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.

Components and Interfaces

Each block in the diagram represents a different component or function within the architecture. Here's a breakdown of each block:

• Service Management and Orchestration Framework: This is the overarching framework for managing and orchestrating the O-RAN network. It includes tools and protocols for service management to ensure that the network is efficient, reliable, and can adapt to changing conditions and demands.

• Non-Real Time RAN Intelligent Controller (RIC): This component handles tasks that require a broader view of the network and can tolerate a longer delay (non-real-time). It performs functions like policy-based guidance, model training for AI, and non-real-time analytics.
• Near-Real Time RIC: This works on a shorter timescale than the non-real-time RIC and deals with functions that need to react more quickly to changes in the network. It handles tasks such as load balancing, radio resource management, and interference management.
• O-eNB: Represents an O-RAN-enabled eNodeB(eNB in LTE and gNB in NR). It is part of the traditional cellular network infrastructure adapted to fit into the open architecture of O-RAN.
• O-CU-CP (Central Unit Control Plane): This component is part of the Central Unit in O-RAN that handles the control plane functions. It is responsible for setting up and managing network connections.
• O-CU-UP (Central Unit User Plane): This is another part of the Central Unit that specifically handles user data traffic. It deals with the transmission and routing of user data across the network.
• O-DU (Distributed Unit): The Distributed Unit processes the real-time data at the edge of the network. It handles functions like real-time radio signal processing and forwarding data between the radio units and the central units.
• O-RU (Radio Unit): This is the radio unit that handles all radio transmission and reception. It converts the digital signals from the O-DU into radio waves and vice versa.
• O-Cloud: This represents the cloud infrastructure that supports the O-RAN architecture. It provides the computational resources and storage necessary for processing, managing, and analyzing data within the network.

The interfaces shown in the diagram you provided can be categorized into three groups: 3GPP interfaces, O-RAN interfaces, and interfaces marked for future study. Here's an explanation for each type of interface

• 3GPP Interfaces : These are standardized interfaces defined by the 3rd Generation Partnership Project (3GPP), which is responsible for setting standards for mobile telecommunications. In your diagram, these interfaces are marked with solid black lines. They include:
• X2-c and X2-u: These interfaces connect different types of base stations. The "X2" interface in LTE architecture facilitates communication between two eNBs (evolved Node Bs), handling control (X2-c) and user (X2-u) plane traffic.
• NG-u and NG-c: These are part of the 5G architecture, connecting the gNodeB (gNB) to the 5G Core Network. NG-u handles user plane data, while NG-c deals with the control plane.
• Xn-c and Xn-u: Similar to X2, but these interfaces are specific to 5G NR (New Radio). Xn-c is for the control plane, and Xn-u is for the user plane, linking different gNBs.
• O-RAN Interfaces : These interfaces are specific to the O-RAN architecture, facilitating open and interoperable connections between different parts of the RAN. They are depicted with dashed green lines in your diagram. These include:
• E1 Interface: This interface connects the O-CU-CP (Central Unit Control Plane) to the O-CU-UP (Central Unit User Plane). It facilitates communication and coordination between these two components of the Central Unit, managing control and user plane functionalities within the network.
• E2 Interface: The E2 interface links the Near-Real Time RIC with other key RAN components such as the O-CU-CP, O-CU-UP, O-DU (Distributed Unit), and O-eNB (enhanced Node B). It enables the Near-Real Time RIC to execute control functions and real-time optimization across these network elements.
• A1 Interface: This interface is established between the Non-Real Time RIC and the Near-Real Time RIC. It is used for policy management and strategic guidance, allowing the Non-Real Time RIC to send higher-level directives and objectives to the Near-Real Time RIC.

Reference : Specification