# Link-State Aware Hybrid Routing in the Terrestrial-Satellite Integrated Network +
## 5 Simulation and Analysis
### Satellite Routing Performance  
#### 5.2.1 Transmission Delay  
Since the congesting satellite may lead to significant queuing delay, in the TSR method, packets would be transferred with more hops than traditional routing algorithms to alleviate congestion and guarantee low communication delay. To verify the effectiveness of the TSR method, the average end-to-end delay is presented in Figure 7. The delay of DSP is the highest among all simulated methods, which is mainly because it always transmit data through the shortest path and long queuing delay begins to appear at the selected path with heavy loads. In SSLB, the traffic of the congestion node is partially diverted to the neighbor node, which may aggravate the load of the neighbor node and cause new link congestion. As a result, some packets may aggregate at some satellites and finally increase the end-to-end delay. In the NSR method, the shortest path is estimated according to the transmission delay, then each intermediate node could adjust the routing path according to the real-time status of neighbor satellites. With the increase of congestion nodes, the number of re-routing process is rising, so that the end-to-end delay of NSR also rises. Since TLR dynamically detours traffic to alternative paths in response to congestion, the delay is substantially lower than the DSP, SSLB, and NSR. However, as the propagation delay of TLR is constant, the packets may be sent through a route with high propagation delay, so the end-to-end delay is higher than the TSR method. To this end, the TSR is proven to be effective in alleviating traffic congestion and delay reduction.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Values** \\ \hline Orbital altitude & 1000 km \\ \(\theta\) & 87.4\({}^{\circ}\) \\ \(N\) & 13 \\ \(M\) & 12 \\ Minimum elevation angle & 10\({}^{\circ}\) \\ Phase offset between interplane satellites & 360/12/2 = 15\({}^{\circ}\) \\ Bandwidth of ISL & 15 Mbps \\ Bandwidth of ground link & 15 Mbps \\ Queue type & FIFO \\ Buffer queue size & 100 packets \\ Packet sizes & 1500 byte \\ \hline \hline \end{tabular}
\end{table}
Table 4: Simulation parameters.  
Figure 6: Simulation topology in a time interval.  
To further investigate the congestion control performance, the average queuing delay is plotted in Figure 8. In the simulation, the source UT sends data with the rate of 1.5 Mbps. The results indicate that the DSP algorithm gains a high queuing delay at some satellites, because it only considers the propagation delay when choosing the route, and some packets may aggregate at specific satellites. Since NSR and SSLB can adjust routing paths according to the real-time transmission delay of neighbor satellites, the average queuing delay of NSR and SSLB is substantially lower than the DSP. The queuing delay is calculated in TLR, thus the peak points become lower. However, due to the dynamic topology, a certain ISL in the middle is disconnected. In TLR, packets can only wait for link reconnection or TTL timeout to be discarded at the intermediate node, resulting in higher queuing delay. In the proposed TSR method, the dynamic satellite topology is represented by using the space-time graph model, and the path with the minimum delay is obtained, which can alleviate congestion. Thus, the peak values of queuing delay decline remarkably.