# Time-Sensitive Networking Mechanism Aided by Multilevel Cyclic Queues in LEO Satellite Networks
## 4 Time-Sensitive Networking Technique Aided by Multilevel Cyclic Queues
### Priority-Based Multilevel Cyclic-Queue Mechanism  
In order to enhance the forwarding capability of satellite nodes for TSF, we propose a multilevel cyclic-queue mechanism based on the CQF mechanism. As shown in Figure 3, this mechanism includes time-sensitive and non-time-sensitive queues. Non-time-sensitive queues use the same forwarding mechanism as that of the CQF. Time-sensitive queues are multiple, and we assumed that \(Q_{4}\) to \(Q_{7}\) were time-sensitive.Their priority is adjusted periodically. As shown in the figure, at time slot \(T_{0}\), the priorities of \(Q_{4}\) to \(Q_{7}\) were 4, 5, 6, and 7, respectively. At the next time slot, the priorities of \(Q_{4}\) to \(Q_{7}\) changed into 5, 6, 7, and 4, respectively, i.e., the priority increased by 1. In contrast, the highest-priority queue became the lowest-priority queue (4 in the example). Therefore, the period of this priority change was four time slots. We assumed an ISL with \(n\) time-sensitive queues and \(m\) non-time-sensitive queues. The priority of each queue in time slot \(T\) can be expressed as follows:  
\[P(q)= \begin{cases}q,&q\in[0,m-1]\\ (T+q)\%n+m,&q\in[m,m+n-1]\end{cases}  \tag{3}\]  
where \(q\) denotes the queue number. For LEO ISLs, the number of \(q\) can be dynamically configured according to the level of time-sensitive services. The entire orchestration of traffic flows is performed in the SDN controller of the LEO satellite network. Figure 3: Multilevel cyclic-queue mechanism. Figure 2: Priority-based queuing model for CQF (\(\alpha\): open, \(c\): closed). The core principle of the proposed technique is its utilization of a cycle priority mechanism that ensures that all TSFs are forwarded within the allotted time frame. This mechanism works by assigning a priority value to each TSF on the basis of its level of urgency. The cycle priority mechanism then ensures that TSFs are forwarded in the order of priority, with high-priority TSFs being forwarded before those with a lower priority. In summary, the proposed priority-based multilevel cyclic-queue mechanism represents a significant improvement over existing CQF techniques as it provides differentiated forwarding actions for diverse time-sensitive services. Multiqueue cache TSFs with different priorities use a cycle priority mechanism to guarantee that all TSFs are forwarded within their deadlines. This approach enhances system performance by dynamically allocating resources on the basis of the priority level of each service, thereby optimizing the handling of time-sensitive services. In addition to priority scheduling, the inbound scheduling mechanism of services is entirely different from CQF. First, the SDN controller calculates the flow path and then the urgency degree on the basis of the deadline feature of the TSF flow. Traffic flows are mapped to different-priority queues of satellites according to the value of the urgency degree. As shown in Figure 4, if the value of urgency degree is low, meaning that the TSF should be forwarded immediately, the controller queues the flow to the queue corresponding to P(6). Then, it is guaranteed that the service is forwarded in the following two time slots. If the value of the urgency degree of the flow is high, the traffic flow can be inserted into the queue corresponding to P(4). The TSF can be guaranteed to be forwarded in four time slots. Therefore, the method can use different queuing mechanisms for different priority levels. Furthermore, the highest-priority queue, i.e., the queue corresponding to P(7), only sends a packet in a time slot and cannot queue the arrived packets. This gating mechanism can guarantee that, in each time slot, the traffic of the highest-priority queue can be completely forwarded. After completing the calculation, the SDN controller sends configuration messages to each satellite node to configure the look-up table for each satellite.