# SOLIOT--Decentralized Data Control and Interactions for IoT
## 2 State of the Art
### Related Work  
The ongoing trend towards digitizing the industrial domain and the thereby created need for standardization has led to the formation of a set of industry consortia. Most relevant among them are the International Internet Consortia with their reference architecture [9] and the German Plattform Industrie 4.0. While the first covers a broader picture across the related domains, the later one focuses on the IoT entity called an Industry 4.0 components and their digital representation, the Administration Asset Shell [10]. Further similar initiatives have been formed, among them the Italian Piano Industria 4.0 and the French Industrie du Futur are closely related to the Asset Shell concept [11]. The International Data Space on the other hand concentrates on the data exchange level and promotes workflows and patterns for usage control even after the sensitive data has left the own network [12]. The embedding of network and Internet-enabled devices into nearly any production-related asset, requiring digital communication between devices from different manufacturers, is definitely not a new development. Machine-to-machine protocols intend to solve this challenge, with the OPC UA stack [13] as the most widely adopted solution. OPC UA defines the interactions, data structures and information models for entities on the shop floor, also often referred to as Operational Technology (OT). AutomationML is further often used to describe the specific logic in OPC UA-driven production workflows as e.g., described by [PERSON] et al. [14]. Especially in the automation domain, communication and control networks relying on OPC UA provide vendor-independent machine-to-machine architectures. The OPC UA protocol stack specifies the data representation, remote interactions with resources and functionalities and its own information model. OPC UA-driven architectures have a strong focus on the operational technology domain, for instance intended for shop floor devices. [PERSON] et al. [15] present a layered architecture for Digital Twins based on OPC UA. The interactions are enabled through either OPC UA sockets or REST APIs, similar to the SOLIOT approach. Generally, the interactions close to the asset or devices are executed through OPC UA binary communication and the ones between the Digital Twin and higher-level applications rely on REST over HTTP. The complete potential of Digital Twins however is the seamless connection throughout the islands and silos, and cross networks, domains and companies. OPC UA networks usually require gateways and proxies, thereby forming bottlenecks and enforcing deep understanding of the protocol's abilities and restrictions. In addition, as both [PERSON] et al. [16] and [PERSON] et al. [17] have mentioned, the lack of formal concept definitions hampers the further integration with higher-level applications. Earlier approaches to transfer established web technologies to the industrial setting combine service-oriented architectures [18; 19]. The method-driven semantic allows the integration through rich service functions using SOAP and the standardized WS* stack. However, the complex requirements and side effects led to redesign of the CBS, less viewed as a collection of method calls and more as a virtual resource with distinct state transitions. Web agents and representations of avatars have been suggested to by [PERSON] et al. [6] as such a resource-oriented extension of CBS with Web capabilities. The one-to-one relation of an Avatar and its cyber-physical object are intended to lift CBS to a homogeneous, Web-driven integration layer. Their interfaces appear through REST APIs and make use of semantic descriptions, both of their capabilities and the offered and consumed data. The main focus of the Avatar approach is the interoperability challenge of IoT scenarios, with less detailed specifications on data protection and security. Virtual Representations [20] go into a similar direction, with read/write capabilities also on functions and services.