Abstract Recent surges in the development of underwater acoustic networks (UANs) have lead to a rapid acceptance of this technology in scientific, commercial, and military applications. However, limited work has been performed on developing secure communication mechanisms and techniques to protect these networks. Security mechanisms are wildly studied in terrestrial networks, and various defense mechanisms have been developed as safeguards. Because of the difference in communication mediums and physical environments, the existing solutions for terrestrial networks cannot be directly applied for UANs. In this paper, we study the effects of denial‐of‐service jamming attacks on UANs using real‐world field tests. We develop our own jammer hardware and signals in order to analyze the characteristics of different jamming attack models on a network. Our tests are performed on multiple commercial brand acoustic modems and an orthogonal frequency division multiplexing modem prototype. We show that UANs can be easily jammed using carefully timed attacks, which are energy efficient. Copyright © 2012 John Wiley & Sons, Ltd.

Underwater Acoustic Networks (UANs) are a transformative technology that is helping expand military, commercial and scientific applications in deep sea environments. The nature of network deployment in these deep sea environments has spurred a new paradigm in network routing known as pressure routing. Pressure routing is based on geographic routing but only uses limited location information, namely depth information, to route data from the sea floor to the surface. Recent work has shown that existing pressure routing protocols are vulnerable to malicious intrusions. In this paper we propose a resilient pressure routing protocol that seeks to reduce the effectiveness of malicious attackers, such as spoofing attacks. We evaluate our proposed protocol in a simulation environment and show that it reduces the effectiveness of spoofing attacks and maintains routing performance with minimal trade-off.

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). Although many time-synchronization protocols have been proposed for terrestrial wireless sensor networks, none of them can be directly applied to UWSNs. This is because most of these protocols do not consider long propagation delays and sensor node mobility, which are important attributes in UWSNs. In addition, UWSNs usually have high requirements in energy efficiency. To solve these new challenges, innovative time synchronization solutions are demanded. In this paper, we propose a pairwise, cross-layer, time-synchronization scheme for mobile underwater sensor networks, called DA-Sync. The scheme proposes a framework to estimate the doppler shift caused by mobility, more precisely through accounting the impact of the skew. To refine the relative velocity estimation, and consequently to enhance the synchronization accuracy, the Kalman filter is employed. Further, the clock skew and offset are calibrated by two runs of linear regression. Simulation results show that DA-Sync outperforms the existing synchronization schemes in both accuracy and energy efficiency.

Recently, various Medium Access Control (MAC) protocols have been proposed for underwater acoustic networks. These protocols have significantly improved the performance of MAC layer in theory. However, two critical characteristics, low transmission rates and long preambles, found in the commercial modem-based real systems, drastically degrade the performance of existing MAC protocols in the real world. A new practical MAC design is demanded. Toward a proper approach, this paper analyzes the impact of the two newly found modem characteristics on the random access-based MAC and handshakebased MAC, which are two major types of MAC protocols for underwater acoustic networks. We further develop the nodal throughput and collision probability models for representative solutions of these two MAC protocol types. Based on the analysis, we believe time sharing-based MAC is very promising. Along this line, we propose a time sharing-based MAC and analyze its nodal throughput. Both analytical and simulation results show that the time sharing-based solution can achieve significantly better performance.

Time synchronization is an important requirement for many services provided by distributed networks. A lot of time synchronization protocols have been proposed for terrestrial Wireless Sensor Networks (WSNs). However, none of them can be directly applied to Underwater Sensor Networks (UWSNs). A synchronization algorithm for UWSNs must consider additional factors such as long propagation delays from the use of acoustic communication and sensor node mobility. These unique challenges make the accuracy of synchronization procedures for UWSNs even more critical. Time synchronization solutions specifically designed for UWSNs are needed to satisfy these new requirements. This paper proposes Mobi-Sync, a novel time synchronization scheme for mobile underwater sensor networks. Mobi-Sync distinguishes itself from previous approaches for terrestrial WSN by considering spatial correlation among the mobility patterns of neighboring UWSNs nodes. This enables Mobi-Sync to accurately estimate the long dynamic propagation delays. Simulation results show that Mobi-Sync outperforms existing schemes in both accuracy and energy efficiency.

A mobile underwater wireless sensor network is a system of mobile underwater sensors which are acoustically networked. Existing ocean studies using independent mobile sensors have led to many interesting discoveries. Therefore, the networking capability will extend the capability of existing approaches and enable novel applications because it provides more control and coordination of the sensors. However, network mobility and environment uncertainty create numerous challenging issues which need to be addressed before such a paradigm becomes reality. Network deployment is among the most important problems because it has a significant impact on other research areas. In this paper, we consider the reuse-based deployment scheme in which a mobile underwater sensor network operates in a bounded area and a sensor, when reaching beyond the area, is deployed back into it. We propose a deployment framework to study the connectivity and coverage of such networks and show its effectiveness through two case studies. Our framework is based on the idea that the reuse of network nodes stabilizes these network metrics. By modeling them, we can estimate the number of nodes needed to achieve a requirement on network coverage or network connectivity only by solving algebraic equations rather than running more simulations.

In this paper, we investigate reliable data transfer for multi-hop underwater acoustic networks. Motivated by experiences from real-world field tests, we propose a Practical Coding-based Multi-hop Reliable Data Transfer (PCMRDT) protocol. For the per-hop reliable data transfer, PCMRDT combines random linear coding and selective repeat to achieve high reliability and efficiency. We analyze the data recovery capability for random linear coding so that we can set an appropriate coding rate. In addition, PCMRDT utilizes a multi-hop coordination mechanism to eliminate collisions and decreases average end-to-end delay over multiple hops. Simulation results show that PCMRDT can significantly reduce the network delay with high energy efficiency.

In underwater wireless sensor networks (UWSNs), one efficient way to alleviate the burdens of high propagation delay and high error probability during transmission is to deploy surface-level gateways, which utilize radio waves to forward information to a control station. Usually, deployment of the gateways is considered as an optimization problem with the objective to best satisfy certain parameters. In this work, we propose a prediction assisted dynamic surface gateway placement algorithm for mobile underwater sensor networks, called “PADP”, which intends to maximize the coverage within a specific period of time. PADP applies a tracking scheme “IMM” to predict sensor nodes' positions, adopts branch-and-cut to solve the optimization problem, and employs a disjoint-set data structure to handle connectivity. Simulation results show that PADP outperforms the existing static gateway deployment scheme.

Time synchronization and localization are basic services in a sensor network system. Although they often depend on each other, they are usually tackled independently. In this work, we investigate time synchronization and localization problems in underwater sensor networks. We propose a joint solution for localization and time synchronization, in which the stratification effect of underwater medium is considered, so that the bias in the range estimates caused by assuming sound waves travel in straight lines in water environments is compensated. By combining time synchronization and localization, the accuracy of both are improved jointly. Additionally, an advanced tracking algorithm IMM (interactive multiple model) is adopted to improve the accuracy of localization in the mobile case. Furthermore, by combining both services, the number of required exchanged messages is significantly reduced, which saves on energy consumption. Simulation results show that both services are improved and benefit from this scheme.

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). We propose a pairwise, cross-layer, time synchronization scheme for mobile underwater sensor networks, called TSMU. Facilitated by the Kalman Filter, the proposed method greatly improves the dynamic propagation delay estimation by exploring the Doppler effect. Simulation results show that TSMU outperforms existing synchronization schemes in both accuracy and energy efficiency.