## 3 State of the Art in Digital Twins and Cyber-Ranges  
### Digital Twin Concept and Definition  
It is a tough task to define what a Digital Twin is. The fact that Wikipedia has nine definitions for digital twins is a clear indicator of this problem. To some authors, a DT is a simulation model that mirrors physical systems and allows simulations; to others it is something that can mirror the state of a real asset, allowing its monitoring, control and alteration as is common in IoT [51] The differences in the definition seem to be related to the focus that each author has in the DT concept. [PERSON] et al [52] gave a comprehensive overview of the DT definitions available in the literature from its first appearance in 2012 to 2016. However, even in recent years the definition of DT still seems to change according to the authors and their purpose. [PERSON]'s team [53] with their focus on industrial process lines, defined the DT concept as the functional system of a continuous process optimization, formed by the cooperation of the physical and the digital production lines. Thus, DT continuously collects and evaluates process information in order to shorten production cycles, accelerate the introduction of new products and reduce process inefficiency. In addition, for [PERSON] et al [54] DTs are related to optimization, with a DT being a \"simulation with ability of real-time control and optimization of products and production lines.\" However, [PERSON] et al [55] saw the DT as a method or tool to be used in the simulation and modeling of the behavior and status of entities. According to [PERSON] et al [56] DTs are deeply associated with the Industry 4.0 development. For the authors, a DT is a one-to-one representation of a real-world element (such as a machine, component or part of the environment) or a real subject (person, software or system). The DT comprises the virtual representation of this element, its behavior and its communication facilities. In their work, the authors presented a simulation technology, based on the concept of \"Experimentable Digital Twins,\" which combine DTs with simulation technologies to bring DTs to life. [PERSON] and [PERSON] [57] developed a DT that assumes the employee communication and coordination tasks with the production system. The usual concept of a DT that emulates the properties and behavior of a system was adapted by the authors to act as a representative for a human employee in a cyber-physical production system (CPPS), since the property and behavior of the human DT need to be based on user feedback and recorded patterns instead of actual measured data. According to [PERSON] et al [58] the DT is the semantic, functional and simulation-ready representation of each shop floor CPS. It can define performance specifications and behavioral and functional models. Thus, it can be said, according to the authors, that the DT is a composite concept that aggregates the following elements:  
* CPS Prototype is a model that defines the structure and the associate semantic for a certain class of CPS. * CPS Instance is a computer-based representation of an instantiation of a CPS prototype. As the DT is an instance of a CPS prototype, it can be said that a CPS instance is a computer-based representation of its DT. * Behavioral Models are simulation models related with the semantic representation of a CPS prototype and instance. Each DT can address different behavioral models to allow multi-disciplinary simulations. * Functional Models allow the analysis of data from the shop floor. The result of the data analysis is used to enrich the DT, for example to enable predictive maintenance. Therefore, as already referred, several definitions of DTs exist in the literature and each author adapts the definition according to their purpose. However, all the definitions have something in common: the simulation of something in a specific environment. The definition presented in the Industrial Internet Consortium White Pape [59] seems to be general and consider all the definitionspresented before.