E2AP

The interface that is designed for communication between DU/CU and Near-Real Time RIC is called E2. The protocol for the communication over E2 interface is called E2AP. This kind of naming conventions is similar to 3GPP naming convention (e.g, NGAP as Protocol for NG interface, X2AP as Protocol for X2 interface, S1AP as Protocol for S1 interface etc). The algorithms happening inside of RIC would be up to each individual implmentation and out of the scope of this note, but E2AP (protocol and message structure) should be same for every RIC and CU/DU.

The types of Messages (signaling messages) running over E2 interface is defined in O-RAN specification as follows.

< O-RAN.WG3.E2SM-KPM - Table 5-1: Relationship between E2SM services and E2AP Information elements >

RAN Function specific E2SM Information Elements	E2AP Information Element reference	Related E2AP Procedures
RIC Event Trigger Definition	E2AP [O-RAN.WG3.E2AP] section 9.2.9	RIC SUBSCRIPTION
RIC Action Definition	E2AP [O-RAN.WG3.E2AP] section 9.2.12	RIC SUBSCRIPTION
RIC Indication Header	E2AP [O-RAN.WG3.E2AP] section 9.2.17	RIC INDICATION
RIC Indication Message	E2AP [O-RAN.WG3.E2AP] section 9.2.16	RIC INDICATION
RIC Call Process ID	E2AP [O-RAN.WG3.E2AP] section 9.2.18	RIC INDICATION RIC CONTROL
RIC Control Header	E2AP [O-RAN.WG3.E2AP] section 9.2.20	RIC CONTROL
RIC Control Message	E2AP [O-RAN.WG3.E2AP] section 9.2.19	RIC CONTROL
RAN Function Definition IE	E2AP [O-RAN.WG3.E2AP] section 9.2.23	E2 SETUP RIC SERVICE UPDATE

As you notice from the table above, the elemenary E2AP procedure can be grouped into only a few types : SUBSCRIPTION, INDICATION, CONTROL, UPDATE. The overall procedure of these types can be illustrated as follows (NOTE : The diagram on the left column is from The O-RAN SC RIC). As you would notice here, almost all the procedure except CONTROL procedure is initiated by SUBSCRIPTION. Another common thing to be noticed is that most of the procedure (except CONTROL) is initiated (Requested) by RIC and RAN is supposed to do some action (e.g, Report, Perform Policy, Insert) as per requested by RIC.

Functional Procedure	Description
	Subscription (Report): The RIC initiates the process by sending a subscription request to the RAN. This request specifies the types of reports or indications the RIC wants to receive from the RAN (e.g., performance metrics, events, alarms). The subscription acts as a filter, ensuring the RIC only receives relevant information. Acknowledgment (Ack Subscription/Report): Upon receiving the subscription request, the RAN sends an acknowledgment back to the RIC, confirming the subscription's successful establishment. This step establishes a reliable communication channel for the subsequent reporting process. Trigger Detection (RAN): The RAN continuously monitors the network for the occurrence of events or conditions that match the criteria defined in the subscription. These triggers could be related to various aspects, such as QoS degradation, congestion, or specific user activity patterns. Indication (Subscription, Report) (RAN to RIC): Once a trigger is detected, the RAN generates an indication message, encapsulating the relevant report or information as per the subscription. This message is then transmitted to the RIC, providing it with real-time insights into the RAN's state. Continue Call Processing (RIC): Upon receiving the indication, the RIC processes the report and takes appropriate actions based on the information. These actions could involve adjusting RAN configurations, initiating optimizations, or triggering other control procedures to maintain optimal network performance.
	Indication (Optional): The RAN may send an optional INDICATION message to the RIC, conveying relevant information about an event or network state. This step is not mandatory but provides the RIC with additional context for subsequent actions. Event Detection (RIC): The RIC continuously monitors the network and detects specific events that warrant intervention. These events could relate to various aspects, such as quality of service degradation, congestion, or changes in traffic patterns. Action Execution (RIC): Upon detecting an event, the RIC performs appropriate actions to address the situation. These actions could involve adjusting configurations, initiating optimizations, or triggering other control mechanisms. Control Message (RIC to RAN): The RIC sends a CONTROL message to the RAN, encapsulating the necessary commands or instructions based on the detected event and the intended action. This message communicates the desired changes to the RAN. Resume/Initiate Call Processing (RAN): The RAN receives the CONTROL message and takes corresponding steps to execute the instructions. This could involve modifying parameters, reallocating resources, or adapting its behavior to align with the RIC's directives. Control Acknowledgement (RAN to RIC): The RAN sends a CONTROL ACK message back to the RIC, confirming the successful reception and execution of the CONTROL message. This acknowledgement establishes a reliable communication loop, ensuring the RIC is aware of the RAN's response.
	Subscription (Policy): The RIC initiates the process by subscribing to specific policies from the RAN. These policies define rules or conditions that the RAN should adhere to for optimizing network performance, managing resources, or handling specific events. Acknowledgement (Ack Subscription (Policy)): The RAN confirms the successful reception of the subscription request by sending an acknowledgement back to the RIC. This establishes a communication channel for subsequent policy-related interactions. Detect Trigger (RAN): The RAN continuously monitors the network for the occurrence of events or conditions that trigger the subscribed policies. These triggers could be related to various factors like congestion, QoS degradation, or specific traffic patterns. Perform Policy (RAN): Upon detecting a trigger, the RAN executes the corresponding policy as defined in the subscription. This could involve adjusting configurations, reallocating resources, or modifying network behavior to comply with the policy's objectives. Continue Call Processing (RAN): After executing the policy, the RAN resumes its normal call processing operations. The policy enforcement is seamlessly integrated into the ongoing network functions without causing disruptions.
	Subscription (Insert): The RIC initiates the process by subscribing to a specific type of "Insert" indication from the RAN. This indicates the RIC's interest in receiving notifications when certain events related to user equipment (UE) or data insertion occur in the RAN. Acknowledgement (Ack Subscription (Insert)): The RAN acknowledges the subscription request, confirming that it will send indications for the specified "Insert" events. Detect Trigger (RAN): The RAN continuously monitors the network for trigger events that match the criteria of the "Insert" subscription. These triggers could be related to specific UE activities, data thresholds, or other predefined conditions. Indication (Subscription, UE, Insert): Upon detecting a trigger, the RAN sends an "Indication" message to the RIC. This message includes information about the subscription, the UE involved, and the specific "Insert" event that occurred. Halt or Suspend Call Processing (Optional): Based on the received indication, the RIC may decide to send a command to the RAN to halt or suspend the ongoing call processing for the involved UE. This step is optional and depends on the RIC's policy or the nature of the trigger event.

Reference