# A Novel Energy-Efficient Contention-Based MAC Protocol Used for OA-UWSN
## 5 Simulation Results
### Simulation Results  
#### 5.2.1 OA-CMAC Protocol Throughput  
According to the difference in the optical handshaking success rate, this work simulated the relationship between the packet sending rate and the throughput of the OA-CMAC protocol and compared it to the OA-MAC protocols, as shown in Figures 5-7. In Figure 5, when the optical handshaking success rate was 20% and the network load was low, the OA-CMAC protocol had a postponed access window and the time slot utilization was low, so the throughput was lower than the OA-MAC. As the load increased, the throughput of the OA-MAC protocol based on node slot allocation tended to be stable, while the OA-CMAC protocol node randomly competed, the slot utilization rate increased, and the throughput doubled. In Figure 6, when the optical handshaking success rate was 50%, the data packet transmission rate was low. When the network load was low, the OA-CMAC protocol had a postponed access window, and the time slot utilization was low, the OA-MAC throughput based on time slot allocation had greater advantages. With increases in the data packet transmission rate and the network load, the OA-MAC protocol data conflict rate was higher. However, the OA-CMAC protocol adopted a postponed access mechanism, which effectively avoided collisions and had a high channel utilization rate. In Figure 7, where the optical handshaking success rate was 80%, even if the packet transmission rate was low, the throughput of the OA-CMAC protocol was higher than the other two protocols because optical communication could perform high-speed and large-capacity data transmission. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Fixed node & 8 \\ Mobile node & 4 \\ Sink node & 2 \\ Control center & 1 \\ Topological area & 500 m \(\times\) 500 m \\ Distance between fixed nodes & 30 m \\ Distance between fixed node and mobile node & 20 m–36 m \\ Simulation time & 8000 h \\ Number of simulations & 1000 \\ Initial energy & 3600 kJ \\ Acoustic bit rate & 10 kbps \\ Optical bit rate & 1 Mbps \\ \hline \hline \end{tabular}
\end{table} 
Table 4: System simulation parameters. The optical handshaking success rates were configured as 20%, 50%, and 80%. When four packets were transmitted every 10 s, the throughput was 75 kbit/s, 100 kbit/s, and 150 kbit/s, respectively. With the increasing success rate of optical handshaking, the amount of data transmitted from optical communications was continuously increasing, and the throughput of the OA-UWSN was continuously improving. The simulation results showed that, with the increase in the optical handshaking success rate, the throughput of the OA-CMAC protocol was multiplied. Even if the optical handshaking success rate was low, when the network load was high, the throughput of the OA-CMAC protocol was higher than the other two protocols. Therefore, the OA-CMAC protocol is suitable for high-speed and high-load networks. Figure 5: Comparison of three protocol throughputs when the optical handshaking success rate was 20%. Figure 6: Comparison of three protocol throughputs when the optical handshaking success rate was 50%. In Figure 8, we compared the total amount of data from the OA-CMAC to the optical-acoustic simultaneous handshake MAC protocol (OAs-CMAC), which exchanges RTS/CTS messages over both acoustics and optics at the same time. Figure 8 shows the average results of 1000 simulations. Optical and acoustic simultaneous handshakes were performed within 0.4 s in OAs-CMAC. If the optical handshake was successful within 0.4 s, optical communication was performed. Otherwise, acoustic communication was performed, and at the same time, the optical handshake continued. If the optical handshake was successful within the next 1 s, the optical communication was switched. Otherwise, the acoustic communication was performed continuously.