# Structuring Reference Architectures for the Industrial Internet of Things
## 2 Related Work  
In the following, the definitions as collected by Reference [2] are used. The terms 'Industrial Internet' (as defined by Reference [3]), 'Industry 4.0' (as defined by Reference [4]), 'Industrial Internet of Things' (as defined by Reference [2]) or even 'Cyber-physical Systems' (as defined by Reference [5]) all have different variations. In the following, in order to increase readability, we will stick to 'Industrial Internet of Things' and 'IIoT' as the unifying terms. Several works aim to create a framework for software architecture descriptions. ISO/IEC 42010 [6] proposes Architecture Descriptions structured by a list of so-called _concerns_ being addressed by several _architecture views_. An architecture view is a projection and therefore a simplification, of the abstract architecture in order to describe specific topics. For instance many IIoT reference architectures cover both interoperability and security related aspects. Though there are many inter-dependencies, describing both concerns in one view decreases readability and significantly increases the complexity. In contrast to the general agreement on the classification into views, only a minority explicitly states the regarded concerns and the followed conventions throughout the presented views. Therefore, it must be stated that the proposed structure of ISO 42010 is not followed--a development, which significantly hampers the comprehension of core aspects and limits effective comparisons. [PERSON] et al. [7] further strengthen this fact. They also demand a first-class treatment of stakeholders and their concerns. [PERSON] et al. provide taxonomies for the industry sector and its various domains and subdomains [2]. Additionally, the authors create hierarchies for connectivity, characteristics of IT, IIoT devices and user interactions. However, the proposed terms are not connected with the frameworks, which are currently developed. Therefore, the proposed taxonomies can be seen more as parallel activities and less as compliant to the prominent IIoT reference architectures. A number of surveys on IIoT and related architectures have been published recently. [PERSON] and [PERSON] [8] provide a concise overview of the most relevant frameworks and initiatives. While outlining the main approaches from an industry-based view, an in-depth analysis is missing. [PERSON] and [PERSON] and [PERSON] [9] outline a collection of IIoT topics, mentioning key drivers and enabling technologies but lack the link to guidelines how to use those. Even though a short wrap-up on lessons learned is outlined, the important organizational players especially for industrial standardization are not covered. [PERSON] et al. outline the key requirements and technologies for Industrie 4.0 applications [10]. They present the relations between IoT-enabled manufacturing, cyber-physical systems, cloud manufacturing and intelligent manufacturing mainly through AI applications. They briefly compare the main efforts from the US, Germany/EU, Japan and China and argue for an agent-based, generic framework covering all previous frameworks. However, they do not present a unifying framework that could fulfill this demand. [PERSON] et al. group the developments and discussions in their review mainly according to smart manufacturing and Industrie 4.0. In addition to Reference [10], they also discuss human-machine interaction with a special focus on safety and the prevention of hazards for the involved workers. Furthermore, they draw the conclusion that the description of the \"variety of technical standards from various disciplines\" [11] requires clear, widely-known reference models. However, the authors only refer to the Reference Architecture Model for Industrie 4.0 but do not make the link to other ones. Similarly to the previously mentioned reviews, [PERSON] and [PERSON] highlight comparable issues and implications as the others--for instance cyber-security and data privacy as new challenges.