## 2 Applications and Background of Uwsn
### Background of UWSN  
#### 2.2.1 Underwater Acoustic Communication  
Path loss designates the underwater acoustic communication. Mostly the transmitting distance and frequency of the transmitted signal define the path loss [22]. Absorption loss occurs when acoustic energy is transferred into thermal energy. This fact implies that the channel bandwidth has to be set on the basis of the desired transmitting distance. Absorption loss increases with the increase in the operating frequency and the interval between the receiver and the transmitter [22]. The power constraint automatically sets a limit on the availability of the bandwidth. Consequently, longer the communication link lesser the bandwidth, shorter the communication link more the bandwidth. Besides lower the bandwidth, lesser the data rate [23; 24]. As for example, transmission over a distance of 30 km is feasible to perform by a single hop of 30 kHz bandwidth or the same transmission distance can be covered with much higher bandwidth of 300 kHz by 10 hops instead. So higher bandwidth (increased throughput) can be obtained by trading-off complicated relays in transmission. The shorter the bandwidth the longer the sensor transmission range. On the contrary, the transmission range decreases with the increases of the bandwidth. These variations in transmission ranges correspond to different bandwidths and operating frequency bands. The bandwidth of the underwater acoustic  
Figure 1: Underwater Pipeline Monitoring with UWSN.  
communication varies from less than 1 kHz to more than 100 kHz [2]. On the other hand, the operating frequencies for underwater acoustic channels usually vary from 10 Hz to 1 MHz [25]. Acoustic modems often use operating frequency bands centered at a unit of kHz which cover the distance on the order of kilometers (km) [26]. Besides, particular underwater applications need particular transmission range and bandwidth depending on the task it performs. The data rate of the acoustic communication varies from 31 kb/s to 125 kb/s depending on the channel encoding and the number of transmitters and receivers [27]. Table 1 presents the acoustic channels of UWSN in terms of operating frequency band, bandwidth, direct transmission range, and applications [25; 26; 2].  
Opportunity for Free Space Optical (FSO) waves in the underwater environment is limited as the optical frequency band faces acute water absorption and heavy back scatter. Its attenuation is very high and it is almost 1000 times that of air even in the clearest water. Furthermore, the attenuation of the turbid water is one hundred times that of the densest fog [35]. So, in underwater environments, the main drawback of FSO is very limited transmitting distance due to high attenuation.  
Acoustic communication can be considered as the most versatile underwater communication technique due to its low attenuation. Acoustic medium works out more perfectly with higher depth and steady temperature of the water. But in case of using acoustic waves in shallow water, temperature gradients and surface ambient noise can affect the performance adversely. Multi-path propagation due to reflection and refraction prospects might be another reason. Sound speed in water is 4 times than that of in air. But as the depth, temperature and Practical Salinity Unit (PSU) of water increase, the sound speed also increases [24]. Even though its speed in water is much slower than that of Electromagnetic (EM) waves. But it is the most reliable underwater communication medium which is feasible for practical deployment presently.