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# <center><i class="fa fa-edit"></i> E2 Interface</center>
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### Explanation
The E2 interface is an open interface between two endpoints, i.e., the near-RT RIC and the so-called E2 nodes.The E2 allows the RIC to control procedures and functionalities of the E2 nodes. Moreover, this interface also enables the collection of metrics from the RAN to the near-RT RIC, either periodically or after pre-defined trigger events. Both control and data collection procedures can pertain to one or more cells, slices, [QoS](https://help.stonesoft.com/onlinehelp/StoneGate/SMC/6.7.0/GUID-F6E6FD9D-768E-4419-ADC2-12D26E792096.html#:~:text=QoS%20(Quality%20of%20Service)%20allows,bandwidth%20management%20and%20traffic%20prioritization.) (quality of service) classes, or specific UEs.

To support the above operations, the O-RAN Alliance uses a variety of unique identifiers. Specifically, O-RAN uses identifiers based on 3GPP specifications for the gNB, slice, and QoS class. Regarding specific UEs, the O-RAN Alliance has instead introduced a common user identifier (i.e., the UE-ID) across its specifications. This provides a consistent and uniform user identity across the system without exposing sensitive information related to the user.

The E2 interface has been logically organized in two protocols: E2 Application Protocol (AP) and E2 Service Model (SM).

The E2 AP is a basic procedural protocol that coordinates how the near-RT RIC and the E2 nodes communicate with each other, and provides a basic set of services, as shown in Figure.
O-RAN E2 interface, application protocol, services and packet.

E2AP messages can embed different E2 SMs, which implement specific functionalities (i.e., the reporting of RAN metrics or the control of RAN parameters). The E2 interface runs on top of the SCTP protocol.