# Digital Twin Technology-Based Networking Solution in Low Earth Orbit Satellite Constellations
## 5 Networking Protocol Based on Digital Twins
### Topological Twinning  
In order to address the constellation scenarios associated with different orbit altitudes (orbit stratification) and constellation configurations (near-polar orbit, inclined orbit), this research focuses on generating a physical world representation of low Earth orbit satellite movements and their relative positional relationships through orbit calculations. It investigates twin network traffic steering techniques, wireless channel characteristics, and the transformation and mapping of network layer features. These efforts aim to accurately characterize the topological connectivity, dynamic latency, and bandwidth of the constellation network, ultimately achieving the simulation of the real target network's topology and link characteristics. Specifically, there are two types of twin simulations: topological twinning and link twinning. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Comparison Items** & **Container** & **KVM Virtualization** \\ \hline Startup Speed & Milliseconds to seconds & Minute level \\ Deployment Mode & Container + application & Operating system + application \\ Running Location & On hypervisor & On host operating system \\ Image Type & Container image & Virtual machine image \\ Image Size & Small (MB or even KB) & Larger (GB level) \\ Performance & Weaker than host machine & Close to host machine’s local process \\ Resource Utilization Rate & Low & High \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the characteristics of containers and KVM virtualization. The topological twinning [28; 29] implemented in this study includes a static topology and dynamic topology. A static network topology refers to a network composed of multiple nodes, where the attributes of links and their connectivity remain unchanged. In contrast, in a dynamic network, the attributes of links and their connectivity change over time. For a given scenario, the static network topology can be decomposed into a collection of nodes, ports, and links, while the dynamic network topology can be decomposed into a collection of static network topologies at different time instances. To achieve a highly realistic simulation of the low Earth orbit constellation network topology, the dynamic topology simulation adopts the design principles of software-defined networking (SDN). It separates the control plane from the forwarding plane and utilizes the orbit information of the real constellation obtained through STK software simulation as input. It precomputes a collection of network topologies within a time period. When the simulation reaches a moment at which the network topology changes, the topology information at that specific moment is dynamically parsed and deployed to the forwarding plane through flow tables, enabling the dynamic variation of the low Earth orbit constellation network topology, as shown in Figure 4.  
Due to the periodicity and predictability of the low Earth orbit constellation network topology, the real-time positions of each satellite at the simulation time can be calculated based on the two-line element sets (TLEs) of each satellite in the low Earth orbit. By utilizing the position information of the satellites, the visibility between any two adjacent satellites, i.e., the connectivity status of inter-satellite links, can be determined. If the inter-satellite link is not visible, the connectivity status of the underlying platform's links is examined. If the link is already connected, it is disconnected. If the link is not connected, its status remains unchanged. If the inter-satellite link is visible, the distance between adjacent satellites is calculated, and the link delay between any two low Earth orbit satellites at the simulation time is determined. The connectivity status of the underlying platform's links is checked. If the link is already connected, it remains connected, and only the link delay needs to be reset. If the link is not connected, it needs to be established, and the current link delay is set.