# Time-Sensitive Networking Mechanism Aided by Multilevel Cyclic Queues in LEO Satellite Networks
## 3 LEO Satellite Network Management Architecture
### System Model  
An LEO satellite network model consists of \(N\) disjoint orbital planes. Each plane contains \(M\) satellites with an angle of \(2\pi/M\). All of the planes are in inclined orbits with the same angle. ISLs are divided into two types, inter- and intraplane ISLs. Interplane ISLs operate in an unstable state with two planes moving, while intraplane ISLs maintain a steady state. In our system model, the satellites had four ISLs: two intraplane and two interplane ISLs.  
We considered an LEO network undirected graph \(\mathbb{G}=(\mathbb{V},\mathbb{E})\), where \(\mathbb{V}\) denotes the satellite nodes, and \(\mathbb{E}\) is the ISLs. Then, we defined \(e_{a,b}=[v_{a},v_{b}]\in\mathbb{V}\) as the ISL connecting nodes \(v_{a}\) and \(v_{b}\). The predictable and periodic trajectory of the satellite was considered in a temporal graph model (TGM) to improve the satellite routing performance. There are many studies on the topological description model using periodic characteristics in satellite networks [24; 25]. The basic idea of a TGM is to divide the system period into discrete periods denoted by \(\{t_{1},t_{2},...,t_{m}\}\). Thus, all topological states in the system period can be represented as a set of static topology graphs; thus, \(\{G_{1},G_{2},...G_{m}\}\), with the subgraph at \(t_{i}\) denoted by \(\{\mathbb{V}_{i},\mathbb{E}_{i}\}\). The evolving topology comprises a series of static topological graphs that can be transferred periodically from one graph state to the following graph. The TGM accurately describes the topology at any point in the period. Therefore, we used this model to describe the satellite topology. In this model, we defaulted the routing policy for all traffic to the shortest path. If multiple shortest paths existed, a random path was selected.  
Figure 1: LEO satellite network management architecture.  
For the CQF mechanism of LEO satellite networks, eight queues, \(Q_{0}\) to \(Q_{7}\), were adopted for each ISL or satellite-terrestrial link (STL). Moreover, \(Q_{6}\) and \(Q_{7}\) were used for the TSFs. The TSF is denoted by \(f^{*}\), while regular traffic is denoted by \(f\). The \(i\)-th TSF can be represented by  
\[f^{*}_{i}=\{src,dst,ddl,period,band,\overline{path},\overline{L_{prop}}\} \tag{1}\]  
where \(src\) and \(dst\) are the source node and destination node, respectively; \(ddl\) is the deadline time for the service; \(band\) and \(path\) represent the flow's average bandwidth requirement and path, respectively. The \(\overline{path}\) includes many links denoted by \([v_{i},v_{j}]\), namely, links between nodes \(v_{i}\) and \(v_{j}\). The length of \(\overline{path}\) is \(p\). \(period\) is the packet period of \(f^{*}\). In this model, we defined each flow's degree of urgency denoted by \(U\). \(U\) denotes the ratio of the flow deadline to the path length given by  
\[U=(ddl)/p-L_{prop}, \tag{2}\]  
where \(L_{prop}\) denotes the average propagation latency of ISLs. Due to the ISL's periodic mobility, the propagation delay of each ISL at a specific time can be predicted by ephemeris. However, multiple flows in a node could cause contention, since the time-sensitive bandwidth of a satellite is limited, which makes the flow run out of service time. Since satellite networks accommodate an increasing amount of IoT traffic and increasing number of industrial control scenarios, TSF is likely to become the leading service of LEO satellite networks.