Open RAN  

RAN Control

RAN is the part that directly communicates with UE and some functions (e.g, various PHY and low MAC  procedure) are autonmously handled by RAN itself, but there are many of the RAN functionalities that are controlled by other network components that are located out side of RAN.

The most critical components that controls the RAN would be 3GPP core network components (e.g, AMF, UPF in 5G). The connection between RAN and AMF is established by the network interface called NG and the protocol for NG connection is called NGAP. In O-RAN based network, the control mechanism by 3GPP core network is mandatory and almost always the first controller to be implmented.

Another components that controls RAN in addition to 3GPP core network is RIC (Radio Inteligence Control). While 3GPP core network is for controlling the essential functionalities of RAN, RIC is more for RAN optimization.

There are two types of RIC which are called Near-Real Time RIC and Non-Real Time RIC. Near Real Time RIC is the type of controls that should be complete with around 10 ms (one radio frame) and Non Real Time RIC is controlling RAN function in relatively longer time span.

The interface between Near-Real Time RIC and RAN (CU+DU) is called E2 and the protocol for E2 communication is called E2AP.

The interface between Near-Real Time RIC and Non-Real Time RIC is called A1 and the protocol for A1 communication is called A1AP.

Reference : Specification