# mmS-TCP: Scalable TCP for Improving Throughput and Fairness in 5G mmWave Networks
## 5 Discussion  
mmS-TCP is based on NewReno, a conventional ACK-based congestion control algorithm. Therefore, the RTT fairness problem, which is the biggest drawback of ACK-based CCAs, may occur (the performance of a flow with a short RTT is high). However, the time-based CUBIC TCP, which improved on the shortcomings of existing CCA, may also be affected by RTT and cause fairness issues [41]. Of course, the fact that it is based on the classic CCA is a clear disadvantage; to compensate for this, we are considering a CUBIC-based CCA design for mmWave networks in our future work.  
X-TCP [25] and MillProxy [24], introduced in Section 2, have the disadvantage of being difficult to design and deploy. In particular, because both proposals are based on a cross-layer method, there are difficulties in their actual implementation. Meanwhile, the redesigned mmS-TCP in this study has the advantage that it is able to provide sufficient performance in mmWave networks through a simple modification of the existing algorithm, S-TCP, and that it is easy to design and deploy.  
Before understanding the behavior of TCP on mmWave links, important RAN parameters must also be considered. To support a high-speed wireless network, the deployment of multiple micro base stations is required rather than the wide coverage of a large signal tower. The dense deployment of micro base stations and fast handovers between base stations present various challenges for transport layer protocols. In particular, the handover mechanism is a factor that can greatly affect end-to-end performance depending  
Figure 14: Throughput comparison of five flows according to the CCAs: (**a**) CUBIC; (**b**) S-TCP; (**c**) mm-TCP; (**d**) H-TCP; (**e**) HS-TCP; (**f**) Yeah.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Flow 1** & **Flow 2** & **Flow 3** & **Flow 4** & **Flow 5** & **Average** & **Jain’s** \\  & **[Mbps]** & **[Mbps]** & **[Mbps]** & **[Mbps]** & **[Mbps]** & **Total Throughput** & **Fairness Index** \\ \hline S-TCP & 141.8775 & 217.5223 & 147.088 & 181.7001 & 165.7131 & 853.901 & 0.975083 \\ mmS-TCP & 161.6046 & 174.928 & 184.4512 & 181.9639 & 183.1542 & 886.102 & 0.997718 \\ CUBIC & 169.239 & 129.0921 & 138.1982 & 201.5095 & 179.7331 & 817.772 & 0.974036 \\ HS-TCP & 168.977 & 172.6625 & 180.5496 & 175.9145 & 148.8744 & 846.978 & 0.995843 \\ H-TCP & 99.5897 & 121.7243 & 164.707 & 164.5096 & 133.5064 & 684.037 & 0.967242 \\ Yeah & 112.3956 & 127.9662 & 178.8458 & 176.2211 & 163.2704 & 758.699 & 0.96984 \\ \hline \hline \end{tabular} Notes: S-TCP: scalable TCP; mmS-TCP: scalable TCP for mmWave; HS-TCP: highspeed TCP; H-TCP: Hamilton TCP; TCP: transmission control protocol.  
\end{table}
Table 3: Comparisons of Jain’s fairness index, the throughput of each flow, and the average total throughput.  
on user mobility. Additionally, because the mmWave band has already been used for satellite communication, it can be expected that the terrestrial and satellite communication frequency bands will gradually overlap [9]. Therefore, as the use of the mmWave band expands, consideration should begin as to whether to use existing transport layer protocols for integrated communications.