# Dynamic Routings in Satellite Networks: An Overview
## 4 Existing Works on Dynamic Satellite Routings
### Dynamic Routings in the Multi-Layer Satellite Networks
#### 4.2.2 QoS-Based Dynamic Routings
\begin{table}
\begin{tabular}{c c l l} \hline \hline
**Network Structure** & **Reference** & **Proposed Algorithm/Scheme** & **Features/Advantages** \\ \hline \multirow{8}{*}{MEO-LEO} & [72] & A new dynamical hierarchical distributed QoS & QoS metric information composed of delay and bandwidth \\  & [74] & A sustainable heuristic QoS routing algorithm & Utilized heuristic algorithms to optimize routing \\ \cline{2-4}  & [77] & An adaptive QoS dynamic routing & Collected QoS status between the link from the source node to the destination node, set QoS parameters based on specific satellite scenarios \\ \cline{2-4}  & [86] & A routing algorithm based on priority and probability & Low-priority services forwarded to MEO satellites, considered both bandwidth and latency QoS parameters \\ \hline GEO-MEO-LEO & [73] & A multi-QoS optimization routing & Effectively balanced network traffic load, avoided the constraints of onboard network resources \\ \hline GEO-HEO-LEO & [75] & A QoS-aware transport routing & Build multi-cast trees with the LCT approach \\ \hline \multirow{8}{*}{GEO-LEO} & [78] & A routing algorithm based on broadband services & Related weights are introduced, improving communication quality \\ \cline{2-4}  & [79] & A QoS-aware routing scheme & Used the link throughput and end-to-end delay as path cost factors; the GEO and the LEO satellite layer are relatively independent \\ \cline{2-4}  & [82] & A QoS routing scheme with weights & Adopted feature vector to set weights for QoS state values, balanced the network traffic \\ \hline DTN-deployed satellite networks & [80] & A new QoS-enabled routing strategy & Utilized STAG to implement a routing scheme \\ \hline STAG-based mission model & [81] & A QoS multiple routing algorithm & Considered QoS for multiple transport tasks; flow and latency requirements are accounted for \\ \hline BRSN reputation model & [83] & A dynamic QoS routing scheme & Optimized heuristics from a holistic perspective, scheme included route lookup and routing table update \\ \hline \hline \end{tabular}
\end{table} 
Table 9: The summary of the works related to the QoS-based dynamic routings. bandwidth utilization based on service priority. The work in [90] proposed a two-step scheduling technique to provide fairness, QoS, and performance for satellite terminals. However, these scheduling techniques have two issues. First, the system cannot identify the priority of each QoS. Second, the channel state cannot be monitored. Therefore, a scheduling strategy must provide QoS-based distinction.The packet scheduler can determine the QoS priority depending on the channel status of each target user. To improve the performance of the digital video broadcasting-satellite second generation extension (DVB-S2X) satellite platform, the authors proposed two new two-stage scheduler (TSS)-method scheduling algorithms [91]. The first stage of the TSS uses an adaptive priority scheduler (APS) to meet QoS requirements and better distribute IP packets in the MAC frame queue. An improved version of proportional fair (PF) queuing, called PFPP, is considered in the second stage. The approach decreases packet data, improving the packet drop rate, E2E latency, and delay jitter. The approach uses distinct scheduling pairs in the first and second phases of the scheduler to evaluate the system's packet loss rate, packet latency and delay jitter. The scheduling scheme improves packet drop rate, delay jitter, and packet delay, according to simulations. In particular, the method balances QoS, link traffic allocation, and throughput in a satellite environment.