# An On-Demand Scheduling-Based MAC Protocol for UW-WiFi Networks
## 5 Experiments and Results
### Ocean Field Trials Implementation  
We code the proposed SDDA MAC protocol in SeaLinx, a network protocol stack designed in [28]. Additionally, the network protocol stack SeaLinx is implemented in the embedded operating system of underwater acoustic modems, which are provided by Smart Ocean Technology Co., Ltd. In the sea trials, each node is equipped with one aforementioned acoustic modem, which operates at a frequency band between 21 and 27 kHz. To fully verify the performance of the SDDA protocol, we conduct two categories of experiments. In the first category of experiments: for the software parameters, we experiment with different numbers of data packets in respect of throughput. The number of data packets is 5, 10, 15, 20, 25, and 30, respectively. The length of the data packet is 544 B. Apart from those, we also trial with varying packet generation rate from 0.02 to 0.2 packet/s with regard to other performances mentioned above. The length of the data packet is 300 B. All the test data are recorded in the log files. In the other category of experiments: to testify the utility of the proposed protocol, we conduct experiments on different classes data of nodes, including image, CTD (conductivity, temperature, and depth), and UUV (unmanned underwater vehicle) status data, of which the image data size is 8 KB, the CTD data size is 50 B, and the UUV status data is 120 B.  
performance for S-FAMA. As shown in Figure 8, the SDDA protocol achieves the best performance in terms of end-to-end delay, in that each node can burst out all data packets in sending queue by every channel reservation in SDDA, while TDMA can only send one packet in each slot, and there are a great many of handshake clashes in S-FAMA, which greatly enlarges the end-to-end delay. Figure 9 demonstrates the packet delivery ratio performance of these three protocols. From Figure 9, we can see that the packet delivery ratio of SDDA and TDMA is essentially constant, linked to the packet loss rate specified in the simulation settings, but that of SDDA is higher on account of its acknowledgement-repeat mechanism. While in S-FAMA, the packet delivery ratio is relatively low, and there is a lot of fluctu  
Figure 8: End-to-end delay comparison. Figure 7: Channel utilization comparison. packets collisions become heavier and heavier, leading to multiple retransmissions of data packets failures, and thus data packets that fail to be retransmitted will be discarded. Based on the above comparison experimental results, the performance of SDDA is significantly superior to the other two. However, in terms of energy efficiency, TDMA is the most energy-saving without transmitting additional control packets; compared to S-FAMA, SDDA only needs a small amount of control packets and completely avoids conflict to save energy. As can be seen from Figure 7, when the packet generation rate is greater than 0.1, the channel utilization of SDDA exceeds 90%, and the proportion of control packets is less than 10%. SDDA's extra energy consumption except data transmission is not high. Compared with TDMA, although SDDA consumes slightly more energy, its channel utilization is two times higher and end-to-end delay is three times lower, and to some extent, it guarantees the reliability of data transmission. _B. Ocean Field Experiment Results_  
In this subsection, we evaluate the performances of the SDDA protocol through two types of sea trials. In experiment I, we compare the results of real sea tests with those of simulation in regard to throughput, channel utilization, end-to-end delay, and packet delivery ratio of SDDA protocol. In experiment II, we verify the practicability of SDDA by collecting various underwater data in the built UW-WiFi network system. **Experiment I:**  
Figure 10 shows the results of ocean trial and the simulation on network throughput of the SDDA protocol in the same settings.