Underwater Wireless Sensor Networks (UWSNs) are finding different applications for offshore exploration and ocean monitoring. In most of these applications, the network consists of a significant number of sensor nodes deployed at different depth levels throughout the area of interest. Sensor nodes on the sea bed cannot communicate directly with the nodes near the surface level, so they require multihop communication assisted by an appropriate routing scheme. However, this appropriateness not only depends on network resources and application requirements, but environment constraints are involved as well. These factors all provide a platform where a resource aware routing strategy plays a vital role in fulfilling different application requirements with dynamic environment conditions. Realizing this fact, much of the attention has been given to construct a reliable scheme, and many routing protocols have been proposed in order to provide efficient route discoveries between the source and sink. In this chapter, the authors present a review and comparison of different algorithms proposed recently for underwater sensor networks. Later on, all of these have been classified into different groups according to their characteristics and functionalities.
TopIntroduction
The ocean is vast as it covers around 140 million square miles; this is more than 70% of the earth’s surface. Not only has it been a major source of nourishment production, but with the passage of time it has also taken a vital role for transportation, presence of natural resources, defense and for purposes of entertainment. With the increasing role of oceans in human life, discovering these largely unexplored areas has gained more importance during the last decades. On one side, traditional approaches used for the underwater monitoring missions have several drawbacks. At the same time, these inhospitable environments are not feasible for human presence as unpredictable underwater activities, high water pressure and vast areas are major reasons for un-manned explorations. As a result of these reasons, Underwater Wireless Sensor Networks (UWSNs) are attracting the interest of researchers, especially those who belong to terrestrial sensor networks.
Figure 1.
A general scenario of the mobile UWSN architecture
Sensor networks used for underwater communication are different in many aspects from traditional wired or even terrestrial sensor networks. Firstly, energy consumption is different because some important applications require a large amount of data, but very infrequently (Heidemann, Wei, Wills, Syed & Yuan, 2006). Secondly, these networks are usually working on a common task instead of representing independent users. The ultimate goal is to maximize the throughput rather than fairness among the nodes. Thirdly, for these networks, there is an important relationship between the link distance, number of hops and reliability. For energy concerns, packets sent over multiple short hops are preferred instead of long links, as multi-hop data deliveries are proven more energy efficient for underwater networks than the single hop (Jiang, 2008). At the same time, it is observed that packet routing over more number of hops ultimately degrades the end-to-end reliability function especially in the fragile underwater environment. Finally, most of the time, such networks are deployed by a single organization with economical hardware so, strict interoperability with the existing standards is not required. Due to these reasons, UWSNs provide a platform that supports a review of the existing structure of traditional communication protocols.
Current research in UWSNs aims to meet the above criterion by introducing new design concepts, developing or improving existing protocols, and building new applications. This chapter examines different underwater routing protocols and algorithms proposed in recent years. The main purpose of this study is to address the issues like data forwarding, coverage and localization in UWSNs under different conditions. We present a survey of more than twenty routing protocols and algorithms of different types proposed for different applications.
Figure 2.
A possible system design for an UWSN (Shi, 2006)