# Dynamic Routings in Satellite Networks: An Overview
## 4 Existing Works on Dynamic Satellite Routings
### Dynamic Routings in the Multi-Layer Satellite Networks
#### 4.2.3 Traffic Balancing Dynamic Routings  
Traffic-balanced dynamic routing is similarly a key research topic in multi-layer satellite networks. Such routings can enhance the throughput of the entire satellite network and decreases the packet loss rate of the network. As the satellite network topology is predictable and periodic, the satellite can predict traffic congestion and adjust its routing strategy. Table 10 summarizes the works related to the traffic balancing dynamic routings. The hierarchical QoS routing protocol (HQRP), designed in [92], was the first proposal for multi-layer satellite networks. On this basis, LEO satellites are responsible for modifying routes, while the MEO satellites are distributing routes, thus enabling the rapid upgrading of routes. However, HQRP assumes that the ISLs of the inter-satellite links between LEO layers are independent of each other. Since MEO satellites are charged with the communication of the entire satellite network, there are significant constraints on the operational efficiency of the system. Multi-layer satellite networks can be improved by combining the benefits of satellites at different altitudes. In MLSN, packets were transmitted from the information source to their destination through each node of the space network. Any changes between nodes in the satellite network would affect the flow control of the upper system. Table 11 shows the the layers' performance factors, technologies, and mechanisms. Therefore, the design of the MLSN should also consider the cross-layer optimization principle and adjust the initial weight of different service links. Therefore, the work in [93] presented the multi-layered satellite IP Networks algorithm. The scheme employs MEO satellites for LEO topology design and GEO for LEO routing, reducing communication costs and algorithm complexity. However, the dispersed MEO satellites prevent LEO satellite links from receiving real-time dynamic topology information, causing congestion. Distributed scheduling is the best load-balancing strategy for each region. Moreover, the algorithm is only for instantaneous communication for local and adjacent satellite networks. When a group of satellites is subjected to a large amount of traffic simultaneously, it is likely to cause local congestion in the network. Thus, the work in [94] introduced an explicit load balancing (ELB) routing strategy to handle local traffic congestion. In the routing scheme, satellite nodes only obtain the traffic state of their neighbors to route the global network. However, the approach has not explicitly proposed an optimal traffic allocation strategy to avoid link congestion problems, and the ability to solve severe network congestion is greatly limited. Inspired by traffic lights, the congestion status of satellite nodes can be indicated by the status of traffic lights, similarly [95]. While delivering packets on a predetermined path, the satellite link can dynamically adjust the network routing based on the real-time colors of the data center traffic lights. Each packet can eventually obtain a near-optimal transmission path by planning and dynamically adapting routing paths in real-time. The approach takes full advantage of the real-time queuing delay of the satellite link and the congestion between the current and next-hop satellite nodes. The packet loss rate and queuing delay are diminished, and the transmission efficiency of the network is enhanced. However, such a method avoids the link congestion by sending packets to inactive satellites, leading to link congestion on other satellites. The work in [96] suggested a routing protocol to balance delay-sensitive, video application, and other transmission traffic. Latency-sensitive traffic has the highest priority and can be transmitted first when passing over a congested link. Images transmitted on LEO satellites are detoured when passing through congested areas. Other data can be transmitted by GEO satellites.