# Digital Twin-Driven Virtual Network Architecture for Enhanced Extended Reality Capabilities
## 2 Background and Related Work
### Extended Reality  
The extended Reality (XR) term comprises a diverse set of technologies that facilitate the expansion of traditional reality information. Among these technologies are included: Virtual Reality (VR), defined as an interactive virtual environment; Augmented Reality (AR), which augments the physical world with virtual information; and Mixed Reality (MR), which combines both VR and AR technologies [12]. One of the pioneers of addressing the concept of Augmented Reality was [13] in 1992 and it was introduced in the context of airliner manufacturing envisioning that workers are able to access augmented digital information stored as CAD files in real time, enabling an immersive object visualization. The steps for deploying immersive applications are: sensing, environment tracking, digital content rendering, alignment and registration, visualization and interaction, and synchronization. By means of AR, the user's visual field is augmented with relevant, necessary and dynamically changing information to facilitate task comprehension. Virtual Reality was presented for the first time in 1965. VR is defined as a realistic, immersive and responsive experience that enables users to interact with a virtual environment involving vision, audition, smell or touch senses to give the perception of being in a real environment; this is achieved by means of headsets, gloves, glazes, etc. VR's primary objective is to provide realistic and immersive simulations and reduce the production times in industries by facilitating workers' collaboration and communication [14]. To deploy XR systems, in [15] is presented a framework to facilitate the physical-virtual environments' integration comprised of an agent controller, virtual environment layer, physical environment layer, user interface and data synchronization mechanism, while in [16], the importance is emphasized of ensuring the synchronization state of the virtual environment and communication among clients to achieve an improved collaborative experience. On the other hand, paper [17] proposes an architecture based on cloud infrastructures, tailored Software Development Kit (SDK) environments and a core engine which is in charge of data acquisition, management, algorithm deployment and interface enablement. A different approach that can be leveraged in virtual environments is presented in [18], which consists of Inverse Augmented Reality (IAR) that leverages agents in the virtual domain. In this research paper, the XR functionalities are intended to be deployed in a Digital Twin Network environment; the supporting framework is presented in Sections 3.2 and 3.3. XR has the potential to enhance virtual environments. In [19], it is stated that XR can introduce a DT's interactive visualizations, immersive DT model control, collaborative model visualization and interaction, among others. In [20], the DT-XR resource-constrained scenario is introduced, which can be addressed with different technologies including: cloud/edge computing, data compression and resource allocation based on AI techniques. In [21], a framework which integrates DT with VR is presented and it is constructed in a cloud infrastructure, with sensor interfaces and a data analysis pipeline based on AI techniques. Reference [22] discusses the potential of integrating DT with XR for enhancing maintenance and fault detection processes with novel applications such as healthcare, education, urban planning, etc. Reference [23] analyzes the cyber-security enhancement; since the integration of digital and physical environments incurs blurred boundaries, leveraging DT and XR provides a unified reference framework. Reference [24] presents a framework composed of a DTN, XR service management and a resource allocation entity. References [25; 26] present similar frameworks based on layers, with the XR and DT layers being the core of such frameworks. While the DT layer enables real-time monitoring, the XR layer is intended to manage the immersive services, based on NFV technology.