# Dynamic Routings in Satellite Networks: An Overview
## 4 Existing Works on Dynamic Satellite Routings
### Dynamic Routings in the Single-Layer Satellite Networks
#### 4.1.3 Virtual Topology
The strength of FSA is that it can maximize the use of the network's resources. However, the algorithm also has some limitations. First, the shortest route is not a selection criterion for routes. Secondly, implementing the dynamic link assignment method in LEO satellite networks is difficult. In conclusion, the method is operationally complex. The first study of the multi-path routing problem for satellite networks is compact explicit multi-path routing (CEMR) [46]. The basic idea of CEMR was to utilize the Path-ID path identifier to encode the path. Based on that, a new satellite network model based on a dynamic virtual topology was developed. In the model, when the time interval is small enough, the cost of each satellite link is considered constant over that time interval. The multi-path routing mechanism of the CEMR mainly includes route discovery, route maintenance, and traffic distribution. In the algorithm, the sum of propagation delay and queuing delay constitutes the time delay. In addition, the idea of global path planning of the CEMR algorithm is utilized to achieve explicit multi-path routing for LEO satellite networks. Meanwhile, the CEMR algorithm can reduce the LEO satellite network delay, improve the system throughput, and balance the load since the routing algorithm and grouping policy are not covered. Grouping policies without improvements may generate too many snapshots, and routing algorithms cannot be applied in this case. For that, the work in [47] modified the ant algorithm and combined it with routing methods. The improvement in the ant colony algorithm generally includes two different approaches to an ant system, with an elitist strategy as the elite approach and the rank-based version of the ant system as the ranked approach. However, the rank-based method takes too long to run and fails to adapt to the rapid topology changes in the satellite network. Therefore, this scheme uses ant systems with an elitist strategy as elite to improve the ant colony algorithm. The improved ant colony algorithm shortens the convergence time and reduces the generation of snapshots. In addition, the solution updates only the pheromone concentration on the optimal route to prevent the algorithm from falling into local optimality. On this basis, the global optimal routing path can be obtained by reasonable adjustment of each parameter. The experiments prove that the routing algorithm combined with the grouping strategy has a better performance. However, the VN-based routing scheme is incapable of enabling multiple satellites to serve a single coverage area of the network simultaneously. In [48], a multi-state virtual network (MSVN) topology was designed to address the issue. The topology allowed multiple satellites to cover a single area simultaneously. In addition, the scheme investigated the potential switching mechanisms in fixed satellite systems. Meanwhile, the soft handover algorithm for MSVN-based satellite networks (MSVN-SHO), the VN-HO, and the MSVN-SSHO were proposed. Despite the increased cost, the switching algorithm based on the MSVN system has the benefit of being faster and smoother compared to the VN-HO switching algorithm. The work in [49] presented a routing algorithm based on virtual topology snapshots, namely, the DDRA. The DDRA guarantees the benefits of the topology snapshot algorithm while offering small transmission latency and high throughput rates. In addition, the DDRA can adapt well to network changes and avoid communication delays caused by queuing and link failure. Experiments prove that the DDRA outperforms the traditional topological fast routing and distribution path selection methods. Among the currently available routing algorithms for satellite networks, little research has been conducted on the trade-off between the survivability of routes and the computational overhead of routes. For this reason, a new routing protocol with traffic prediction, namely, the distributed traffic balancing routing protocol (DTBR), was proposed in [50].