A1 AP Termination module for ONOS SD-RAN (µONOS Architecture)
The onos-a1t is the A1 termination node in the near-RT RIC for A1 interface to communicate the near-RT RIC with the non-RT RIC.
It is the proxy that forwards incoming A1 messages from the non-RT RIC to appropriate xApps or outgoing A1 messages from xApps to the non-RT RIC.
As per the O-RAN Working Group 2 specification, onos-a1t should support A1 messages for (i) the policy management, (ii) the enrichment information, and (iii) the machine learning model management.
As of today, since the O-RAN A1 specification only defines the policy management data model, onos-a1t only supports the policy management service.
Regarding O-RAN specifications, onos-a1t supports A1 Application Protocol v03.01 and A1 Type Definitions v02.00.
The onos-a1t interacts with at least three nodes: (i) onos-topo, (ii) A1-enabled xApps and (iii) non-RT RIC.
To begin with, onos-a1t keeps listening the onos-topo to check if there is new A1-enabled xApps deployed and if there are A1-enabled xApps already running.
Basically, the A1-enabled xApps initially stores its A1 interface information, such as supported A1 services (i.e., the policy management, the enrichment information, and the machine learning model management) and A1 interface endpoint (i.e., IP address and port number).
Listening onos-topo, onos-a1t scrapes the A1 interface information and store it into the onos-a1t local store.
With the A1 interface information, onos-a1t starts creating the gRPC session with appropriate xApps to communicate with each other.
A gRPC server is the A1-enabled xApp, whereas onos-a1t acts as the gRPC client (Note that this design is able to support the high availability and reliability by using the replicas for the near future).
In order to communicate with the non-RT RIC, onos-a1t has both an HTTP server and an HTTP client.
And of course, the non-RT RIC has to have both the HTTP server and the HTTP client for the bi-directional communication over HTTP.
The HTTP server in onos-a1t receives the JSON formatted A1 interface message from the non-RT RIC.
The HTTP client in onos-a1t is the client that sends the JSON formatted outgoing A1 interface messages to the non-RT RIC.
The HTTP client and server implementations are auto generated from the OpenAPI definitions provided by the A1 Application Protocol specifications.