## 2 Applications and Background of Uwsn
### Background of UWSN
#### 2.2.4 Reliability  
The main challenging factor for underwater networking system is relaying the collected data to the surface stations. Congestion control mechanisms of terrestrial networks show many difficulties in underwater wireless multi-hop networks [45]. TCP is the kind of mechanism that works based on an end-to-end connection technique. A TCP-handshake of the sending and receiving nodes is needed even before the transmission begins. In the case of UWSN, only a few bytes in each packet have to be transmitted. Transmitting this small data is a problem for TCP as it follows the 3 way handshake mechanism. In the case of acoustic communication, propagation takes longer than that of transmission which leads us to a bandwidth\(\times\)delay product problem [24]. For reliability, TCP needs an end to end ACK and retransmission strategy but it will cause poor throughput along with a longer transmission time. It is considered in TCP that packet data losses are caused by congestion only. So, TCP only focuses on the decrease of the transmission rate but the error prone acoustic channel and the failures of the nodes can also be a reason for data packet losses in UWSN. So, to maintain throughput efficiency, the data transmission rate needs to not be decreased [24]. Other terrestrial protocols like User Datagram Protocol (UDP) usually do not maintain this flow control. Rather it just drops the packet without creating any scope for recovery or re-transmission which results in total loss of the data packet [45]. Data transmission in underwater communication is greatly affected by the following four challenging factors:  
* Noise of the Communication Medium,
* Attenuation of the Channel,
* Channel Bandwidth Limitation, and
* Low Speed of the Acoustic Transmission. The underwater noisy medium greatly degrades the transmission quality and there are four factors which cause this noise--1. movement of the Vehicles 2. waves 3. thermal variations and 4. other turbulence [46, 47]. If ambient noise is designated by \(N\) in \(dB\), then the power spectral density of the noise is as follows:  
\[N=N_{\rm v}+N_{\rm w}+N_{\rm tv}+N_{\rm tb}, \tag{1}\]where,  
\(N_{\rm v}=\) Power Spectral Densities of the Noises due to Movement of the Vehicles,  
\(N_{\rm w}=\) Power Spectral Densities of the Noises due to Waves,  
\(N_{\rm tw}=\) Power Spectral Densities of the Noises due to thermal variations,  
\(N_{\rm tb}=\) Power Spectral Densities of the Noises due to turbulence. And these parameters are modeled as follows [46]:  
\[N_{\rm v} =40+20(s-0.5)+26 log(f)-60 log(f+0.03). \tag{2}\] \[N_{\rm w} =50+7.5^{\rm s}+20 log(f)-40 log(f+0.4).\] (3) \[N_{\rm tw} =-27+log(f).\] (4) \[N_{\rm tb} =27-30 log(f). \tag{5}\]  
Here, \(f\) denotes the frequency in kHz and s denotes the speed of the wind where \(s\in[0,1]\) and it defines the extent of shipping noise. Noise of the ship movements ranges from 20-200 Hz whereas noise due to thermal variation affects the signals above 200 kHz. Absorption loss and spreading loss cause attenuation in underwater communication. Attenuation also varies with the frequency of the signal and also the distance of the signal from the source [47]. So, the attenuation of the signal in the underwater medium is denoted by \(A(d,f)\), where d is the distance of the signal from the source and \(f\) is the frequency of the signal. Equation (6) shows the expression for this attenuation where \(A_{\rm c}\) is the normalized constant, \(\alpha\) is the absorption co-efficient and k is the spreading factor (in practice k = 1.5):  
\[A(d,f)=A_{\rm c}d^{\rm k}\alpha(f)^{\rm d}. \tag{6}\]  
The delay efficiency of UWSN greatly depends on the speed of the acoustic waves. The speed of the acoustic waves in saline water can be expressed as [48]:  
\[ \begin{split} c&=1449+4.591T-5.304\times 10^{-2}T^{2} \\ &\quad+2.374\times 10^{-4}T^{3}+1.34(S-35)\\ &\quad+1.63\times 10^{2}D+1.675\times 10^{7}D\\ &\quad+1.025\times 10^{-2}T(S-35)-7.139\times 10^{3}TD^{3}. \end{split}