## 3 Materials and Methods
### CQF Scheduling Mechanism  
For time-sensitive operations, utilizing a periodic cyclic scheduling and forwarding mechanism is a key technology for deterministic transmission. The satellite's physical layer responsible for forwarding functions includes TSN switches that can handle time-sensitive business transmissions. Leveraging the characteristics of these switches, gate scheduling based on odd-even time slots with time-protected segments is employed for the transmission of time-sensitive operations.  
In the CQF model, TSN switches are configured with a CQF scheduling mechanism that includes two queues (even and odd). The CQF mechanism operates using a single time slot \(d\), with a gate of length \(d\) cyclically alternating its operation within each time slot. In each time slot of length \(d\), one queue receives data, while the other queue transmits data [30]. Furthermore, the timer module in the SW is utilized to control the opening and closing of transmission gates on the forwarding ports.  
In some cases, TSN packets might be transmitted behind lower-priority traffic, and if lower-priority packets are too large, they may not meet the latency requirements of time-sensitive operations, potentially leading to congestion in time-sensitive operations.  
To address this issue, this paper proposes to allocate a period \(d_{offset}\) (typically equivalent to the transmission time of a standard Ethernet frame) before the scheduled transmission of time-sensitive operations, during which no lower-priority traffic can be transmitted.  
Figure 5: Long-distance optical links and the division of primary and secondary parallel subnets.  
The time axis for gate-controlled switching in CQF forwarding within a single satellite is illustrated in the Figure 6.  
Assuming the timer starts counting at \(T_{0}\), at this moment, Queue 1 in the SW opens for data transmission and closes for data writing, while Queue 2 opens for data writing and closes for data transmission. After a time period \(d\), when the timer module reaches the threshold \(T_{0}+d\), the states of the two queues are flipped: Queue 1 opens for data writing and closes for data sending, and Queue 2 closes for data writing and opens for data transmission. Therefore, the time point \(T_{transfer}\) for the switch in the gate opening and closing transition becomes  
\[T_{transfer}=T_{0}+d+d_{offset} \tag{2}\]  
Based on the key time points in the CQF gate scheduling diagram provided above, the switch in the previous hop satellite, upon receiving frame \(f\) in time slot \(d_{i}\), will definitely be able to forward the arriving frame \(f\) to the switch in the next hop satellite in time slot \(d_{i+1}\). Therefore, the maximum delay \(D_{\text{max}}\) is from the beginning of the first time slot to the end of the last time slot, plus the width of the guard band, as follows:  
\[D_{\text{max}}=2d+d_{offset} \tag{3}\]  
The minimum delay \(D_{\text{min}}\) is from the time node at the end of the starting time slot to the time node at the beginning of the next time slot, as follows:  
\[D_{\text{min}}=0 \tag{4}\]  
Therefore, it can be inferred that the range of frame forwarding jitter \(T_{shake}\) on a single satellite is  
\[T_{shake}=0\,\sim 2d+d_{offset} \tag{5}\]  
Furthermore, it can be further deduced that the delay \(D_{transfer}\) of forwarding utilizing the CQF cyclic gate control mechanism in a multi-hop satellite network can be expressed as  
\[D_{transfer(\text{max})}=(h+1)\cdot d+h\cdot d_{offset} \tag{6}\]  
\[D_{transfer(\text{min})}=(h-1)\cdot d+(h-1)\cdot d_{offset} \tag{7}\]  
In the equation, \(h\) represents the number of hops in the satellite network.  
Based on the odd-even cycle gate scheduling, by combining the data frame guard band and cyclic forwarding mechanism, the forwarding delay of a single satellite becomes predictable and controllable. When extended to the entire network, it enhances the overall predictability and controllability of the satellite network transmission delay.  
Figure 6: Key time points in CQF gate scheduling.