# An Efficient Scalable Scheduling MAC Protocol for Underwater Sensor Networks +
## 6 Performance Evaluation
### Simulation Results
The second set of simulations studies how the load affects the network throughput of those three protocols with only 10 sensor nodes. As shown in Figure 13, when the offered traffic increases, the network throughput increases correspondingly and finally reaches the saturation status. ED-MAC achieves higher network throughput than that of UWAN-MAC and T-Lohi when their traffic loads are the same. This is because ED-MAC uses a depth-based timer that allows each node to prioritise slot reservation depending on the node depth; the greater the depth, the higher the priority to reserve a slot first. In both UWAN-MAC and T-Lohi, the spatial-temporal uncertainty problem is not considered and therefore their network throughput are lower than that of ED-MAC. Specifically, ED-MAC improves 37.8% of the network throughput than that of T-Lohi and 63.9% of the network throughput than that of UWAN-MAC on average. We next compare the throughput of those protocols by increasing the node density while the offered traffic is kept fixed to 0.25 packets per second. Figure 14 shows the throughput of the three protocols as a proportional to the number of nodes. As the number of nodes increases, the network throughput increases correspondingly and eventually reaches a saturation point. ED-MAC reaches higher network throughput and performs better than other two MAC protocols in the same circumstances. This is due to its specific benefits from a high scalable scheduling to handle more data packets than T-Lohi and UWAN-MAC. Moreover, the curve of ED-MAC exhibits significantly increased by achieving five packets per second within 50 nodes, and then a slower decline compared with that of T-Lohi and UWAN-MAC, which proves the effectiveness of the proposed protocol. Thus, the scalability of handling more packets in ED-MAC outperforms that in T-Lohi and UWAN-MAC. The third set of simulations evaluates how the offered traffic affects the energy consumption of the three different MAC protocols with 10 nodes. Figure 15 demonstrates that ED-MAC is much  
Figure 14: Throughput vs. node density. Figure 13: Throughput vs. offered traffic. more energy efficient in this simulation than T-Lohi and UWAN-MAC. At a low traffic load, ED-MAC is still more energy efficient than other protocols due to its lower receiving overhead and less idle overhead per data packet. Another reason for ED-MAC's superior efficiency is that it has a compatible schedule based on its depth criteria, which helps to avoid the possibility of collisions. As the traffic load increases, T-Lohi shows a rapid rise of the energy consumption per packet due to an increasing number of collisions of control packets resulting in incorrect reservations. It is more interesting to observe that when the traffic load increases, the energy consumption of ED-MAC also slightly increases, although it still has a higher network throughput than T-Lohi and UWAN-MAC protocols. We also investigate the effect of increasing the node density in each protocol in terms of energy consumption while the offered traffic is constant. As shown in Figure 16, the energy consumption of the three protocols is drawn as a function of the number of nodes. As the number of nodes increases, the energy consumption increases correspondingly. This is because, when the number of nodes increases, more nodes are involved; therefore, there is more intensive competition to access the channel. ED-MAC consumes the lowest energy among the two other protocols because it adopts energy conservation measures by considering hidden terminal and spatial-temporal uncertainty problems. As the node density increases, T-Lohi consumes significantly more energy per packet due to an increasing number of collisions. Specifically, ED-MAC consumes on average only 61.5% of the energy consumption of UWAN-MAC and 63.8% of the energy consumption of T-Lohi. T-Lohi consumes the highest energy among other protocols, although it conserves energy by employing a wake-up tone receiver that allows very low-power listening for wake-up tones.