RAN Function Exposure

The E2 Service Model defines the standardized contract between an xApp and a specific RAN function. Each E2SM specifies the RAN Function Definition, supported Event Triggers, Actions, and Insert Indications.

• E2SM-KPM: Key Performance Measurement collection and reporting
• E2SM-RC: RAN Control for UE and cell-level configuration
• E2SM-NI: Network Interface management and flow control
• E2SM-CCC: Coordination between multiple xApps for conflict resolution

Each exposed RAN function publishes a RAN Function Definition that describes its capabilities in a machine-readable format. This definition includes the OID (Object Identifier) of the E2SM, supported event trigger styles, report styles, and insert styles.

• Enables plug-and-play interoperability between any vendor's O-CU/O-DU and any xApp
• The Near-RT RIC uses this definition to match xApp subscriptions with available RAN functions
• Eliminates the need for proprietary adapter layers per vendor

RAN Function Exposure supports multiple event trigger styles that determine when the RAN sends data to subscribing xApps:

• Periodic Reporting: Configurable intervals (e.g., every 100ms) for continuous monitoring
• On-Demand Request: xApp explicitly requests a single measurement report
• Event-Triggered: RAN pushes data only when a defined threshold is crossed (e.g., PRB utilization > 80%)
• Insert Indication: RAN suspends a procedure and awaits policy guidance from the xApp before proceeding

xApps interact with RAN functions through a publish-subscribe model over the E2 interface. An xApp sends an RIC Subscription Request specifying the E2SM, event trigger definition, and action definition.

• The RAN node acknowledges with an RIC Subscription Response or rejects with a failure cause
• Multiple xApps can simultaneously subscribe to the same RAN function
• The RIC Indication message carries the actual measurement or event data from RAN to xApp
• Subscriptions persist until explicitly deleted or the E2 connection is reset

Beyond monitoring, RAN Function Exposure enables near-real-time control through standardized action definitions. xApps issue RIC Control Requests that map to specific RAN procedures:

• UE-Level Control: Modify per-UE radio bearer configuration, handover thresholds, or scheduling weights
• Cell-Level Control: Adjust cell individual offsets, transmission power, or antenna tilt
• Slice-Level Control: Reallocate resource block quotas between network slices
• The RAN executes the action and returns a RIC Control Acknowledge with success or failure status

The Insert Indication mechanism allows the RAN to pause a procedure and query an xApp for a decision before continuing. This enables policy-driven control loops without replacing the RAN's native procedures.

• Example: Before admitting a new UE, the O-DU sends an Insert Indication to a QoE Optimization xApp
• The xApp evaluates current load and UE priority, then returns an Insert Indication Response with admission or rejection
• The RAN resumes its procedure using the xApp's guidance
• This preserves the RAN's protocol stack integrity while externalizing decision logic

The standardized data schema that defines what a specific RAN function exposes over the E2 interface. Each E2SM specifies the control actions, event triggers, and report formats for a particular RAN feature.

• E2SM-KPM: Key Performance Measurement—exposes cell-level metrics like throughput, PRB utilization, and active UE count
• E2SM-RC: RAN Control—exposes fine-grained control over UE admission, bearer setup, and handover
• E2SM-NI: Network Information—exposes topology and neighbor relation data

Without a standardized E2SM, an xApp cannot interpret data from a different vendor's RAN.

The mechanism by which an xApp registers interest in specific RAN events and data streams. The xApp sends a RIC Subscription Request to the E2 Node, defining:

• Event Trigger Definition: What condition initiates reporting (e.g., UE throughput drops below 5 Mbps)
• Action Definition: What control action to take or report to generate
• Report Style: Periodic, event-triggered, or on-demand

This publish-subscribe model ensures xApps receive only relevant data, minimizing E2 interface overhead.

The foundational protocol that carries all E2 messages between the Near-RT RIC and E2 Nodes. E2AP defines four core procedure classes:

• RIC Subscription: Setup, modification, and deletion of event subscriptions
• RIC Indication: Asynchronous data reporting from E2 Node to RIC
• RIC Control: Commands from xApps to modify RAN behavior
• RIC Service Update: Dynamic capability exchange when RAN functions change

E2AP runs over SCTP/IP, providing reliable, ordered message delivery with built-in heartbeating for connection health monitoring.

A unique identifier that distinguishes individual RAN functions exposed by a single E2 Node. Each RAN Function ID maps to a specific E2SM and represents a discrete optimization domain.

• ID 1: Key Performance Monitoring (E2SM-KPM)
• ID 2: RAN Control (E2SM-RC)
• ID 3: Network Information (E2SM-NI)

During E2 setup, the E2 Node advertises its supported RAN Function IDs, enabling the RIC to discover available control and monitoring capabilities dynamically.

The asynchronous report sent from an E2 Node to the Near-RT RIC when a subscribed event trigger fires. Each indication contains:

• RIC Requestor ID: Which xApp subscription triggered the report
• RAN Function ID: Which RAN function generated the data
• Action ID: Which specific action within the subscription
• Indication Data: The actual measurement or state payload

Indications are the primary data feed driving xApp decision loops, enabling near-real-time responses to changing radio conditions.