1. Introduction to Underwater Wireless Communication Network

Underwater wireless communication networks  (UWCNs) are constituted by sensors and autonomous underwater vehicles (AUVs) that interact to perform specific applications such as underwater monitoring (Fig. 1). Coordination and sharing of information between sensors and AUVs make the provision of security challenging. The aquatic environment is particularly vulnerable to malicious attacks due to the high bit error rates, large and variable propagation delays, and low bandwidth of acoustic channels. Achieving reliable intervehicle and sensor-AUV communication is especially difficult due to the mobility of AUVs and the movement of sensors with water currents. The unique characteristics of the underwater acoustic channel, and the differences between underwater sensor networks and their ground based counterparts require the development of efficient and reliable security mechanisms. This article discusses security in UWCNs. It is structured as follows. The following section explains the specific characteristics of UWCNs in comparison with their ground-based counterparts. Next, the possible attacks and countermeasures are introduced. Subsequently, security requirements for UWCNs are described. Later, the research challenges related to secure time synchronization, localization, and routing are summarized. Finally, the article is concluded.


Figure 1. Underwater sensor network with AUVs.



Underwater sensor networks have some similarities with their ground-based counterparts such as their structure, function, computation and energy limitations. However, they also have differences, which can be summarized as follows.Radio waves do not propagate well underwater due to the high energy absorption of water.Therefore,underwater communications are based on acoustic links characterized by large propagation delays. The propagation speed of acoustic signals in water (typically 1500 m/s) is five orders of magnitude lower than the radio wave propagation speed in free space. Acoustic channels have low bandwidth. The link quality in underwater communication is severely affected by multipath, fading, and the refractive properties of the sound channel. As a result, the bit error rates of acoustic links are often high, and losses of connectivity arise. Underwater sensors move with water currents, and AUVs are mobile. Although certain nodes in underwater applications are anchored to the bottom of the ocean, other applications require sensors to be suspended at certain depths or to move freely in the underwater medium. The future development of geographical routing is very promising in UWCNs due to its scalability and limited signaling properties. However, it cannot rely on the Global Positioning System(GPS) because it uses radar waves in the 1.5 GHz band that do not propagate in water. Since underwater hardware is more expensive, underwater sensors are sparsely deployed. Underwater communication systems have more stringent power requirements than terrestrial systems because acoustic communications are more power-hungry, and typical transmission distances in UWCNs are greater;  hence, higher transmit power is required to ensure coverage. The above mentioned characteri-stics of UWCNs have several security implications.UWCNs suffer from the following vulnerabilities. High bit error rates cause packet errors.Consequently, critical security packets can be lost. Wireless underwater channels can be eavesdropped on. Attackers may intercept the informat-ion transmitted and attempt to modify or drop packets. Malicious nodes can create out-of-band connections via fast radio (above the water surface) and wired links, which are referred to as wormholes. Since sensors are mobile, their relative distances vary with time. The dyna-mic topology of the underwater sensor network not only facilitates the creation of wormholes but it also complicates their detection. Since power consumption in underwater communi-cations is higher than in terrestrial radio communicate-ons and underwater sensors are sparsely deployed, energy exhaustion attacks to drain the batteries of nodes pose a serious threat for the network lifetime.



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