# A Glider-Assisted Link Disruption Restoration Mechanism in Underwater Acoustic Sensor Networks
## 3 The Glider-Assisted Link-Disruption Restoration Mechanism
### The Network Model  
In the water volume D to be monitored, the glider-assisted link disruption restoration problem considered in this paper is based on the pre-existing hybrid deployment of static and dynamic nodes. Assume that the original sensor nodes deployment has satisfied the coverage and connectivity conditions.  
As shown in Figure 1, the entire volume is divided into several sub-volumes and the network is a clustered structure. Each sub-volume contains three kinds of nodes including a cluster head node \(h_{i}\), a glider \(g_{i}\) and multiple static sensor nodes \(S_{i}=\left\{s_{i}^{1},s_{i}^{2},\ldots,s_{i}^{j}\right\}\). A static node is anchored to the sea floor by cables and floats in the water aided by buoyancy devices. Its function is to collect and forward the environmental data to complete the conventional monitoring tasks. All static nodes have the same sensing range and communication range. The cluster head node is the only one in each sub-volume. The cluster head node is responsible for receiving the data forwarded by the static and dynamic nodes. In addition to this, the cluster head node can collect the status information of whether the link is disrupted in its sub-volume. Every cluster head node then sends the collected information to the cluster head nodes of the upper sub-volume so that the collected data is transmitted to sink nodes layer by layer. Finally, the sink node will send the combined data to a satellite or shore base station through radio communication. The dynamic node in our paper is a glider. On the one hand, the glider has a common function with static nodes. It can collect and forward environmental data. On the other hand, to a certain extent the dynamic nodes can freely move and act as an advanced sensor node. Every glider would periodically pass through the cluster head nodes to forward the collected data information to the cluster head node in its sub-volume.  
Although the static nodes in the network are fixed to the bottom by cables, the location of the static nodes will be disturbed due to the external forces such as water currents and contact with marine animals. Compared with the nodes that are not fixed, the anchoring nodes are relatively less affected by the water flow. Their position change volume is in a spherical coronal surface, which also returns to the original position as the water flow changes over time. The change of the node position will directly affect the quality of underwater acoustic links. In addition, the quality of the underwater acoustic links will also be affected by the sea surface fluctuation. Therefore, different sea surface fluctuations may cause different signal propagation paths. These time-varying factors will temporarily reduce the quality of underwater acoustic links. In severe cases, the communication link will be disrupted and the network connectivity will be degraded. If the network just waits for the environment to return to its original state, it will inevitably cause link disruption for a longer time, resulting in longer transmission delays and greater waste of network resources caused by data retransmission.  
In addition to the problem of link disruption caused by the change of underwater acoustic channel, there are several other reasons why the acoustic links can be disrupted in UASNs. These problems include the failure of nodes, the changed direction of hydrophones due to currents and so on. In this paper, we only cover the link disruption during the networking process of sensor nodes, that is, we do not consider the case that underwater node is disrupted. We assume that the sensor nodes in our network model carry omni-directional hydrophones and we do not consider the change of hydrophone direction.