## 2 Queue Scheduling and Optimization Mechanism of Large-Scale LEO Satellite Communication Networks  
### CQF Mechanism  
The CQF mechanism adopts the gating method from the TAS mechanism, adjusting the state of queues to achieve the forwarding of time-sensitive traffic. CQF utilizes two queues for forwarding time-sensitive service traffic; in a CQF slot, packets in the Open-state queue are forwarded to the next node, while incoming packets are directed to the queue in the Close state. The status of the two queues cycles periodically with the progression of the slot. Within a slot, all packets in the Open-state queue must be completely forwarded.  
If the slot duration is \(T_{c}\), the propagation delay is \(T_{D}\), and the number of hops for end-to-end packet transmission is \(h\); disregarding packet processing delay, the maximum end-to-end transmission delay is then \(D_{max}=(h+1)T_{c}+T_{D}\), and the minimum delay is \(D_{min}=(h-1)T_{c}+T_{D}\). The transmission mechanism of CQF and its maximum and minimum delay jitter are shown in Figure 3.  
Figure 3: (a) Transmission mechanism of CQF in large-scale LEO satellite communication networks. (b) Maximum and minimum delay jitter.  
The CQF mechanism requires that all nodes in the network must complete slot synchronization and strict alignment of slots, and the sum of transmission delay and propagation delay in the network must be less than the length of the slot. To apply the CQF mechanism to large-scale LEO satellite communication networks, this study utilized large-scale deterministic network forwarding technology (LDN). AnLDN assigns a slot ID to each slot at the nodes. In an LDN, when a packet leaves a node's output port, it carries the sending slot ID of that node. When the downstream node receives this packet, it maps the upstream node's sending slot ID to its own sending slot ID, and then decides which queue the packet should join based on the sending slot. Therefore, with each node using LDN technology, the processing time is roughly the same, and the need for precision in time synchronization is reduced.  
The length of the CQF queues significantly impacts the forwarding performance of time-sensitive traffic [15]. When CQF queues are too short, although the single-hop queuing delay for traffic transmission is minimal, there is not enough space within the switch ports to buffer more incoming flows, leading to packet loss due to memory overflow. When CQF queues are too long, the single-hop queuing delay for traffic transmission becomes excessively high. This results in an overly long end-to-end transmission delay for the flow, potentially making the flow unschedulable and worsening the network system's schedulability performance. When there is a high load of time-sensitive traffic on the switch and the maximum tolerable end-to-end transmission delay for various services is short, the traditional CQF mechanism struggles to meet the transmission needs of time-sensitive traffic. Therefore, it is necessary to analyze and optimize the selection of queue lengths and numbers according to the specific scenarios of the network. The influence of the CQF queue length on transmission effect is shown in Figure 4.