# Digital Twin and Internet of Things--Current Standards Landscape
## 3 Standards for Digital Twins and the Internet of Things
### Asset Administration Shell  
The Asset Administration Shell (AAS) is a concept developed by the Plattform Industrie 4.0, a network of companies, associations, trade unions, science and politics in Germany [37]. It is part of the Reference Architectural Model Industry 4.0 (RAMI4.0) [38] and driven by the idea to manifest the concept of DT in factories and industrial production plants. The standard is divided into three parts. Part 1 [39] introduces the AAS meta-model and specifies how to serialize the content of an AAS, Part 2 defines how to interact with an AAS instance at runtime and Part 3 covers the infrastructure aspects and how to discover and interconnect multiple AASs [40]. This matches exactly our identified basic set of common functions: resource description, resource discovery, and resource access. At the time of writing, only Part 1 is officially released. Part 2 is available to the authors in a draft version.  
An AAS is, just like a DT, a digital representation of a resource. Resources are referred to by the term 'asset', hence the name AAS. Figure 3 shows a class diagram of the AAS meta model. They main element is the Asset AdministrationShell, representing an Asset and composed of multiple Submodels. A Submodel is a container for SubmodelElements that can be properties, operations, collection, files, etc. The intention behind grouping sets of elements together into submodels and not adding those directly to an AAS is to allow standardization of submodels. By supporting multiple types of resource identifiers, IRI, IRDI and custom types, the standard provides much flexibility when referring to external resources e.g., for semantic annotations. The used type system is adapted from XSD complemented by the RDF data type langString. AAS descriptions can be serialized using XML, JSON, RDF, OPC UA data model and AutomationML. There is no explicit support for neither geo-spatial, temporal nor historical data although the standard is generic enough so this could be implemented on a higher level.  
Figure 3: Class diagram depicting the metamodel of the Asset AdministrationShell concept developed by Plattform Industrie 4.0 (based on Figure 11 from [39]).  
Resource discovery for this standard cannot be evaluated, as Part 3 is not yet released. However, Part 2 and [40] indicate that there will be an AAS registry to register AAS instances with some level of liability and a minimum set of metadata, e.g., endpoints. As an extension, AAS directories or even AAS catalogs may be built with further meta data elements that can be queried to discover resources.  
For resource access, the standard will define a set of technology-specific APIs that each AAS instance has to implement. These APIs are derived from conceptual, technology-neutral interface specifications, which are currently being defined in Part 2. For the design of Industrie 4.0 systems and applications, they will be aggregated into services [41] by system and software architects, optionally extended with other, possibly proprietary interfaces. Therefore, the following information may be subject to changes in the future. At the core of the interfaces are basic CRUD (create, read, update, delete) operations for submodels and submodel elements. The interfaces will be defined for multiple communication protocols: HTTP(S), OPC UA, and probably MQTT.