Introduction
Underwater acoustic networks (UANs) are an emerging research topic because of the key role that this technology will play in military and commercial applications including disaster prevention, tactical surveillance, offshore exploration, pollution monitoring, and oceanographic data collection. A key challenge in the design of UANs stems from the characteristics of the underwater acoustic (UW-A) channel, which exhibits high path loss, noise, multipath, high and variable propagation delay, and Doppler spread. Therefore, reliable communication links are practically feasible only at low data rates. Additionally, the propagation challenges in the underwater environment result in temporally and spatially varying UW-A channel coefficients, which drives research efforts toward the design of specialized protocols at different layers of the network protocol stack - often with a cross-laver aooroach.