# DCO-MAC: A Hybrid MAC Protocol for Data Collection in Underwater Acoustic Sensor Networks
## 6 Numerical Results and Discussions  
We use Aqua-sim [23] based on NS-3 to evaluate the performance of the proposed DCO-MAC protocol, and compare the DCO-MAC protocol with some existing underwater MAC protocols. The network topology used in the simulations, as shown in Figure 10, is a tree-topology-based UASN, where the level-1 nodes and level-2 nodes are distributed in the transmission range of the sink node and the corresponding level-1 nodes, respectively. Figure 10: Network topology used in simulations. The simulation parameters are listed in Table 1.  
Firstly, we compare the performance of the CT-MAC protocol, in terms of the normalized throughput and the packet delay, with two contention-based protocols, slotted floor acquisition multiple access (SFAMA) and T-Lohi. Three contention-based protocols are performed in a sub-topology, as illustrated in Figure 10, which consists of a level-1 node A; four level-2 nodes, A\({}_{1}\), A\({}_{2}\), A\({}_{3}\), A\({}_{4}\); and an interference node, B\({}_{4}\). Figure 11 shows the normalized throughput of three MAC protocols, CT-MAC, SFAMA, and T-Lohi. From Figure 11, we observe that, for the normalized throughput of the CT-MAC protocol, the simulation results match the analytical results relatively well, which means that the performance analysis is relatively 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 propagation delay in the performance analysis. Moreover, we also observe that the normalized throughput of the three MAC protocols increases first as the traffic load increases. After a peak value is reached, the normalized throughput decreases along with the increase of the traffic load. The reason for this phenomenon is that these three MAC protocols are contention-based. When the traffic load is large enough, the probability of a successful channel reservation decreases as the traffic load increases. Furthermore, the normalized throughput of the CT-MAC protocol is smaller than that of the SFAMA and T-Lohi protocols when the traffic load is light. The reason for this is that the level-1 node should spend some time to invite the other level-2 nodes in order to join in the transmission after a successful channel reservation using an RTS packet in the CT-MAC protocol. When the traffic load is light, the number of level-2 nodes with data packet(s) to be transmitted is small. A few extra data packets are added as more channel reservation time is used. Hence, the normalized throughput of the CT-MAC protocol is relatively small. However, the normalized throughput of the CT-MAC protocol is larger than that of the SFAMA and T-Lohi protocols when the traffic load is large. The reason for this phenomenon 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. Figure 12 shows the packet delay of three MAC protocols, CT-MAC, SFAMA, and T-Lohi. From Figure 12, we observe that the packet delay of the three MAC protocols increases as the traffic load increases. After the traffic load reaches a specific value, the packet delay increases rapidly along with the increase of the traffic load. The reason for this phenomenon is that these three MAC protocols are contention-based. When the traffic load is large enough, the probability of a successful channel reservation decreases as the traffic load increases. Without the reserved channel resource, the data packets in the level-2 nodes should wait to transmit, and the packet delay therefore increases.