The widespread adoption of the Wireless Sensor Networks (WSNs) in various applications in the terrestrial environment and the rapid advancement of the WSN technology have motivated the development of Underwater Acoustic Sensor Networks (UASNs). UASNs and terrestrial WSNs have several common properties while there are several challenges particular to UASNs that are mostly due to acoustic communications, and inherent mobility. These challenges call for novel architectures and protocols to ensure successful operation of the UASN. Localization is one of the fundamental tasks for UASNs which is required for data tagging, node tracking, target detection, and it can be used for improving the performance of medium access and network protocols. Recently, various UASN architectures and a large number of localization techniques have been proposed. In this paper, we present a comprehensive survey of these architectures and localization methods. To familiarize the reader with the UASNs and localization concepts, we start our paper by providing background information on localization, state-of-theart oceanographic systems, and the challenges of underwater communications. We then present our detailed survey, followed by a discussion on the performance of the localization techniques and open research issues.

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Dense WDM technologies make effective use of the vast fiber bandwidth and offer an added dimension to all-optical networks. Wavelength conversion at key network nodes is emerging as a fundamental functionality that can allow transparent interoperability, contention resolution, wavelength routing, and, in general, better utilization of the network resources under dynamic traffic patterns. In this contribution we offer an overview of the enabling technologies and extend the treatment to the network application of these converters. Attention is given to semiconductor optical amplifiers and their use in wavelength converters. Converters based on four-wave mixing as well as those based on nonlinear optical loop mirrors are evaluated, paying special attention to signal integrity and architectural as well as performance issues. The use of wavelength converters in wavelength routing networks is explored together with the application of these devices in contention resolution and in the routing wavelength assignment problem. Future directions are outlined at the system as well as network levels.

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