# SOUNET: Self-Organized Underwater Wireless Sensor Network
## 5 Simulation Results
### Analysis of Results
#### 5.3.1 Network Connectivity  
The network connectivity (\(C\)) is defined by:  
\[C=\frac{N_{connect}}{N_{unwake}}, \tag{1}\]  
where \(N_{unwake}\) and \(N_{connect}\) are the number of nodes that are awake, which means alive but not sleeping in isolation and the number of nodes that are connected to the sink, respectively. Obviously, the network with a larger \(C\) can cover a wider area. Figure 11 shows how the connectivity changes at the beginning of node deployment from 0 s to 120 s. At the beginning, SOUNET and S\({}_{13}\) achieve 100% connectivity very quickly within several seconds while Seaweb requires much more time. The  
Figure 10: Various network stages of SOUNET: **(a)** After initialization; **(b)** After the event 1; **(c)** After the event 2; **(d)** After the event 3; **(e)** After the event 4; **(f)** After the event 5; **(g)** After the event 6; **(h)** After the event 7; **(i)** After reorganization.  
Seaweb's connectivity reaches only 15% until 120 s and ends up at 100% after 15 min. The quick achievement of 100% connectivity results from the _HELLO_ packet flooding in the network initialization while the breadth-first search algorithm [34] is used in Seaweb.  
Figure 12 shows how the connectivity changes during the whole simulation time in which the incidents occur every three hours as planned from 0 h to 27 h, respectively. As the incident of node loss occurs at 3 h, 6 h, 9 h, 15 h, and 21 h, the connectivity sharply decreases since the group of nodes associated with the lost nodes is disconnected at once. However, since the proposed scheme allows the disconnected nodes to detour through the new parents, the connectivity is recovered shortly while in the other schemes the connectivity remains unrecovered. Although the formation of closed-loops are possible during the detour process, the nodes can detect and eliminate them. In our scenario, a group of nodes 2, 3, and 9 and another group of nodes 4-8, 12-15, and 22 could form such closed loops after event 1 and 2, respectively.  
It is shown that the first group of nodes 2, 3, and 9 can resolve the loop immediately after the event occurs while the second group of nodes 4-8, 12-15, and 22 may take more time to resolve it. Normally, as a group of isolated nodes is bigger, it takes more time to recognize isolated or closed loops. The node  
Figure 11: Variation of network connectivity at the beginning of simulation (0 s to 120 s).  
Figure 12: Variation of network connectivity during the entire simulation time (0 h to 27 h): (**a**) Considering only packet collisions; (**b**) Considering both packet collisions and bit errors.  
addition at 12 h and 18 h also temporally decreases the connectivity since the denominator, the number of nodes that are awake, \(N_{\textit{auxke}}\) increases. However, in the proposed scheme the added nodes quickly repair the damaged connectivity by building a new connection with the isolated nodes or by rebuilding the existing connection so that the paths to the sink become shorter. Thus, the recovery of connection is very quick. Meanwhile, both Seaweb and S\({}_{13}\) recover the connectivity only when the network is reorganized at 24 h, not being capable of dealing with the incidents of loss or addition of nodes. Even at the network reorganization, the recovery of Seaweb and S\({}_{13}\) remains below 90%. When we add bit errors that reflects the harsh underwater channel condition, as well as packet collisions (Figure 12b), SOUNET's connectivity fluctuates with time because nodes continuously change their parent node. On the other hand, performances of S\({}_{13}\) and Seaweb decreases considerably. For Seaweb, in particular, the initial connectivity at 0h and 24h suffers a significantly large drop (compared with Figure 12a) since many control packets for network initialization are lost. SOUNET clearly outperforms the others and it highlights SOUNET's robustness to the unreliable and peculiar characteristics of underwater channel.