# Digital Twin and Internet of Things--Current Standards Landscape
## 4 Summary  
Table 1 provides an overview on the classification of the different IoT and DT standards analyzed. Although the standards were developed independently by different SDOs, they ended up with quite similar approaches/solutions to some aspects. To enable resource description, providing a meta-model is the way to go. NGSI-LD and STA also provide some cross-domain model, primarily defining terms needed for support of geo-spatial and temporal data. At first glance, the standards seem to use quite different types of resource identifiers. Considering that IRI \(\supset\) URI \(\supset\) URL, URI \(\supset\) DTMI, and URL \(\cap\) DTMI = \(\varnothing\) it seems that URI might be a good compromise for a unified identifier type in the future. An even more flexible, but also more complex, approach is the one used in the AAS by defining meta-level constructs to describe any kind of identifier. All standards supporting resource interlinking (or at least having it on the agenda) reflects the fact that relations between resources are essential for IoT and DT. Besides, using different terms to describe the kind of elements that make up a resource, there seems to be more-or-less general consent that a resource should at least contain properties and services. Only half of the standards support resources defining custom events, but almost all support at least notification upon property changes. For serialization of resource descriptions, JSON is the common denominator between all standards. Since version 1.1, JSON-LD [60] supports interpreting basic JSON as JSON-LD when using HTTP and setting the header appropriately. Additionally, JSON-LD is compatible with RDF, the cornerstone of the Semantic Web. For this reason, almost all standards make use of JSON-LD to provide, on the one hand, easy-to-use JSON-based access and, on the other side, fine-grained access with full Semantic Web support. Again, the AAS provides a more generic approach supporting different kinds of serialization formats. This is partially because it also supports different protocols for resource access and some protocols require a certain serialization format, e.g., OPC UA. Resource discovery is something on the agenda of most of the SDOs and will be properly addressed in the future. No standards currently supporting resource discovery allow direct communication between the consumer and the resource, i.e., they already have a central repository-like component by design. Furthermore, there is no consensus about which query language to use. Query languages are a complex subject itself. In fact, AAS and WoT are currently still evaluating different query languages that might be suited for them. Probably the main issue is the trade-off between the expressivity of a language and the complexity to implement it. Additionally, query languages are also often tied to specific data formats and/or protocols. SPARQL is a rather complex but powerful query language for RDF with SQL-like syntax. As most standards already support RDF data, SPARQL is one of the most promising candidates for a unified query language. Furthermore, there are already existing extensions for geo-spatial  
Figure 8: Class diagram depicting the metamodel of the Web of Things Thing Description (based on Figure 1 from [55]). queries in SPARQL, e.g., GeoSPARQL. Another promising approach are URL-based query languages like OData/STA and JSONPath. The query language of OData/STA has proven quite powerful in real-world applications but is much simpler to implement than SPARQL. JSONPath is adapting the concept of XPath from XML to JSON. It has been around for over ten years but has never been properly standardized by an SDO. Recently, efforts have been made initiated by the IRTF Thing-to-Thing Research Group (T2 TRG) to pursue an official standardization and to push JSONPath as a query language in the IoT context [34; 61]. Regarding resource access, the most prominent paradigm is to define a new API that each resource has to implement. The only standard adapting to another paradigm is WoT.