E2 Interface
O-RAN interface between the near-RT RIC and O-CU/O-DU, carrying control messages and telemetry for real-time RAN optimization.
E2 is the link that makes the near-RT RIC useful — it connects the RIC to the RAN nodes (the O-CU and O-DU, collectively called E2 Nodes) and carries both directions of the conversation: telemetry up, control and policy down. Without it the RIC would be blind and mute. It's the channel an xApp uses to subscribe to measurements and then act on them within the near-real-time loop.
What makes E2 more than a single protocol is its service models. E2SM definitions specify what you can monitor and what you can control for a given function — there are service models for things like KPM (key performance measurement, the reporting side), RC (RAN control), and others. An xApp is effectively written against one or more of these service models, so the set a node supports determines what optimisation is even possible. When a third-party xApp doesn't work against a particular RAN node, mismatched or unsupported E2 service models are one of the first things to check.
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