## 3 Digital Twin, from Concept to Architecture Model
### Digital Twin Models, Frameworks, and Architectures  
Several DT models, frameworks, and architectures have been proposed in recent years in order to structure different perspectives and facilitate common understanding. Taking into account the application domains, different approaches (some of which are presented below) can be identified. DT models can be analyzed from the perspective of a combination of models and algorithms; therefore, certain aspects can be treated differently. [PERSON] et al. [18] proposed a reference model based on physical objects, the Digital Twin, and implicitly the level of communication between the two parties. [PERSON] et al. [19] demonstrated that peer-to-peer data transmission between a physical and virtual counterpart can be achieved through CPS, and [PERSON] et al. [20] presented a reference model of DTs for risk reduction in processing facilities. Being in a continuous development process [21], ISO 23247-2 [22]focuses on providing a general architecture for the entities and domains of the DT reference structure in the production area [23]. Table 4 shows a comparison of the results proposed in [18; 19; 23] based on a set of characteristics and criteria. An architecture model containing a general description of the levels defining the Digital Twin was proposed by the German Association of Electrical and Electrotechnical Manufacturings [23] and aims to develop a general framework that defines business models and products [21]. The reference architecture addresses the fundamental structure of the process, including the design, development, and implementation phases, and can be used in Industry 4.0 applications. In Figure 8, a unifying model is presented. The model is based on the reviewed architectures, following the guidelines from [23]. Digital Twin components are highlighted, and the relation between the physical entities is detailed. The first level is represented by the level of abstraction within the Digital Twin (the physical, communication, digital, and cybernetic layers of the application). The physical layer contains real-world objects. The communication layer contains elements that enable communication between physical and digital entities. The digital layer contains an abstraction from the real world. Data security elements were included at the cyber layer level. The application layer contains elements that favor connections with other Digital Twins. In addition, elements have been introduced that highlight the service process evolution life cycle. It contains iterative improvements that are brought to the process/service throughout its life at various levels. For example, for a product/service, improvements can be made during the life cycle at the cyber layer to secure and encrypt the data. The Agile methodology is used to introduce these small improvements, which are carried out at short time intervals. The integration dimension of the Digital Twin is brought to the fore (Digital Shadow, Digital Model, Digital Twin, or Predictive Digital Twin). The Digital Model involves making a digital copy of the physical object; between the two entities, there is only an offline synchronization of behaviors. In the case of digital shadows, the digital component reflects in real time the changes that occur on the physical object, but it can only influence