# Dynamic Routings in Satellite Networks: An Overview
## 4 Existing Works on Dynamic Satellite Routings
### Dynamic Routings in the Single-Layer Satellite Networks
#### 4.1.2 Virtual Nodes  
In the initial state of the satellite network, the virtual node strategy refers to the projection point of each satellite as a virtual node. Each virtual node has an individual identity. The identity of the virtual node closest to the satellite is used as the identity of the satellite when the satellite is in motion. The identity varies with the satellite motion to shield the satellite network's topology dynamics. The core idea of such a routing algorithm is to set a fixed logical address for a virtual satellite node. When the satellite moves to the location of the virtual node, its logical address becomes the logical address of this virtual node. As we know, the concept of virtual nodes was first presented in [33]. Numerous researchers have developed their routing schemes based on the approach. Table 5 summarizes the works related to the virtual node-routing algorithms. The distributed routing algorithm (DRA) based on satellite networks was proposed in [34]. The algorithm takes the polar orbit constellation as the research object, adopts its spatial symmetry to partition the surface space into several specific areas, and sets the corresponding logical spots. Since the polar-orbiting constellation is symmetrical, there will inevitably be a satellite within its range of movement that matches its logical loci, thereby avoiding its high dynamics. However, the method requires satellites with high computing and storage capacity. The routing path generated by such a routing algorithm is \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Network Type** & **Reference** & **Proposed Algorithm/Scheme** & **Main Contributions** \\ \hline GEO Satellite network & [29] & A extended range routing algorithm & Considered two new metrics, reliability and routing tables \\ \hline  & [27] & A Unicast routing algorithm similar to Mobile IP & Provided low latency and high-quality video delivery \\ \hline  & [28] & A routing algorithm based on geographic information & Reduced network signaling overhead significantly \\ \hline  & [32] & A geographical routing scheme & Reduced end-to-end network latency and reroute rates \\ \hline  & [30] & An addressing scheme based on geographic IP & Reduced the number of abnormal users, decreased routing costs \\ \hline  & [31] & An improved geographic addressing scheme & Reduced system packet loss \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The summary of the works related to the topological planarized routing algorithms. not necessarily the optimal route based on the global network topology state information. Multiple shortest paths may exist between two satellites in a satellite network; therefore, determining the optimal shortest path is essential for maximizing network resources. A priority-based adjustable routing (PAR) was proposed in [35]. The approach employs link utilization and historical data to balance the load. PAR takes a distributed routing approach based on the utilization of ISLs and cached data. In addition, the scheme further proposes the enhanced PAR (e-PAR) algorithm to prevent redundant traffic data and better utilize ISLs. The PAR algorithm selects the link with minor output usage. Through an in-depth study of the e-PAR algorithm, the scheme can enhance the stability and performance of the system by adjusting the relevant parameters of the algorithm. The virtual node (VN) approach is used to eliminate the network layer switching problem in fixed LEO satellite systems. However, the physical network is dynamic, and the underlying switching can lead to large packet loss. Therefore, designing a switching algorithm with low packet loss and latency is a crucial issue. Adopting a suitable switching strategy can significantly enhance the performance of a system, particularly in satellite networks with extensive delays. The work in [36] presented a virtual network-based satellite link layer switching algorithm, namely, the virtual node handover operating (VN-HO) algorithm.