
The gNB-CU includes an RRCReestablishment message and transfers to the gNB-DU. If the UE requests to re-establish RRC connection in the last serving gNB-DU, the DL RRC MESSAGE TRANSFER message shall include old gNB-DU UE F1AP ID.
The gNB-CU triggers an UE Context Modification procedure by sending UE CONTEXT MODIFICIATION REQUEST message, which may include DRBs to be modified and released list. The gNB-DU responses with the UE CONTEXT MODIFICATION RESPONSE message.
The gNB-DU triggers an UE Context Modification procedure by sending UE CONTEXT MODIFICIATION REQUIRED message, which may include DRBs to be modified and released list. The gNB-CU responses with UE CONTEXT MODIFICATION CONFIRM message.
The gNB-CU may add additional TNL Endpoint(s) to be used for F1-C signalling between the gNB-CU and the gNB-DU pair using the gNB-CU Configuration Update procedure. The gNB-CU Configuration Update procedure also allows the gNB-CU to request the gNB-DU to modify or release TNLA(s).
The gNB-DU may add additional TNL association(s) to be used for F1-C signalling using a gNB-CU endpoint already in use for existing TNL associations between the gNB-CU and the gNB-DU pair. The gNB-DU CONFIGURATION UPDATE message including the gNB-DU ID shall be the first F1AP message sent on an additional TNLA of an already setup F1-C interface instance after the TNL association has become operational.
The F1AP UE TNLA binding is a binding between a F1AP UE association and a specific TNL association for a given UE. After the F1AP UE TNLA binding is created, the gNB-CU can update the UE TNLA binding by sending the F1AP message for the UE to the gNB-DU via a different TNLA. The gNB-DU shall update the F1AP UE TNLA binding with the new TNLA. The gNB-DU Configuration Update procedure also allows the gNB-DU to inform the gNB-CU that the indicated TNLA(s) will be removed by the gNB-DU.