# Digital Twin-Driven Virtual Network Architecture for Enhanced Extended Reality Capabilities
## 3 Architecture's Proposed Approach
### DTN Topological Architecture  
The DTN represents an ON (physical or virtual) instance. As outlined in Section 1, the ON replication and DTN deployment are hosted by the Digital Twin Manager (DTM) entity. In addition, the DTM is responsible for the DTN's self-management and the ON-DTN communication and synchronization. The synchronization is defined in this paper as an asymmetric process described as follows:  
1. Synchronization from ON to DTN is mandatory: this ensures that any ON modification is reflected in the DTN. 2. Synchronization from DTN to ON is not required: since the DTN is designed to be enhanced with respect to the ON, any modification deployed in the DTN (topology, configuration, services, external connections, etc.) does not affect the ON. 3. The DTN can request resources to the ON via the DTM: for instance, when DTCDs request resources located in the ON, the DTM can request access to such resources and download them in the DTN. 4. Synchronization must be assured even in disadvantaged scenarios: QoS may be compromised in terms of latency, percentage of packet loss, throughput, reliability, etc. However, synchronization is an essential process and must be ensured even in disadvantaged conditions. Regarding network management, the DTM has to guarantee the autonomy of the network management, enabling a \"zero-touch\" network. Figure 1 illustrates the DTN instance; notice that the DTN has been enhanced with extended Reality Services and an external network connection. In addition to the enhancements provided by the XR services and external connections, this paper introduces the DTN's ability to modify the ON's topology within the DTN in order to improve the inner communication and the access to resources. Figure 2 depicts the ON's topology, while Figure 2 depicts the DTN's improved topology; notice that redundant links have been added, while \"Files\" resources have been relocated to \"PC-2\" within the DTN topology. The supporting architecture has to offload the expensive twinning and XR services' provisioning computations from the DTCDs. The proposal presented in this paper exploits data centers' computational capabilities. Figure 3 illustrates the perspective from the DTCDs to access data center services via a standardized API. Basically, a server in the data center needs to host two of the main architectural entities: the DTM which is responsible for DTN building and maintenance, while the XRM is responsible for providing XR network services. The systems' interaction standardization offered by API platforms can be leveraged to facilitate access to these modules. Similarly, on the ON domain, it must be deployed as a supporting entity defined as Original Network Manager (ONM) to facilitate ON-DTN communication and to handle the twinning process for the DTN's creation/updating. The complete network's topological architecture can be observed in Figure 4.  
Figure 1: Proposed DTN + XRF topological architecture. Figure 3: DTN + XRF access from Digital Twin Client Devices. Figure 2: Example of topology improvement in the DTN. Since this paper considers a space station hosting the data center, it is mandatory to consider the space communication scenario. In space communications, several communication constraints arise including latency, high packet loss and packet error percentages, limited bandwidth, link disruptions, jitter, interferences and noise, and synchronization issues. In addition, the _TCP_ protocol is not suitable due to the _TCP_ window increase, which can lead to significant QoS degradation. The _TCP_ window (Mb) equation is introduced next, where _RTT_ is the Round Trip Time (seconds), while _Bw_ is bandwidth (Mbps). \[Bw=\frac{WINDOW_{TCP}}{RTT} \tag{2}\]  
In the aforementioned scenario, it is necessary to consider the _RTT_, which can range from tens of seconds to hundreds of seconds. Consequently, according to Equation (2), the _TCP_ utilization will result in a notable bandwidth degradation. Then, it is fundamental to mitigate the cited communication constraints.