# DCO-MAC: A Hybrid MAC Protocol for Data Collection in Underwater Acoustic Sensor Networks
## 6 Numerical Results and Discussions
It is because the probability of collision is very small, and the nodes with data packets to be transmitted can always reserve a channel successfully, and the data packets are transmitted to the receiver immediately. When the traffic load increases, the probability of collision increases, and the data packets should wait a period before they can be transmitted to the receiver, which results in the increasing of the end-to-end packet delay. For the RIPT protocol, the data packets can be transmitted  
Figure 14: The normalized throughput of the DCO-MAC protocol. Figure 15: The end-to-end packet delay of the DCO-MAC protocol. only when the receiver initiates the transmission, which results in a relatively large wait delay. For the CH-MAC protocol, TDMA is performed in the level-2 sub-network, and no transmission collisions happen. Although CSMA/CA is performed in the level-1 sub-network, the end-to-end delay is smaller than that of the SFAMA when the traffic load is large. For the DCO-MAC protocol, the average packet delay of the CT-MAC performed in the level-2 sub-networks is relatively small, as shown in Figure 12. In addition, a reservation-based MAC protocol is performed in the level-1 sub-network to avoid the collision. Therefore, the end-to-end packet delay is smallest among the four MAC protocols. Figure 16 shows the energy overhead of the DCO-MAC, SFAMA, RIPT, and CH-MAC protocols, where the energy overhead is defined as the energy consumption per data packet transmission. From Figure 16, as the traffic load is low, the energy consumption of the four MAC protocols decreases slightly along with the increase of the traffic load. The reason for this is that the number of data packets that are successfully transmitted increases as the traffic load increases. After a minimum value is reached, the energy overhead increases along with the increase of the traffic load. The reason for this is that as the traffic load is large, and the probability of collision increases along with the traffic load, which results in the increase of energy overhead for the contention-based MAC protocol. As SFMA, CAMA/CA in CH-MAC, and CT-MAC in DCO-MAC are contention-based protocols, the energy overhead of these three MAC protocols increases along with the increase of the traffic load. For the RIPT protocol, the receiver schedules the data transmission to avoid collision at the receiver. Hence, the energy overhead is the lowest among the four MAC protocols. From Figure 16, we also observe that the energy overhead of the DCO-MAC is lower than that of the SFAMA and CH-MAC protocols. The reason for this phenomenon is that, in the DCO-MAC protocol, the contention-based MAC protocol is performed in the level-2 sub-networks with a low traffic load, and a reservation-based protocol is used to avoid the collision in the level-1 sub-network with a high traffic load. Hence, the collision probability in the DCO-MAC is smaller than that of SFAMA and CH-MAC, which results in a lower energy overhead. Fairness affects the network survival time, and is an important performance metrics of the MAC protocol. Here, using [PERSON]'s fairness index (FI) [20], the fairness of MAC protocols is defined as follows:  
\[\text{FI}=\frac{\left(\sum_{i=1}^{n}m_{i}\right)^{2}}{n\sum_{i=1}^{n}m_{i}^{2}} \tag{25}\]  
Figure 16: The energy overhead of the DCO-MAC protocol. where \(n\) denotes the number of nodes, and \(m_{i}\) denotes the number of data packets transmitted by node \(i\) during the simulation period. The value of FI ranges from 0 to 1. FI = 1 if the number of data packets transmitted by all of the nodes is the same, which means that the fairness of the considered MAC protocol is the best. Figure 17 shows the fairness index of the DCO-MAC, SFAMA, RIPT, and CH-MAC protocols. From Figure 17, we observe that the fairness index of the SFAMA and CH-MAC protocols decreases as the traffic load increases. In conventional MAC protocols, the distance between the two nodes is a key factor in the channel competition because of the large propagation in the UASN.