# Digital Twin-Driven Virtual Network Architecture for Enhanced Extended Reality Capabilities
## 4 Architecture Modeling Approach
### System Metrics  
In this section, relevant system metrics identified to assess the reliability of the proposed system will be introduced. 1. DTN resilience and Failure Protection: This refers to the metrics employed to measure the Failure Protection capability. This measurement will be approached with commonly used metrics including Mean Time to Recovery (MTTR), which is the average time taken by networks to recover from failures, and the Mean Time Between Failures (MTBF), which is the average time between successive network failures; both metrics are measured in seconds. Furthermore, it must be noted that in the ON-DTN communication disruption, the DTN remains operational. With regard to failure protection, the monitoring process is essential to heal the DTN's operation. 2. Response Time: This refers to the response time between Edge Routers after receipt of a request and it is measured in seconds. The response time is dependent on the channel conditions (bandwidth, disruption time, etc. ), and the Edge Routers' channel selection capability. Response time is defined by Equation (3) where \(T_{edges}\) is the processing time at the Edge Routers, \(T_{prop}\) is the propagation time and \(T_{tx}\) is the  
Figure 16: Reading operation of an XR service. Figure 17: Writing operation of an XR service. transmission time. Note that \(T_{edges}\) and \(T_{tx}\) may vary according to the type of request and the traffic involved, while \(T_{tx}\) is dependent on the Edge Routers' distance. \[T_{response}(s)=T_{edges}+T_{prop}+T_{tx}\] (3)
3. DTN Synchronization Time: This refers to the necessary time to achieve model synchronization between the ON and the DTN and it is measured in seconds. Synchronization Time is dependent on the response time metric and the complexity of the network models employed. The synchronization time is defined by Equation (4), where \(T_{response}\) is the response time and \(T_{model}\) is the time to process the network model according to the model's complexity. \[T_{SYNC}(s)=T_{response}+T_{model}\] (4)
4. Zero Touch Capability: This defines the level of zero touch capability; in [51], several metrics are employed including automation percentage, operational cost, Quality of Service (QoS) and so forth. The advantage of leveraging the DTM's capabilities is that it introduces a high autonomy tier in the DTN; to guarantee the autonomous DTN operation, each DTM sub-module must be provided with enough intelligence. In the case of network re-configurations in the DTN, a DT super user is necessary to introduce those modifications; however, the re-configuration and re-building process will be autonomous. In a similar fashion, the XR Service management is autonomous by means of the XRM entity, 5. DTN scalability: This qualitative metric allows us to realize how the DTN can be scaled up or down and enhanced. The XRM entity enables the DTN's enhancement with novel and immersive functionalities; those DTN-enhanced capabilities are initially not present in the ON. The DTN can be easily scaled up or down topologically by means of the re-configuration process to modify the DTN network model. Similarly, with the re-configuration DTN process, external gateways can be added to the DTN network model with the purpose to provide the deployed DTN connection to external networks 6. System Complexity: This refers to the number of operations that are necessary to execute in the supporting entities to achieve an optimal system operation. In addition, with regard to the entities visualized in Figures 6-8, it can be appreciated that the supporting elements are relatively simple: such entities do not require a large amount of modules, while the modules' behavior and objective are clearly established as well as modules' interactions. 7. DTN portability and Deployment Efficiency: This is a qualitative metric to realize the ease of the DTN's deployment; the DTM's behavior provides a high level of ease of deployment.