# Structuring Reference Architectures for the Industrial Internet of Things
## 3 Methodology for Aligning Reference Architectures
### Considered Concerns  
This section briefly outlines the subset of collected concerns, based on Reference [21; 22], categorized in a hierarchy with interoperability, trustworthiness and business related concerns as the top level entities. Following ISO/IEC 42010, concerns are the aspects or challenges of an IIoT system that are covered by a certain section of a reference framework. A comprehensive list of relevant concerns allows the identification of blank spaces but also to outline different definitions and scopes of reference frameworks. The hierarchy of concerns creates a taxonomy-like structure. Lower level concerns influence a higher level one in terms of supporting its satisfiability. For instance, having a system with secure communication, this fact indicates a higher level of security and therefore trustworthiness of the system. Modeling such relations explicitly outlines previously intangible connections. Furthermore, formalizing information in the graph enables automated reasoning processes to further populate facts and extend the contained facts. The thereby created network supplies a defined vocabulary, which is used to match the distinct terminology of the various frameworks and to map their assertions. In addition, the formalized relations between the concerns themselves enable the precise assignment of contributions of very specific, lower-level concerns to more general, higher-level ones. To the best of our knowledge, no other work contains a comparable  
Figure 2: Structure of the IIoT knowledge graph. The original scheme [19] is extended by the set of concerns and architecture frameworks. structure of IIoT concerns and their respective interconnections or applies its structuring of the domain based on a formalized dimensions like ours. In the following, the most central concerns with a selection of their direct sub-concerns are introduced. _Interoperability_ or connectivity is the fundamental building block for enabling IIoT ecosystems (see Figure 3). Two IIoT components are interoperable when they can work together without any restrictions or additional adjustments. In this sense, interoperability contains integration aspects as endpoint descriptions and communication patterns. Therefore, both syntactic and semantic interoperability have to be established. _Syntactic interoperability_ characterizes all aspects in order to exchange data, for example, network protocols and data formats. _Semantic interoperability_ targets a shared understanding of the meaning of the data, for example, by information models. _Trustworthiness_ and the highly related _security_ of data exchange ecosystems determine the ability to prevent unintended or unauthorized access, change or destruction and therefore behave as expected. _Access control, provenance tracking, identity management_ and _authorization_ but also _reliability_, _availability_ and _resilience_ influence security and are required for a trustworthy system (see Figure 4). Access control frames all methods necessary to ensure data exchange only for entitled parties. Assigning the rights is part of an _authorization_ process by defining roles or policies. Provenance tracking denotes any mechanism to document the source of a data object and any kind of transformation or modification. Identity management faces the challenge of supplying unambiguous identifier and mechanisms for third parties to verify an identity claim (authentication). As IIoT also interacts with the physical world, functional _safety_, as a system's ability to prevent physical damage is an important requirement. Any productive IIoT system must have security concerns at its core. Therefore, security related concerns are judged more and more as non-optional but fundamental for any ecosystem. _Business_-related concerns outline, for instance, requirements around _data economics_ and _governance_ and value-adding features.