Home > Technical Articles > Open Midhaul F1 interface: F1-C and F 1-U


F1 is the interface that connects a gNB CU to the gNB DU. This interface is applicable to the CU-DU split gNB Architecture. The control plane of the F1-C allows signaling between CU and DU while the user plane of the F1-U allows the transfer of application data. The F1 interface is the functional split of 3GPP between the CU(centralized unit) and the DU (Distributed unit).

Control plane status (F1-C)

It is possible to connect and disconnect UE’s i.e especially the disconnect works reliably after a detach request. A UE might however not be correctly freed if synchronization is lost. The following messages are currently implemented: F1 Setup request and F1 setup response, initial UL RRC message transfer, UL/DL RRC message transfer, F1 UE context release request/command/complete. 

Data plane status (F1-U)

The current implementation uses Google Protocol buffers to encapsulate RLC SDU’s and transports them over a configurable UDP tunnel.     

F1 Interface characteristics

  • F1 interface is an Open interface i.e the endpoint can be from different vendors. 
  • It supports the exchange of signaling and data information between the endpoints.
  • F1 is the point-to-point interface between the endpoints, which means a point-to-point logical interface. 
  • It supports control and user plane separation. 
  • It separates the radio network layer and the transport network layer. 
  • It enables the exchange of UE-associated information and non-UE-associated information. 
  • gNB-CU can also be separated into the control plane and user plane. 

F1-C functions

  1. F1 interface management function: The error indication function is used by the gNB-DU or gNB-CU to indicate to the gNB-CU or gNB-DU that an error has occurred. The reset function is used to initialize the peer entity after node setup and after a failure event occurred. This procedure can be used by both the gNB-DU and the gNB-CU. The  F1 setup function allows exchanging application-level data needed for the gNB-DU and gNB-CU to interoperate correctly on the F1 interface. The F1 setup is initiated by the gNB-DU. The gNB-CU Configuration update and gNB-DU configuration update functions allow to update application-level configuration data needed between gNB-CU and gNB-DU to interoperate correctly over the F1 interface and may activate or deactivate cells. 

The F1-setup and the gNB-DU configuration update functions allow informing the S-NSSAI(s) supported by the gNB-DU. 

  1. System Information management function: Scheduling of system broadcast information is carried out in the gNB-DU. The gNB-DU is responsible for transmitting the system information according to the scheduling parameters which are available. It is responsible for the encoding of the Master information block. In case a broadcast of SIB1 and other SI messages are needed, the gNB-DU is responsible for the encoding of SIB1 and the gNB-CU is responsible for the encoding of other SI messages. 
  2. F1 UE context management function: The F1 UE context management function supports the establishment and modification of the necessary overall UE context. The establishment of the F1 UE context is initiated by the gNB-CU and accepted or rejected by the gNB-DU based on admission control criteria. The modification of the F1 UE context can be initiated by either CU or DU.the mapping between QoS flows and radio bearers are performed by CU and the granularity of bearer-related management over F1 is radio bearer level. 
  3. RRC message transfer function: This function allows the transfer of RRC messages between CU and DU. These messages are transferred over F1-C. the CU is responsible for the encoding of the dedicated RRC message with the assistance information provided over the radio interface. 
  4. Paging functions: The DU is responsible for transmitting the paging information according to the scheduling parameters provided. CU provides paging information to enable the DU to calculate exact PO and PF. 

F1-U functions

  1. Transfer of user data: This function allows the transfer of user data between gNB-CU and gNB-DU. 
  2. Flow control function: This function allows to control the downlink user data flow to the gNB-DU. 

Procedures of the F1 interface

Interface Management Procedures 

  • Reset procedure
  • Error indication procedure
  • F1 setup procedure
  • gNB-DU Configuration Update Procedure
  • gNB-CU configuration update procedure 

Context Management Procedures 

  • UE context setup procedure
  • UE context release request
  • UE context modification procedure
  • UE context modification required
  • UE inactivity notification procedure

RRC Message transfer procedures

  • Initial UL RRC Message transfer procedure
  • UL RRC message transfer procedure
  • DL RRC message transfer procedure 

System information procedures

  • System information Delivery command procedure
  • Paging procedures
  • Paging procedures
  • Resource coordination procedures
  • gNB-DU Resource coordination procedure 
  • User plane procedures