# Load-Balancing Routing for LEO Satellite Network with Distributed Hops-Based Back-Pressure Strategy
## 6 Simulation Performance and Analysis
### Result Discussion
#### 6.2.1 Constant Bit Rate Traffic  
In constant bit rate (CBR) traffic, the traffic generation rate changes gradually from 2 Mbps to 4 Mbps at the interval of 0.25 Mbps to test the performance of routing protocols under different link loads. The buffer size of each satellite is 50 packets. Simulation results are demonstrated in Figure 9.  
The average delay under different routing protocols is shown in Figure 9a. The proposed DHBP uses hops count as the backlog metric. Compared with DBPR, which utilizes the link length as the metric, DHBP reduces the average delay by 14.67% at a traffic rate of 3.5 Mbps. OSPF uses the shortest path based on Dijkstra and only obtains information from neighboring satellites to consolidate routing tables. When the link load is heavy, a large number of packets will be dropped, resulting in the network performance degradation. SALB achieves the highest delay because it postpones data sending upon link congestion. Meanwhile, SALB only select the next hop from no more than two neighbors, which aggravates link congestion. The average number of forwarding per packets with different protocols is demonstrated in Figure 9b, which measures the transmission cost of each packet. Through restricting the propagation area to a rectangular region between the source and destination of all packets, DHBP has a significant advantage over the comparison protocols. Although the topology of the Manhattan network provides a large number of redundant links for the LSN, we can see that the transmission cost is reduced by restricting transmission within a certain region. The average number of forwarding per packets is significantly less than DBPR, OSPF, and SALB when the CBR rate is less than 3.25 Mbps. When the CBR rate is close to 4 Mbps, DHBP has a similar performance to that of OSPF. Therefore, we can see that, when the traffic is heavy, the proposed scheme still has a low transmission cost. Figure 9c shows the change of network throughput with traffic generation rate under different routing protocols. As the CBR rate increases, the throughput of several routing protocols shows an upward trend. In comparison, DHBP has the largest network throughput, while OSPF has the smallest. When the traffic generation rate is less than 2.4 Mbps, DHBP and DBPR have a similar throughput. As the link load continues to increase, the throughput of DHBP tends to its maximum of about 24 Mbps. Compared to DBPR, SALB, and OSPF, when the CBR rate is 4 Mbps, the maximum throughout of DHBP is improved \begin{table}
\begin{tabular}{c c c} \hline \hline
**Number** & **Source** & **Destination** \\ \hline
1 & Perth & Beijing \\
2 & Madrid & Guan \\
3 & Lagos & Seoul \\
4 & Cairo & Tokyo \\
5 & Istanbul & Sulawesi \\
6 & New Delhi & Johannesburg \\
7 & Urumqi & New York \\
8 & Maldives & Vilyuchinsk \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Satellite network traffic pairs. by 61.65%, 23.74%, and 10.47%, respectively. We can see that the DHBP can always improve the network throughput, especially when the traffic is heavy. Figure 9d shows the data delivery ratio with different CBR traffic. When CBR rate advances, the data delivery ratio of DHBP, OSPF, DBPR, and SALB decreases due to heavier transmission load. In comparison, DHBP achieves the best data delivery ratio and load-balancing ability since it considers both back-pressure routing and destination-hops-delay, which balances network traffic without exorbitant cost. When the CBR rate reaches 4 Mbps, the data delivery ratio is improved by 11.59%, 14.93%, and 40% compared with DBPR, SALB, and OSPF, respectively. SALB estimates the link states and updates routing tables through an efficient shortest path tree algorithm between two successive handovers, which shows a higher data delivery ratio than that of OSPF. However, SALB cannot provide adequate available paths to adapt to heavy load.