## 3 Design of SROA
### SROA Protocol Description
#### 4.2.3 Average End-to-End Delay

The average end-to-end delay for different protocols with the same CR of 1500 m is shown in Figure 10. We can observe that, as the network size increases, the average end-to-end delay decreases. In general, with the increasing of sensor nodes deployed underwater, it means the deployment of the network is denser and all four protocols can route packets along shorter paths from the source node to the sink node. Therefore, when the network size is 500, the delay of the network is minimal. Furthermore, the SROA protocol appears to have a lower average end-to-end delay than other protocols. The average delay of SROA is 7.4 s when there are 200 nodes in the network and the CR is set to 1500 m, whereas QELAR, GEDAR, and HHVBF are 7.88 s, 8.56 s, and 9.1 s, respectively. This is due to the fact that SROA uses a Q-value-based waiting mechanism to coordinate relay nodes, thus reducing retransmissions and collisions. Moreover, the routing paths to the sink node are shorter because of the on-site architecture where the data center is deployed close to the data source. The average delay of HHVBF is the highest among the four protocols. Owing to the hidden terminal problem, the happening of collisions results in the increasing of the average end-to-end delay. The average latency for GEDAR is the second as it utilizes

Figure 9: Comparison of average end-to-end delay for different architectures. #### 4.2.4 Packet Delivery Rate

Figure 11 compares the packet delivery rate of SROA to that of QELAR, HHVBF, and GEDAR. It can be observed that the PDR of all four protocols rises as the network size increases. Since relay nodes have more available neighboring nodes for relaying data packets, the packet delivery rate improves. We can also see that SROA has a greater PDR than the other methods. For example, SROA's PDR reaches 96.6% when the network size is 500, which is greater than that of GEDAR, QELAR, and HHVBF. One of the reasons is that the reward function of the SROA protocol considers not only link quality while determining routing decisions, but also related factors such as residual energy and end-to-end latency, ensuring high PDR globally. However, grouping the sensor nodes into several clusters and the replication mechanism of cluster head makes the transmission more reliable. Moreover, based on the on-site architecture, the data packet travels fewer hops to the sink node, improving the packet delivery ratio. For the QELAR, since the sender may choose a path with fewer hops to improve the packet delivery rate, the PDR of it increases. As GEDAR considers the expected packet advance, more than one node participates in the packet forwarding. Therefore, in the figure, GEDAR's PDR is substantially larger. The PDR of HHVBF is the lowest, as it does not take the packet error rate into account, resulting in unnecessary retransmissions and low PDR. #### 4.2.5 Energy Consumption

Energy is very precious and important in the underwater environment; each routing protocol should consider the efficiency of energy consumption seriously. The comparison of energy consumption with a network size ranging from 100 to 500 is shown in Figure 12. From the figure, we can find that the total energy consumption of these protocols increases as the network size grows. Generally, it also can be seen clearly that the energy consumed by the SROA is less than other protocols. Specifically, when network size is 500, the SROA consumes 23.8%, 32.1%, and 44.2% less energy than QELAR, GEDAR, and HHVBF. Since the SROA is a cluster-based protocol, the energy distribution of nodes in each cluster is more uniform which extends the network lifetime. Moreover, the SROA uses waiting mechanism based on opportunistic routing to forward packets which reduces the retransmissions, thus SROA consumes less energy in comparison with QELAR and GEDAR. Besides, among the

Figure 10: Comparison of average end-to-end delay for the four protocols. four protocols, the energy consumption of HHVBF is the maximum and grows faster with the increase of nodes.