# Digital Twin-Driven Virtual Network Architecture for Enhanced Extended Reality Capabilities
## 2 Background and Related Work
### In-Space Communications and Tolerant Networks  
To enable the original network (Earth) and Digital Twin Network (space station) communication, satellite communication systems can be adopted in order to establish links and redundancy. In [35], a flexible satellite communication system, based on NFV and SDN technologies, is addressed and presents advantages such as flexible service delivery, efficient traffic scheduling, enhanced data forwarding, etc. On the other hand, Ref. [36] leverages both the lens of Free-Space Optical Satellite Networks (FSONs) for achieving low latency and longer communication distances, and the combination of temporary and permanent links to improve connectivity. Reference [37] discusses the outage probability in optical satellite communications due to atmospheric conditions. Along with satellite systems, the identification of an accurate transport protocol is essential to achieve ON-DTN communication. Reference [38] introduces an algorithm based on bandwidth and Round Trip Time (RTT) detection at regular intervals to minimize TCP window reduction; on the other hand, ref. [39] emphasizes the TCP failure in interplanetary communications due to high latency and intermittent connectivity due to disrupted links. Then, TCP is a risky approach as is proven in Section 3.1 by Equation (2). In [40], several space transport protocols are discussed. The Space Communication Protocol Specification-Transport Protocol (SCP-TP) is based on TCP with the improvement in window adjustment based on the RTT and asymmetric channel considerations. The TP-Planet protocol adaptively selects the channel rate and deals with intermittent communications. The Bundle Protocol for Delay/Disrupting Tolerant Networking (DTN) architecture introduces the custody transfer concept and leverages bundles for intermittent connections. The Saratoga protocol is based on UDP for IP packets, selective negative ACKs and optimistic data transfer. The Packet Interleaving File Delivery Protocol (PIFDP) is based on encoding/decoding packets assuring that the receiver is able to reconstruct the lossy packet. In addition, Ref. [41] underscores the Delay/Disrupting Tolerant Networking (DTN) architecture as a solution for future satellite networking scenarios and highlights the node's long-time storing capacity, the custody transfer, priority scheduling delivery and congestion controls based on selectively discarding bundles or custody non-acceptance. Despite the accurate ON-DTN transport protocol identification, different techniques can be adopted with the aim to guarantee communication robustness and reliability. In lossy-network scenarios, work [42] proposes the utilization of queue mechanisms to deal with packet congestion, thus increasing delivery ratio, while in [43] is addressed lossy channel and high delay networks by modeling channels as finite First In First Out(FIFO) queues to prevent operational failures. Regarding time-varying networks, work [44] presents an algorithm based on \"Bottleneck Bandwidth and Round-trip propagation time\" (BBR): which was deployed by Google, and enhanced with Deep Learning (DL) techniques to dynamically adjust the algorithm gain. On the other hand, in [45] is discussed the latency reduction by adaptively selecting the channel transport protocol based on the current channel conditions (Bw, latency, packet loss rate, etc.) and machine learning algorithms that consider historical decisions. Reference [46] introduces an analytical method to estimate the delivery time in asymmetric space channels. The file delivery estimation is represented by Equation (1) where: \(T_{File\text{-}delivery}\) is the total time for delivering a file, \(N_{Round}\) is the total number of attempts to transmit the file, \(T_{Bundle}\) is the time to transmit a bundle computed as \(Bundle_{size}/Channel_{rate}\), \(T_{RTO}\) is the Re-transmission Time Out, \(T_{Prop}\) is the channel propagation time, \(T_{Trans\text{-}last}\) is the time to transmit the last bundle and \(T_{Disrupt}\) is the total disruption channel.