# A Glider-Assisted Link Disruption Restoration Mechanism in Underwater Acoustic Sensor Networks
## 1 Introduction  
For decades, marine technology has attracted a lot of interest. In the marine technology field, underwater acoustic sensor networks (UASNs) have made significant achievements in many aspects of underwater environmental exploration, such as ocean data collection, monitoring of water pollution, underwater disaster warning [1], and so on. Therefore, the study on UASNs has received increasing attention. However, due to the harsh communication environment, acoustic communication among nodes in UASNs has much more difficulties than that in terrestrial sensor networks. The first one is that the communication among underwater sensor nodes must rely on acoustic channels instead of radio channels for long-range communications. That is because radio signals undergo rapid attenuation and that makes the communication range decrease sharply in UASNs. For example, the reported transmission range of Berkeley Mica 2 Motes is 120 cm at 433 MHz [2]. Therefore, the acoustic signals have become the typical signal carrier due to their relatively lower attenuation. Many studies have been done on the realization of underwater communications. The method proposed by [PERSON] and [PERSON] in [3] shows the design and implementation of an omni-directional underwater acoustic micro-modem and itprovides the development of the cylindrically shaped micro-modem guaranteeing an omni-directional beam pattern in three dimensions [4]. However, the fact that the speed of acoustic signals is five-fold slower than the radio wave makes the end-to-end delay longer. The second one is that many factors make acoustic links among the sensor nodes dynamically changed with the time flow. One of the factor is that the location of a node tends to change over time due to water currents, drift and contact with marine organisms. Another factor is that the speed of sound is influenced by salinity, density and temperature of the water medium, and these factors are heterogeneous, which makes the channels between nodes vary with time. In the sea, those parameters present characteristics of stratification and thus the speed of sound is also stratified and varies with the depth. These factors have led to new studies focused on the optimization of mobility, coverage and connectivity [5]. Even if networks meet the connectivity conditions at the time of initial deployment, the communication could suffer a sudden disruption among nodes due to the time-varying acoustic communicating links. Furthermore, the sudden disruption of links can lead to the fact that the overall network connectivity and reliability might not be able to meet the requirements of application. All of these characteristics make the communication quality of UASNs unstable and the design of stable UASNs very tough. Therefore, in order to improve the connectivity and reliability of networks, it is important to design a mechanism to flexibly realize link disruption restoration over time. There is the problem that acoustic channels are time-varying, which can cause the links among nodes to be disrupted in UASNs. To solve this problem, many routing algorithms and topology maintenance mechanisms are proposed. The hole avoidance algorithm in [6] pays attention to energy hole and coverage hole avoidance. It can take advantage of redundant overlapping and repair a coverage hole during network operation with the least number of nodes. However, the coverage does not necessarily lead to network connectivity. The reliable and stable communication links also affect the network connectivity and thus affect the quality of entire network. In [7], a distributed reactive shadow zone and delay-aware routing protocol (SZODAR) for UASNs is proposed. It plays a certain role in maintaining the connectivity and reliability of networks. However, like SZODAR, many methods just rely on installing additional autonomously moveable power devices in common sensor nodes to move nodes up and down and achieve the desired effect. That is to say, these methods require more energy consumption when the nodes are adjusted up and down.