534 add architecture overview

doc/architecture/architecture_overview.adoc

@@ -1,11 +1,39 @@
 = Overview of OSI architecture
 
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+OSI contains an object-based environment description using the message format of the https://github.com/protocolbuffers/protobuf/wiki[Protocol Buffer] library developed and maintained by Google.
+OSI defines top-level messages that are used to exchange data between separate models.
+Top-level messages define the ``GroundTruth`` interface, the ``SensorData`` interface and, since OSI version 3.0.0, the ``SensorView``, ``SensorViewConfiguration`` and ``FeatureData`` interfaces.
+
+The following figure shows the interfaces and models involved in modeling a sensor.
+
+.Open Simulation Interface overview
+image::osi-context.png[1100]
+
+
+OSI also defines interfaces for traffic participant models.
+The ``TrafficCommand`` interface allows to send commands to traffic participant models.
+The ``TrafficUpdate`` interface allows to receive their updated state.
+The following figure shows the interfaces of a generic traffic participant.
+
+.Interface of a traffic participant
+image::osi-traffic-participant-principle.png[1100]
+
+Traffic participant models may use other OSI interfaces internally, for example, to model autonomous vehicles.
+The following figure shows a more advanced use case for traffic participants.
+
+.Traffic participant with sensor models, AD function, and dynamic model
+image::osi-traffic-participant-advanced.png[1100]
+
+The ``HostVehicleData`` interface describes the measured internal states of a traffic participant
+OSI currently provides only limited support for data structures describing measured internal states traffic participants.
+Actuator intentions are currently not covered by OSI and must be handled with a different data description format.
+
+All fields in an interface are set to `optional`.
+`required` is not used.
+This has been done to allow backward-compatible changes in the field.
+Additionally, this is the default behavior in Protocol Buffer version 3 that does no longer have the `required` type and therefore ensures update compatibility.
+However, this does not mean that filling the field is optional.
+For the purpose of providing a complete interface, all existing fields should be set, unless not setting a field carries a specific meaning as indicated in the accompanying comment.
+
+All field numbers equal to or greater than 10000 are available for user-specific extensions via custom fields.
+Therefore, no future evolution of OSI will use field numbers equal to or greater than 10000.