## 6 Discussion of the Results  
The simulation results presented in this paper show that achieving good performance in underwater communications in shallow areas is very difficult, since the presence of the multipath effect reduces the theoretical maximum communication capacity. So, the designer of a specific acoustic modem for a particular area must have knowledge of the ocean bottom material, in order to adapt the appropriate value of the data transmission capacity of the acoustic link. In the first part of the paperpaper it was shown the impact of increasing salinity and temperature within a range of values similar to the real ranges of values for these parameters in many underwater environments and coastal areas. We have simulated a network with dimensions of shallow areas and there are taking into account the parameters of the S2C R 48/78 acoustic modem. Capacity and total delay lifetime have been estimated as a function of the temperature and salinity values. The result is that a network in shallow areas where surface temperature is higher results in higher communication capacity utilization compared to a network in shallow area where the surface temperature is lower. Obviously, under higher surface temperature conditions, the network lifetime is longer because more data are transmitted per units of time, the acoustic modem would consume less energy in transmitting \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number of Reflections** & **Basalt** & **Limestone** & **Gravel** & **Sand** \\ \hline
2 & 4573 & 4635 & 4474 & 4279 \\
4 & 4065 & 4014 & 3610 & 3362 \\
6 & 3618 & 3447 & 2798 & 2493 \\
8 & 3197 & 2905 & 2008 & 1648 \\
10 & 2789 & 2375 & 1234 & 823 \\
12 & 2391 & 1856 & 502 & 163 \\
14 & 1998 & 1343 & 64 & 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results of lifetime for different materials. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number of Reflections** & **Basalt** & **Limestone** & **Gravel** & **Sand** \\ \hline
2 & 160.16 & 160.00 & 160.04 & 160.34 \\
4 & 291.55 & 291.45 & 291.77 & 292.18 \\
6 & 424.40 & 424.43 & 425.23 & 425.88 \\
8 & 557.71 & 557.96 & 559.79 & 561.13 \\
10 & 691.29 & 691.91 & 696.47 & 700.96 \\
12 & 825.11 & 826.42 & 842.96 & 885.47 \\
14 & 959.22 & 961.98 & 1111.54 & 2221.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of delay for different materials. packets where the transmission time of a unit individual packages are lower. On the other hand, the nodes of a network in a shallow area where salinity has higher values would utilize a lower acoustic channel capacity for communication between them compared to the capacity that would utilize the nodes in an area with lower salinity. This is because the increase in salinity increases the losses due to absorption, causing the SNR in the receiver to decrease, which would be translated into a decrease of the channel capacity. The total data propagation time from transmitter to receiver, as a combined parameter between transmission and propagation delay, decreases with increasing of both salinity and temperature values. This happens because the acoustic waves propagate more rapidly underwater in both cases. The effect of multipath propagation has also been evaluated in this paper, where the simulations are applied at depths of up to 200 m. The receiver node gets data from both the direct path in less time and the reflection paths formed by the surface and bottom at longer time. Losses in surface reflections increase if surface roughness or surface wind speed increases. On the other hand, losses due to bottom reflections depend on bottom material of the ocean bed. So, another conclusion of this paper, thus related to the effects of the bottom materials, is that the acoustic communication capacity available is higher if the bottom material is less porous compared to the case of a more porous material. For example, such as in sand where the capacity utilized for acoustic communication and the lifetime of a network in an area with such bottom material is lower.