## 2 State of the Art
### Metamodelling Protocols  
Regarding communicating data, several authors have dealt with the concepts of CPS and the Internet of Things (IoT) [38]. The definitions could be:  
* _\"Cyber-physical systems are the integration of computation and physical processes. Embedded computers and networks monitor and control the physical processes, usually with feedback loops where physical processes affect computations and vice versa.\"_[39]
* _\"A global infrastructure for the information society, enabling advanced services by interconnecting (physical and virtual) things based on existing and evolving interoperable information and communication technologies.\"_[40]  
However, communicating data on their own is inadequate to address digital twins' formation since workflows have to be addressed, both technically and in a business sense. It is noted that a variety of protocols and standards have existed for similar reasons; for instance, CPS architecture follows the \"5C\" protocol [41]. A list of standards and classifications would also include international standards ISO 25.XXX for manufacturing processes (e.g., 25.020 for forming), standards of American society for testing and materials ISO ASTM 52900 for additive manufacturing (AM) processes, international electrotechnical commission IEC 61360 classification and EXPRESS, ISO 13584-42: Parts library--Part 42: Description methodology: Methodology for structuring part families, and other standards related to CPS (i.e., ISO 13374 and IEC 61508) [42], specific standards for processes such as ISO: 6520-1 for welding, and standards for technologies implied in the digital twin implementation [43]. Additionally, data manipulation standards and formats have to be mentioned. It is noted that the start towards a holistic approach was made with Computer-Integrated Manufacturing (CIM) [44]. Furthermore, application is highly relevant; indicatively, in an electric vehicle (EV), where different international standards exist such as the IEC 62196 and IEC 61851 regarding charging. The concept of metamodelling is not new, either. The Manufacturing Automation Protocol (MAP) [45] has previously been proposed and Specific Communication Service Mapping (SCSM) [46] has been utilized, while there are standards related to Manufacturing Message Specification (MMS) (IEC 61850-8-1:2011 and ISO 9506-6 [47]), as well as to the Simple Network Management Protocol (SNMP) [48]. More recent works haveinvolved working on the \"ideal of automating production optimization\" [49], while a review of languages [50] has been made, highly useful for holistic metamodeling. More pieces of literature on metamodelling include BPMN standardization [51], automation standardization [52] and coexistence standardization [53]. Both MMS and SNMP are based on Abstract Syntax Notation One (ASN.1), a network management language, which has many implementations, i.e., the one based on extensible mark-up language (XML), known as XER format [54]. Moreover, virtual manufacturing devices [55] have been used on the physical aspect, even leading to commercial applications [56]. Interoperability [57] could be addressed in IoT and CPS utilizing SNMP in manufacturing [56], or a hybrid approach of SNMP and MMS [58]. Additional concepts that could be used are related to the technology classification. The Remote Monitoring Management information base (RMON MIB) (1 and 2) [59] offers a tree-like structure and this is a highly useful approach for the current case. Additionally, it is mentioned that concepts such as object-oriented modelling [60], Process Specification Language (PSL) and Unified Modeling Language (UML) [61], as well as languages such as \"Monitor-Analyze-Plan-Execute over a shared Knowledge\" (MAPE-K) [62] can be used at different phases of a digital twin (operation and design), while the concept of orchestration [63] is highly relevant.