## 3 State of the Art in Digital Twins and Cyber-Ranges
### State of the Art in Digital Twins
The tool was used for a prototypical implementation of the data-driven DT generation approach in [64]. * Ansys developed a DT that can be used to monitor real-time prescriptive analytics and test predictive maintenance to optimize asset performance. The DT can also provide data to be used to improve the physical product design throughout the product lifecycle [87]. * IBM developed a DT framework that helps companies to virtually create, test, build and monitor a product, reducing the latency in the feedback loop between design and operation. It enables identifying and fixing problems and bringing products to market more quickly [88]. * Factory I/O is a software developed by Real Games [89] that allows setting up configurable 3D-simulations by plug in components of a given industrial equipment catalog. To this end, the software provides simulation aspects of digital twins, explicit synchronization between real system and virtual replica is limited to the integration of several Programmable Logic Controller (PLC) for simulating the virtual factory. Siemens offers several services for constructing digital twins, including a machine- human interface [90] that can be used for the construction of a digital twin for humans and a portfolio called Digital Enterprise Suite [91], which includes, e.g., digital twins for material transport equipment. Unlike proprietary products, open-source solutions allow the technology to be freely redistributable and modifiable, supporting manufacturers in combining older equipment with modern sensor-based machines and tools from different vendors. Moreover, open source hardware supports faster prototyping and customization, which helps manufacturers to accelerate the design and improve interoperation across actual lifecycle processes [66]. Although the open source community works hard to develop software and hardware solutions to be applied in Industry 4.0 and smart manufacturing, only few open source DT solutions are available:  
* CPS Twinning is a framework for generating and executing DTs that mirror cyber-physical systems [92]. It is a proof of concept that can be used as first approach to model some environments, but also has some limitations such as inability to generate DTs for wireless devices [93]. * Wrld3d is an open source platform that allows the creation of DTs in a quick and easy manner, using a comprehensive set of self-serve tools, SDKs, APIs and location intelligent services. As a dynamic 3D mapping platform, it allows to create virtual indoor and outdoor environments upon which data from sensors, systems, mobile devices and location services can be visualized within millimeter accuracy [94]. * Mago3D is a platform for visualizing massive and complex 3D objects including building information modeling (BIM) on a web browser. Thus, it is possible to model DTs that creates parallel \"worlds\" in a virtual reality with several sensors [95]. * i-Maintenance toolkit enables to create a DT of an industrial asset in order to obtain information on the status of all components related to the production and maintenance of the industrial process, collect, monitor and analyze life-cycle data. It is composed of a messaging system, a set of adapters to integrate sensor/actuator systems and other software components that are used as a technical foundation for the DT development [96]. * At the intersection between pure proprietary and real open-source DT technology, open source solutions are developed by big companies, which make them limited in scope, due to the commercial interests of the developers [97]:
* Eclipse Ditto is a DT developed by Bosch. It enables the design of DTs in the form of IoT development patterns. It can be seen as an open source foundational layer of Bosch IoT platform [98]. * imodel.js is a platform for creating, accessing, leveraging and integrating infrastructure DTs. As what happens with Eclipse Ditto, it is a commercial initiative connected to the US infrastructure company Bentley.