A1 is the northbound interface in the RIC for policy guidance. The northbound caller (non RT RIC, RIC Dashboard, SMO, etc) creates policy types and instances through A1, and subsequently messages are exchanged with xapps.

You can find the A1 RESTful API in the A1 repo: 

The xApp A1 mediator exposed a generic REST API by which xApps can receive and sent northbound messages, e.g., for receiving policy intents (e.g., “isolate UEs from cell 5”), for receiving machine learning models, or for sending current application state like a neighbor cell relations table learned from the RAN. The A1 mediator will take the payload from such generic REST messages,  validate them, and then communicate the payload to the xApp via RMR messaging.

Link to A1 code https://gerrit.o-ran-sc.org/r/admin/repos/ric-plt/a1 under the file ("a1/openapi.yaml")

You can find the documentation here: https://docs.o-ran-sc.org/projects/o-ran-sc-ric-plt-a1

API Specs

Spec 0.9.0

/en/latest/. Exactly what belongs there and what belongs here is a bit murky. 

Policy Overview

There are two "object types" associated with policy: policy types and policy instances. 

Policy types define the name, description, and most importantly the schema of all instances of that type. Think of policy types as defining a JSON schema for the messages sent from A1 down to xapps.

Xapps do not receive policy types from A1, types are used only by A1 to validate instance creation requests from north of the RIC. However, xapps must register for instances of type ids in their xapp descriptor. 

Xapp developers can also create new policy types if they invent new policies; the exact process of where these are stored is still TBD. For practical purposes, when the RIC is running, a CURL needs to be send from somewhere to load the type. 

Policy instances are concrete instantiations of a policy type. They give concrete values of a policy. There may be many instances of a single type. Whenever a policy instance is created in A1, messages are sent over RMR to all xapps registered for that policy type; see below.

Xapps can "sign up" for multiple policy types using their xapp descriptor. 

Xapps are expected to handle multiple simultaneous instances of each type that they are registered for. 

Xapps supporting A1

Please see: Supporting A1 policies