Contents I. Introduction
The earth is a water planet as 71% of its surface is covered with water. The oceans contain about 96.5% of all earth water. Surprisingly, we know very little, only less than 10%, about the Earth's water bodies despite of its vital role for nourishment, transportation, presence of natural resources, defence and adventurous purposes, while a large area still remains unexplored [1]. Recently, ocean bottom sensor nodes are deployed to facilitate scientific and commercial applications, environment and pollution monitoring disaster prevention, tactical surveillance and assisted navigation [2]. To make these applications viable, it is needed to enable underwater communication among underwater devices [3]. Typical underwater applications require multihop co-operative network where sensor nodes must be able to exchange configuration, location and movement information and to relay data to an onshore station using satellite terrestrial network radio frequency (RF) [4]. However, RF can be propagated through conductive sea water only at low frequencies ranging from 30–300 Hz, requires large antenna and high transmission power [5]. Optical signals also suffer from attenuation and scattering [6]. So, these are not suitable in underwater environment. As an alternative, wireless acoustic sensor network (WASN) appears to be a feasible technology for underwater communication. Acoustic communications are the typical physical layer technology that propagates sound waves to communicate. The typical frequency range is between 10 Hz to 1 MHz. However, the employment of acoustic signals imposes many distinctive challenges. For instance, it suffers from long propagation delays, low bandwidth, frequent loss of connectivity, sound speed variability, limited battery power and much other environmental impairments [7].
