# Digital Twin-Driven Virtual Network Architecture for Enhanced Extended Reality Capabilities
## 3 Architecture's Proposed Approach
### DTM Traffic Flows  
Once the supporting architectural entities, modules and functions have been defined, with the objective to clarify the insights about the internal and inter-entity interactions, different traffic flows are introduced. The traffic flows are divided into two different planes: the control plane and the data plane. It is important to underscore that between the DTM Edge Router and the ON Edge Router, a space communication channel must be established; the use of TCP as the transport protocol is not feasible due to the poor bandwidth according to Equation (2). It is crucial to highlight that in this research, a redundancy ACK mechanism comprising a double ACK (ACK + confirmation ACK) is introduced to guarantee that the DTM and ONM entities are fully aware of their partner entity status; basically, the confirmation ACK guarantees that the requested or sent information has been received, as well as the custody ACK (the initial sent ACK). In addition, the custody ACKand the confirmation ACK will not block future transmissions with the aim to optimize communication between the ON and the DTN. 1. Control Plane: The control plane traffic flows are presented next. Such flows describe the DTN control operation including: creation, synchronization, monitoring, re-configuration and status sharing. 1. DTN creation: The process for creating a new DTN once the space station has been established and the DTM and XRM entities have been deployed is initiated by the DTM Modeler, which lacks network models at this initial phase. The modeler generates a \"Request\" message, which is relayed to the Traffic Classifier and then to the Edge Router. Upon receipt of the request by the ONM Edge Router, a custody ACK is sent to the DTM Edge Router. The request is forwarded to the Network Twinning, which processes it and determines that the ON network model has to be transmitted to the DTM (the processing time is referred to as Twinning Time). The network model travels back to the DTM Edge Router. The network model flows through the Traffic Classifier and is received by the Modeler. The Modeler processes the ON network model and subsequently transmits a \"DTN request\" to the Builder, which is responsible for the DTN building for the first time. Figure 9 illustrates the aforementioned traffic flow. 2. DTN synchronization: The synchronization process is initiated by the ONM Event Control, which has the task of periodically monitoring the ON in order to detect any change in the ON; upon identifying a network change, the module generates an \"Event\" message, which contains the specific ON modification. The \"Event\" is sent back to the DTM Edge Router (the double ACK mechanism is established between the Edge Routers) and is relayed to the Traffic Classifier, and then to the Network Synchronizer, which processes the ON modification and forwards the \"event\" to the Modeler. The modeler is responsible for retrieving the stored network model and subsequently updating it according to the \"event\" information (this process time is referred to as Re-modeling Time). Finally, the Modeler generates a \"Net Regeneration\" request that is sent to the Builder. This traffic flow is shown in Figure 10. Note that this paper differs from other Digital Twin Network oriented papers: the synchronization is not requested by the Digital Twin but by the Original Network. 3. DTN monitoring: The monitoring process is essential to ensure autonomous management and DTN self-healing. The DTM Builder has to periodically report its own statistics (packet error rate, throughput, etc.) to the Status Control module. The Status Control module has the capability to access the stored models enabling the detection of any anomaly or failure; in the event of anomaly or failure detection, the Status Control raises a \"Net Warning\" alert, which is sent back to the Builder. When the Builder receives a warning, it needs to request to the Modeler a re-modeling process as described previously. Figure 11 demonstrates the aforementioned flow. 4.