# DCO-MAC: A Hybrid MAC Protocol for Data Collection in Underwater Acoustic Sensor Networks
## 6 Numerical Results and Discussions
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Control packet length & 20 Bytes \\ Data packet length & 200 Bytes \\ Bit error rate & \(10^{-5}\) \\ Guard time & 0.02 s \\ Maximum transmission range & 3 km \\ Transmission rate & 800 bps \\ \(N_{\text{trans}}\) & 2 \\ Speed of mobile nodes & 2.5 m/s \\ Transmit power & 10 W \\ Receive power & 0.4 W \\ Listen power & 0.08 W \\ Idle power & 0.08 W \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. interference nodes on the performance of the CT-MAC, SFAMA, and T-Lohi protocols, in term of the normalized throughput. From Figure 13, we observe that the normalized throughput of the three MAC protocols decreases along with the increase of the number of interference nodes. The reason is that these three MAC protocols are contention-based. The probability of successful channel reservation decreases as the number of interference nodes increases. Moreover, the normalized throughput of the CT-MAC protocol is larger than that of the SFAMA and T-Lohi. The reason for this is that the level-1 node receives data packets from more than one level-2 node with data packet(s) to be transmitted after a successful channel reservation in the CT-MAC protocol. Secondly, we compare the performance of the DCO-MAC protocol, in terms of the normalized throughput, the average packet delay, energy consumption, and fairness, with three underwater MAC protocols, SFAMA, RIPT, and CH-MAC. Four MAC protocols are performed in a tree topology, as illustrated in Figure 10.  
Figure 14 shows the normalized throughput of the DCO-MAC, SFAMA, RIPT, and CH-MAC protocols. From Figure 14, we observe that, for the normalized throughput of the DCO-MAC protocol, the simulation results match the analytical results relatively well, which means that the performance analysis is accurate. The normalized throughput, in theory, is slightly smaller than that in the simulations. The reason for this phenomenon is that the propagation delay between any two nodes is assumed to be the maximum one in the performance analysis. Moreover, as the traffic load increases, the normalized throughput of the DCO-MAC and RIPT protocols increases, while the normalized throughput of the SFAMA and CH-MAC protocols first increases and then decreases. As SFAMA is a contention-based protocol, the collision probability increases as the traffic load increases, which results in a poor performance in terms of the normalized throughput. As RIPT is a receiver-initiated protocol, the receiver schedules the data transmission of the sensor nodes to avoid the collision, which leads to a better performance. CH-MAC is a hybrid protocol, where TDMA and CSMA/CA are performed in the level-2 sub-networks and the level-1 sub-network, respectively. However, if the traffic load in the level-1 sub-network is heavier than that in the level-2 sub-network, CSMA/CA introduces a high probability of collision. Hence, the normalized throughput of the CH-MAC protocol decreases when the traffic load is large. Compared with the  
Figure 13: The normalized throughput of the CT-MAC protocol vs. the number of interference nodes. CH-MAC protocol, DCO-MAC uses the contention-based MAC protocol in the level-2 sub-networks with a low traffic load, so as to decrease the average packet delay, while it uses the reservation-based MAC protocol in the level-1 sub-network with a high traffic load to avoid the collision. Hence, the normalized throughput of the DCO-MAC protocol is the largest among these four MAC protocols. Figure 15 shows the end-to-end packet delay of the DCO-MAC, SFAMA, RIPT, and CH-MAC protocols. From Figure 15, we observe that the end-to-end packet delay of the four MAC protocols increases as the traffic load increases. Moreover, the end-to-end packet delay of the SFAMA protocol is smallest when the traffic load is low.