# Technical Interoperability for Machine Connectivity on the Shop Floor
## Background
### Technical Interoperability and Enabling Technologies  
Interoperability issues relate to technical, semantic, or organizational aspects [2]. Technical interoperability aspects regard the data exchange between different information technologies at the physical and syntactical level. When considering data for CPS in manufacturing, it is important to differentiate between configuration data and run-time data [7]. Configuration data is generated during the design of the system and describes the physical parts of the system. Run-time data is generated during the operational phase and describes the status of the manufacturing process. Optimal synergies are achieved when these data types can share the same models, which require usage of industrial standards for both design and operations. The automation engineering standard Automation ML (AML) and communication standard OPC Unified Architecture (OPC UA) are examples of standards that can be semantically linked [17]. Another core consideration when interconnecting systems is layout, meaning the way in which each entity relates to others in terms of computation and decision making. The classical control system is hierarchical and deterministic, operating with centralized decision making. In such systems, data only flows vertically between child and parent nodes. The IoT paradigm assumes a decentralized approach where data can flow horizontally. Such systems are event-driven and non-deterministic where a more complex system emerges, which needs new methods to be managed [7]. A practical approach to achieve CPS in manufacturing is to create a semi-hierarchical approach with emphasis on modularization. An example is a solution by [PERSON] and [PERSON] [18], promoting a holonic (modularized) system where each Holon is controlled by a software agent (autonomous unit). Figure 1: A high-level illustration of what technological solutions, or paradigms, that can be loosely connected with each step in the Industry 4.0 maturity index model [10]. Approaches to the layout problem, developed outside of the manufacturing domain, include various platforms and middleware for IoT applications that have gained traction lately. IoT platforms, also called frameworks or middleware, are systems that simplify the integration of things (e.g., sensors, machines, and equipment) through decoupling to enable IoT applications. There are three types of IoT systems: Service-based, cloud-based, and actor-based [19]. Service-based systems are centralized heavy-weight generic platforms often deployed in a cloud environment [20]. Cloud-based systems are application specific services often tied to a specific product. Actor-based middleware systems are flexible and scalable since they support deployment in different layers. Examples of actor-based middleware are the Ptolemy framework, Node-RED, and Calvin [21; 22; 23]. Actors are reusable function blocks that can easily be deployed on different hardware. Some platforms also allow migration of actors to where and when they are needed, as long as the system supports the needed requirements [24; 25]. This allows computations closer to the network edge, thus allowing for a more decentralized system, known as edge-networks or fog computing [26]. IoT systems are natural aggregators of data, often including data analytics capabilities for Big Data applications. When data volumes grow the system requires technologies that can distribute computing and storage capabilities and send a large amount of data between different nodes. There are several open-source projects that can enable these systems, such as Hadoop, Spark, and Kafka [27; 28; 29] for distributed computing and data streaming. For data storage, the NoSQL databases, which are more flexible than the traditional relational databases, is a key technology [4]. The term NoSQL, which should be read as \"not only SQL\", incorporates characteristics like non-relational, distributed, open-source, and horizontally scalable [30].