# A Glider-Assisted Link Disruption Restoration Mechanism in Underwater Acoustic Sensor Networks
## 5 Performance Evaluation
### Evaluation with Different Parameters  
Packet delivery rate (PDR) in the network refers to the ratio between the data packets (Ps) sent by the source node and the data packets (Pr) received by the target node, that is, the statistical measurement of correctly transmitting the data packet. It is an important performance indicator that reflects network reliability and communication quality. Therefore, we simulate the relationship of packet delivery rate and the different parameters \(s/h\) of four different algorithms, where \(s\) is the number of common sensor nodes in the network and \(h\) is the number of cluster heads in the network, that is, \(s/h\) is the number of common sensor nodes in a cluster. In the simulation, the parameter \(s/h\) varies between 0 and 30 with a step of 5. As shown in Figure 4, by comparing the packet delivery rate with the change of the parameter \(s/h\), we can conclude that the packet delivery rate is also closer to the ideal value 1 as the parameter \(s/h\) increases. The rate of increase of data packet delivery is faster at first and slower later because the network connectivity is particularly poor when the nodes are sparsely distributed. In this case, adding nodes will have a greater impact on network connectivity. However, when the number of nodes reaches a certain critical value, for example, when the parameter \(s/h\) in the figure is 20, the rate of packet delivery increases quite slowly because the network is basically connected. At this point if the number of nodes increases, the connectivity is improved less obviously. Therefore, in the following simulations, we perform other simulations based on the parameter \(s/h\) = 20. Compared with OVAR, HHVBF and VBF, the glider-assisted link-disruption restoration mechanism has a higher packet delivery rate with the same \(s/h\) value. For example, when the parameter \(s/h\) is 20, in the glider-assisted link-disruption restoration mechanism, the packet delivery rate has increased by 15%, 23% and 33%, respectively, relative to OVAR, HHVBF and VBF. This is because in the glider-assisted link disruption restoration mechanism, when a certain link in the network is disrupted due to change of underwater acoustic channel or external force in the ocean, the cluster head node in each sub-volume will notify the glider to perform restoring the link in time. For both the links among common nodes and the key nodes, the glider can do the repair, which greatly improves the delivery rate of the data packets in the network. Based on the simulation in Figure 4, our simulation parameter \(s/h\) is 20. As shown in Figure 5, the end-to-end delay of the glider-assisted link-disruption restoration mechanism respectively is 20%, 38% and 43% lower than OVAR, HHVBF and VBF when there is no link disruption and the time is less than 200 s. This is because the glider-assisted link disruption restoration mechanism is based on the shortest path routing protocol. Shortest path routing has less time-delay than VBF and the HHVBF and OVAR protocols. For a sudden disruption of the link after 200 s, all of four algorithms restore the broken link. OVAR, HHVBF, and VBF rerouted the topology in a relatively shorter period of time by bypassing to repair the disrupted link. At this time, the glider-assisted link disruption restoration mechanism takes a relatively longer time to wait for the glider to assist in repairing the disrupted link. However, when the link returns to its original state, the glider-assisted link disruption restoration mechanism does not need to spend more time and energy on reconstructing the network topology to return to the original optimal state. OVAR, HHVBF, and VBF will continue to maintain the suboptimal state of the network for some time from 400 s to 1000 s in the following period, or spend