## 2 Underwater Sensor Networks  
### Underwater Channels  
TWSNs use radio frequency waves, but UWSNs rely on physical means like acoustic waves to transmit signals. In terms of protocols, the networks stacks of TWSNs are not suitable for UWSNs since low bandwidths and large latency result in long end-to-end delays [5]. Acoustic signals have unique characteristics. First, acoustic signals tend to have high transmission losses due to attenuation and geometric spreading. The attenuation is induced by absorption due to the conversion of the acoustic energy into heat. Geometric spreading means the propagation of sound energy because of the expansion of the wave front. Second, it is easy for acoustic signals to be affected by noise. Noise includes man-made noise caused by shipping activities and ambient noise caused by natural phenomena such as tides, currents, and storms.  
The speed of sound in sea-water is a function of the temperature, salinity and depth, as expressed by Equation (1):  
\[\begin{array}{ll}\mathrm{C(D,S,T)=}&1448.96+4.59\mathrm{T}-5.3\times\,10^{-2 }\mathrm{T}^{2}+\ 2.374\,\times\,10^{-4}\mathrm{T}^{3}+1.34\mathrm{(S-35)}\\ &+\ 1.63\,\times\,10^{-2}\mathrm{D}+1.675\,\times 10^{-7}\mathrm{D}^{2}-1.025\, \times 10^{-2}\mathrm{T}\mathrm{(S-35)}-\ 7.139\\ &\times\,10^{-13}\mathrm{TD}^{3}\end{array} \tag{1}\]  
Here, T, S, and D refer to the temperature in degrees Celsius, the salinity in parts per thousand, and the depth in meters respectively. This equation is valid within the temperature range from 2 to 30 \(\mathrm{\ {C}}\), 25 to 40 parts per thousand (%o) salinity and depths from 0 to 8,000 m. Figure 1 shows the change in the speed of the acoustic signal with the temperature and the salinity if the depth is fixed at 50 m. When the temperature of seawater increases, the speed of the acoustic signal also increases steadily. The speed shows a dramatic increase if the salinity increases. The effect of the salinity is higher than that of the temperature.  
Equation (2) determines the attenuation of the acoustic signal [A(\(\mathcal{d}\))] according to the distance (\(\mathcal{d}\)) and the communication frequency (\(f\)) in the ocean [6]: Here, \(\mathcal{d}\) is the distance (m), \(f\) is the frequency (kHz) and \(\alpha(f)\) is the absorption coefficient (dB/km)  
\[\begin{array}{ll}\mathrm{A(\mathcal{d})=\ d\alpha^{d},\alpha=\ 10^{\alpha(f)/10}}\\ \alpha(f)=0.11\dfrac{f^{2}}{1+f^{2}}+44\dfrac{f^{2}}{4100+f^{2}}+2.75\cdot 1 0^{-4}f^{2}+0.003\end{array} \tag{2}\]  
Here, \(\mathcal{d}\) is the distance (m), \(f\) is the frequency (kHz) and \(\alpha(f)\) is the absorption coefficient (dB/km) Figure 2 shows a graph of this attenuation with the distance and the communication frequency. In UWSNs, the available bandwidth (under 50 kHz) is severely limited due to the extreme characteristics of the underwater channel. As the distance increases, the attenuation also increases rapidly.  
Figure 1: The speed of the acoustic signal.  
To improve the efficiency of data transmission using the acoustic signal, the design of the network protocol stack is important. A protocol stack for UWSNs should manage power and promote cooperation among the underwater sensor nodes. The protocol stack consists of physical layer, data link layer, network layer, transport layer, and application layer functionalities, as shown in Figure 3.  
The security service protects the underwater sensor nodes and their communications. The power management plane is responsible for the network functionalities aimed at minimizing the energy consumption. The time synchronization service adjusts the local time of each sensor nodes to the global time. The localization service is responsible for providing absolute or relative localization information to the underwater sensor node, when needed by the protocol stack or by the application.