# A Modified TCP BBR to Enable High Fairness in High-Speed Wireless Networks
## 4 Evaluation of Experiments
### RTT Fairness  
First, we tested the RTT fairness of the algorithm under different conditions using NS3. The experiments were set up with 0.5 BDP and 5 BDP buffers, with 10 ms RTT traffic competing with 50 ms RTT traffic for a 1 Gbps bottleneck bandwidth. The throughput comparison results are shown in Figure 6. For BBRv1 and its variants, the throughput difference between flows increases significantly with buffer size. The throughput difference between the two flows in BBRv2 is smaller than that of BBRv1 and is even smaller than that of CUBIC. While the BBR-PG algorithm has a larger throughput difference in small buffers than BBRv3, the throughput difference in large buffers does not increase further and is significantly smaller than that of BBRv3. Overall, the RTT fairness of BBR-PG is significantly improved compared to BBRv1. Next, the fairness of the CCAs at different buffer sizes is compared. In the 0.1 BDP-100 BDP buffer, the average throughput and fairness index of 10 ms RTT flows and 50 ms RTT flows are provided in Figure 7. The throughput difference becomes larger with the increase in buffer size. In Figure 7a, the RTT fairness of CUBIC deteriorates with the increase in the RTT ratio, and the final fairness index is about 0.89. Comparing Figures 7b and 8d, it can be seen that the fairness of BBRv3 is improved compared to BBRv1, and as the buffer size increases, the throughput of 10 ms RTT flows in BBRv3 is inverted from the throughput of 50 ms RTT flows. In general, BBR-PG has better RTT fairness than the other three algorithms, especially in the larger buffer. Compared with BBR, the fairness index increased by about 53%. Figure 5: Throughput and latency in 5G networks. In addition to buffer size having a significant impact on RTT fairness, RTT differences also significantly affect RTT fairness. As depicted in Figure 8, with the increase in RTT difference, the throughput of the long RTT flow gradually dominates, and the fairness index gradually decreases. The RTT fairness of BBRv3 is improved over BBRv1, which has a fairness index of only 0.59 compared to 0.89 for BBRv3. In Figure 8f, BBR-PG can maintain good RTT fairness, and the fairness index can be maintained around 0.95. On the whole, BBR-PG can maintain high fairness at different RTT ratios. Especially when the RTT ratio is greater than 4, the fairness index of BBR-PG, which improves by 61% compared with BBRv1, is the highest. Figure 6: Average throughput comparison of 10 ms RTT flows and 50 ms RTT flows. Figure 7: Average throughput and fairness index of 10 ms RTT flows competing with 50 ms RTT flows in different buffer sizes. Then, experiments were carried out in a 5G communication environment. The packets were transmitted (downlink communication) from the base station to the mobile terminal. Figure 9 shows the statistical results of the throughput ratio of 10 ms and 50 ms RTT flows and the corresponding fairness index. As observed from the results provided in Figure 9, 50 ms RTT flows have obvious advantages in different buffer sizes. Although the disadvantage of 10 ms RTT flows is alleviated in shallow buffers, their bandwidth share is still far below its reasonable share. Unlike the other five CCAs, CUBIC's 10 ms RTT flows are more advantageous in shallow buffers. BBRv3 and Yinker have improved fairness compared to BBRv1. Specifically, in the 0.5 BDP buffer, the fairness index of BBR-PG is close to 0.992, with an 11% improvement in RTT fairness. In the 5 BDP buffer, the fairness index was approximately 0.966, with a 9% improvement in RTT fairness. This further proves the effectiveness of the BBR-PG algorithm in the simulation results. Meanwhile, an RTT fairness test was carried out in the WiFi network to verify the performance of each CCA. We deployed locally installed centralized CCAs and chose the Beijing node as the cloud server. As we can see from Figure 10, the experiment results of the WiFi network are similar to those of NS3 and 5G.