# Link-State Aware Hybrid Routing in the Terrestrial-Satellite Integrated Network +
## 5 Simulation and Analysis
### Terrestrial-Satellite Routing Performance  
Figures 11 and 12 show the comparison results of transmission delay and throughput between the proposed TSHR, OSPF +, TACR, and HGR, respectively. Obviously, the TSHR outperforms OSPF+,TACR, and HGR in terms of end-to-end transmission delay and throughput. Since the proposed TSR algorithm can dynamically detour traffic to alternative paths in response to congestion, the throughput is substantially higher than the TACR and OSPF+. Although TACR utilizes the topology information provided by the contact graph, it searches future available neighbor nodes. Considering the time-varying network topology, the path planning might be sub-optimal or even invalid. As the background flow increases to the range 5-7 Mbits, the non-congested alternative paths reduce. Therefore, the throughput of the TSHR algorithm starts to increase slowly, and the performance gap between the TSHR and TACR is the minimum. However, as the background flow increases to 11 Mbits, TACR always transmits data through the shortest path and a long queuing delay begins to appear at the selected path with heavy loads, which results in rapid throughput degradation. Thus, the performance gap between the TSHR and TACR increase  
Figure 10: Network throughput vs. data transmission rate.  
Figure 9: Packet drop rate vs. data transmission rate.  
as shown in Figure 12. HGR utilizes greedy forwarding strategy and hyperbolic geometry coordinates mapping to achieve path selection, which fails to consider the congestion; therefore, some packets are discarded before they are sent out. Since the routing table is calculated independently in each satellite in OSPF+, the large route calculation interval will cause severe delay and throughput degradation. Furthermore, the delay grows nonlinearly with the route calculation interval due to the discreteness of the constellation topology change. On the contrary, to avoid high queuing delay, alternative paths are calculated in the proposed TSHR when terrestrial network is about to get congested, the dynamic satellite topology is represented by using the space-time model, then the TSL and ISL with the minimum delay is obtained through bipartite graph mismatch. To sum up, the transmission in the satellite network helps to alleviate the traffic burden and leads to better delay and throughput performance.  
Figure 13 shows the comparison results of transmission delay in network congestion situation and the proposed TSHR. At the beginning of transmission, since there are enough network bandwidths to serve the requested traffic of UTs, the selected path by TSHR is the same as the shortest path of terrestrial routing. At 150 s, to simulate the network congestion, background flows or burst flows are triggered, the TSHR significantly outperforms the terrestrial routing algorithm. The reason is that the terrestrial routing algorithm only finds the shortest path, data is then transmitted over the shortest paths during the entire communication session; hence, there would be a large delay and low throughput when some ITLs are congested. On the contrary, to avoid high queuing delay, an alternative paths is calculated in TSHR when the terrestrial network is about to get congested, which  
Figure 11: Delay vs. background flow arrival.  
Figure 12: Throughput vs. background flow arrival.  
means that the data are transmitted via the terrestrial-satellite hybrid path, namely source UT-TSE-satellite network-TSE-destination UT. To sum up, the transmission in the satellite network helps to alleviate the traffic burden and leads to better delay performance.