# Studying Semi-TCP and Its Application in Marine Internet
## 4 Experimental Simulation and Analysis
### Simulation Results Analysis  
In order to analyze the effect of the threshold value on the performance of the protocols, we varied the threshold settings in the simulation. As a lot of data is flowing at the same time, we summed up the average throughput of each data stream and compared them, with the results shown in Figure 7.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Simulation Parameters** & **Values** \\ \hline Nodes’ moving speed & 7\(\sim\)10 m/s \\ Channel model & TwoRayGround \\ Attenuation model & Ricean, K = 5 \\ Frequency channel & 2.4 G \\ MAC rate & 11 Mbps \\ Sending queue & FIFO, Length 150 Kb \\ Transmission power & 33 dBm \\ Routing protocol & AODV \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic parameters of the scenario.  
Figure 6: Abstract simulation scenario of the ship ad hoc network.  
From Figure 7, it can be seen that the two Semi-TCP protocols have a significant improvement in the throughput of the network compared to TCP-Lite. This is because TCP-Lite misjudges the network congestion status inferred from packet loss for the bad channel quality and mistakenly reduces the sending window, which limits the network performance. The nodes judge themselves to determine whether they are congested or not depending on the congestion threshold. From this figure, we can also see that the change of the threshold value does not have a great impact on the overall throughput of the network. However, as the threshold value increases, the average length of the queue will inevitably increase, and the average queue delay increases as well. The following data statistics are collected from the average queue delay of node J in the J \(\rightarrow\) K data stream. At the same time, the total throughput of Semi-TCP-RTS-V2 is slightly lower than Semi-TCP-RTS, and the results are explained jointly by using Figure 8 below.  
From Figure 8, it is found that the load of node A is much smaller than the sum of the average throughput of the entire network. In fact, there is even a failure of a data stream connection in node A in the Semi-TCP-RTS test. This is because node A is affected by node J and node B while sending data; it is not only acting as an exposed terminal for J \(\rightarrow\) K and B \(\rightarrow\) C data streams, but also is in the B \(\rightarrow\) D  
Figure 8: Load with various CTh settings of node A.  
Figure 7: Throughput with various CTh settings in a congestion network.  
data stream while node D is in its receiving range, which affects the data-sending of node A. Apart from that, the congestion caused by the two data streams of node B restrict node A from sending data, resulting in the above phenomena.  
In addition, it can be clearly observed that in the improved protocol, node A will increase its load when the entire network throughput decreases. This shows that the gap between node A and the other 2 sending nodes has shrunk, meaning that the fairness of the data stream has improved. Because the exposed terminal problem with the Semi-TCP-RTS-V2 has been relieved at a certain level, the load has also been increased. The overall throughput shown in Figure 7 is lower than Semi-TCP-RTS, because it takes time to define the exposed terminal with the improved protocol. At the same time, the other two sending nodes enter into the process of the exposed terminal's judgment when node A is sending packets, yielding the above result.  
As shown in Figure 9, as the threshold is consistent, there is no difference between Semi-TCP-RTS and Semi-TCP-RTS-V2, which is also in line with the expectations.