gNodeB Split Architecture with F1 Interface: -

F1 Interface connect to gNodeB CU to gNodeB DU. This Interface is applicable to the CU-DU Split gNodeB architecture. Control Plane F1-C allow signalling b/w the CU and DU while User Plane F1-U allow to transfer the user data b/w CU and DU. F1 interface is an open interface, and the end point can be from different vendors.

 

Distributed Unit (DU): -

Distributed Unit support for the lower layer of the gNodeB protocol stack such as RLC, MAC, and Physical layer and it’s deployed close to RU Radio Unit on the site in field.

Centralized Unit: -

Centralized Unit provides support for the higher layer of gNodeB protocol stack such as SDAP, RLC, RRC. CU deployed in cloud to integrated with core network UPF.  

Note: - SDAP Layer is not include when CU is connected to the 4G Core Network. SDAP Layer is work only with 5G Core Network.

Note: - Single CU can be connect with single gNodeB but one CU can be connecting multiple DU like 100.

 

Control Plane Functionalities:

F1 Setup: The F1 Setup procedure is used to create a logical F1 connection between the CU-CP and DU. It is necessary to establish an SCTP connection between the CU-CP and DU before the F1 Setup procedure can be initiated. The DU initiates the procedure by sending an F1 Setup Request message, while the CU-CP completes the procedure by returning an F1 Setup Response. The F1 Setup Request is used to inform the CU-CP about DU identity and the set of cells supported by the DU. The F1 Setup Response is used to indicate which DU cells should be activated.

F1 Setup Call flow b/w gNodeB DU and gNodeB CU: -

 

 

·     The purpose of the F1 Setup procedure is to exchange application-level data needed for the gNB-DU and the gNB-CU to correctly interoperate on the F1 interface.

 

·       gNB-DU initiated the procedure by sending F1-Setup request message including the appropriate below data in this request message and send to the gNB-CU.

 

·       gNB-DU Name IE: - gNB-DU include DU identity in this IE so. gNB-CU use this IE as a human readable name of the gNB-DU.

 

·       gNB-DU Served Cells List IE: - gNB-DU include the list of supporting serving cells.

 

Note: - For NG-RAN: - gNB-DU also include gNB-DU System Information IE (MIB, SIB Parameter) and the TAI Slice Support List IE (TAC, PLMN Identity, Slice Support List)

·       gNB-CU include the activated cells list IE in response to gNB-DU to activate these cells and configure the PCI for selective cells in F1 setup response message.

 

·       gNB-DU shall take the received PLMN List into account and only broadcast the PLMN(s) included in this received Available PLMN List IE in response message.

 

Note: - The gNB-DU initiates the procedure by sending a GNB-DU CONFIGURATION UPDATE message to the gNB-CU including an appropriate set of Addition/Modify/Deletion in Application data and send to the gNB-CU.

 

User Plane Functionality:

 The user plane of the F1-U transfers application data between CU-UP and DU. GTP-U tunnels are used to transfer the application data. These tunnels are identified using their TEID. A tunnel is setup for each DRB. The user plane protocol which runs above the GTP-U layer provides various control mechanisms associated with the transfer of downlink data. These control mechanisms include flow control, packet loss detection and successful delivery reporting. The frame formats used by the user plane protocol is referred as PDU Type 0 is sent by the CU, whereas PDU Type 1 is sent by the DU.

 

 

PDU Type 0: The CU-UP uses PDU Type O to add a sequence number to each downlink data packet. The DU uses this sequence number to detect lost packets. The CU-CP can also use POU Type O to provide various discard instructions. If the DU reports Radio link outage, then the CU-UP may attempt re-transmission from the PDCP layer using a second DU. If the second DU reports successful delivery of the PDCP PDU, the CU-UP instructs the original DU to discard the successfully delivered packets to avoid unnecessary transmission

 PDU Type 1: The DU uses PDU Type 1 to report any lost packets and also to control the rate at which downlink data is sent by the CU. it provides a mechanism for flow control to avoid the buffers within the DU becoming too full. The DU signals the sequence number of the highest successfully delivered PDCP PDU, the desired buffer level and the desired data rate. The desired data rate is specified as the number of bytes which the DU would like to receive within a 1 second time interval. The CU uses these information elements to determine the quantity of data to forward towards the DU. The DU can also use PDU Type 1 to indicate ‘Radio Link Outage’ or ‘Radio Link Resume’.