The JSON DataEncoding was developed to allow OPC UA applications to interoperate with web and enterprise software that use this format. The OPC UA JSON DataEncoding defines standard JSON representations for all OPC UA Built-In types.
The JSON format is defined in RFC 7159. It is partially self-describing because each field has a name encoded in addition to the value, however, JSON has no mechanism to qualify names with namespaces.
The JSON format does not have a published standard for a schema that can be used to describe the contents of a JSON document. However, the schema mechanisms defined in this specification can be used to describe JSON documents. Specifically, the DataTypeDescription structure defined in OPC 10000-3 can define any JSON document that conforms to the rules described below.
Servers that support the JSON DataEncoding shall add DataTypeEncoding Nodes called “Default JSON” to all DataTypes which can be serialized with the JSON encoding. The NodeIds of these Nodes are defined by the information model which defines the DataType. These NodeIds are used in ExtensionObjects as described in 220.127.116.11.