# Ocean-Current-Motion-Model-Based Routing Protocol for Void-Avoided UASNs
## 5 Performance Simulation and Analysis
### Performance Comparison  
#### 5.2.1 Influence of the Node Density  
In this section, we investigate the influence of the node density to obtain the performance of these protocols. The number of moving nodes ranges from 300 to 1000 in the network, the node moving velocity ranges from 0.2 to 3 m/s, and the initial energy is 100 J. The visualization of simulation results is shown from Figures 8-10.  
Figure 8 illustrates the comparison of PDR with a different number of nodes. The PDRs of four protocols increase gradually as the number of nodes increases. This is because the probability of a node being selected as a relay node to participate in the data transmission increases with the increasing node density. The OCMR protocol performs better than the VBF, the HH-VBF, and the QELAR in PDR. This is because the candidate forwarding set is formed in the OCMR, and the optimal alternative node will be rapidly selected as the relay node in this set to guarantee the reliability of data transmission when communication is interrupted. The communication link will be reconstructed once the alternative relay node is selected, which reduces unnecessary data collisions and improves the forwarding efficiency, thus increasing the PDR.  
Figure 9 presents a comparison of the average end-to-end delay with a different number of nodes. We can see that the performance metrics of four protocols gradually decrease as the number of nodes increases. This is because more neighbors can be chosen as the relay nodes for data forwarding as the number of nodes increases, which reduces the  
Figure 8: Comparison of PDR with different numbers of nodes.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline UASNs deployment space & 800 m \(\times\) 800 m \(\times\) 800 m \\ Simulation time & 500 s \\ Communication range & 100 m \\ Transmission power & 2 W \\ Receiving power & 0.75 W \\ Idle power & 8 m W \\ Acoustic speed & 1500 m/s \\ Node minimum velocity & 0.2 m/s \\ Packet size & 50 Bytes \\ \hline \hline \end{tabular}
\end{table}
Table 3: Simulation parameters.  
probability of a routing void occurring. Below 600 nodes, the performance of the OCMR protocol is worse than that for the other protocols. This is because more time is needed to extend the protection radius and expand the search coverage to contain more nodes and form a candidate forwarding set in a sparse network. However, when the number of nodes is more than 800, more nodes will be contained in a smaller search coverage, which will reduce the formation time of the candidate forwarding set. However, once the alternative relay node is selected in the forwarding set, the OCMR protocol can suppress the problem of a routing void occurring via the void avoided strategy, which reduces unnecessary data retransmissions.  
Figure 10 depicts the comparison of the average energy consumption with a different number of nodes. As the node density increases, the performance metrics of the VBF protocol exhibit fluctuating characteristics because of the limitation imposed by the single routing pipeline; the OCMR, HH-VBF and QELAR protocols tend to balance as node density increases.  
Figure 10: Comparison of average energy consumption with different number of nodes.  
Figure 9: Comparison of average end-to-end delay with a different number of nodes.  
density increase. That is because the total energy consumption grows at the same rate as the number of packets received by the destination node. The metric of the OCMR protocol is better than that of the VBF, HH-VBF, and QELAR protocols. This is because the OCMR algorithm adopts the protection radius to narrow the search coverage, thus improving the forwarding efficiency. However, reliable routing could increase the number of received packets.