A B S T R A C TThe past 30 years have seen a growing interest in underwater acoustic communications because of its applications in marine research, oceanography, marine commercial operations, the offshore oil industry and defense. Continued research over the years has resulted in improved performance and robustness as compared to the initial communication systems. In this paper, we aim to provide an overview of the key developments in point-to-point communication techniques as well as underwater networking protocols since the beginning of this decade. We also provide an insight into some of the open problems and challenges facing researchers in this field in the near future.protocols for such networks. In this paper, we do not attempt to provide an exhaustive survey of all research in the field, but instead concentrate on ideas and developments that are likely to be the keystone of future underwater communication networks. II. Underwater CommunicationsHigh-speed communication in the underwater acoustic channel has been challenging because of limited bandwidth, extended multipath, refractive properties of the medium, severe fading, rapid time variation and large Doppler shifts. In the initial years, rapid progress was made in deep water communication, but the shallow water channel was considered difficult. In the past decade, significant advances have been made in shallow water communication.

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Communication between a set of underwater systems such as remote sensors, autonomous underwater vehicles and control vessels would enhance the effective use of such systems tremendously.As electromagnetic waves do not propagate well underwater, acoustics plays a key role in underwater communication. Although point-to-point acoustic links can be established via numerous modulation schemes, an acoustic communication network demands multi-user communication. In such an environment, orthogonal modulation schemes could provide a solution for multiple simultaneous acoustic links.As an alternative to orthogonal schemes, random shared access technology has proven successful in many wireless networks.Through numerical simulations, we compare the performance of orthogonal and random shared access for underwater networking.Over the past few decades, numerous networking protocols have been developed for use in wired and wireless networks. Due to significant differences in the characteristics of electromagnetic and acoustic channels, these networking protocols require modifications to perform well in underwater networks. As sound waves are much slower than electromagnetic waves, the latency in communication is typically much higher. Due to the multi-path propagation and ambient noise, the effective data rates are lower and packet loss is much greater. In this paper, we simulate variants of some popular protocols for underwater use, focusing on the Physical and Datalink layers of the OSI protocol stack. The aim is to select an appropriate Physical Layer and Datalink layer model for a small underwater network to be implemented.

Abstract-This paper presents results from a study on using multiple communication channels simultaneously for effective networking in a small AUV network. AUVs can be highly mobile, leading to time-varying inter-node distances and a dynamic network topology. We try to exploit this mobility by using multiple acoustic modems operating at different frequency bands and suited for different ranges. We utilize a MAC protocol based on MACA that uses RTS / CTS / DATA / ACK handshaking along with carrier sensing. Data packet trains are used to greatly enhance the performance of this protocol and results show that this feature makes it a very viable protocol for underwater networks in general. The protocol exchanges AUV position information and uses this information to allocate traffic to the different modems. The study is oriented towards the use of multiple AUVs in highly co-operative missions where effective peer to peer data exchange is vital. The term "channel" is used to represent very different capability modems, and is different from the standard context of multi-channel communications where a single transceiver has the option to choose between multiple channels, as in FDMA or CDMA. In our model, we have multiple and very different transceivers being used simultaneously. We name the protocol MACA-MCP since it utilizes Multiple Channels and Positioning information.

-As electromagnetic waves do not propagate well underwater, acoustics plays a key role in underwater communication. Due to significant differences in the characteristics of electromagnetic and acoustic channels, networking protocols for underwater systems differ from those developed for wired and wireless radio networks. Various schemes have been proposed for one or many aspects of underwater networks. However, no widely accepted common framework exists for underwater acoustics to unify these proposed schemes into a functional underwater network. The availability of such a framework will enable easy integration of independently developed techniques and thus accelerate the pace of research in underwater acoustic networking.In order for a common framework to be successful, it needs to have a wide acceptance. To gain such an acceptance, a framework needs to take into account a wide variety of different constraints and requirements for various underwater applications. This requires inputs from various research groups and end users. To help define the use cases and a common framework for underwater networking, a joint effort has been initiated between acoustic communication research groups at the Acoustic Research Laboratory (National University of Singapore), Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology. In this paper, we discuss the first draft of the framework specifications from this effort. We welcome feedback from the underwater acoustic research community and potential end users of underwater networking systems.

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