# A Two-Stage Routing Protocol for Partitioned Underwater Wireless Sensor Networks
## 4 Results and Discussion
### Partitioned Network
#### 4.2.2 Death Rate  
Figure 16a-d show the number of dead nodes for 25, 50, 75 and 100 nodes, respectively, over the routing cycles intervals of 1-20, 21-40 and 41-50. For 25 and 50 nodes, PH shows a smaller number of dead nodes compared to NPH. Comparison of the death rate figures of the partitioned networks with those of unpartitioned networks reveals that the difference between the death rates of PH and NPH is reduced, i.e., the death rate of PH has increased whereas the death rate of NPH has reduced in the case of partitioned networks. The increase in the death rate of PH is caused due to the additional transmissions (in case of partitioned networks) in the form for localization requests and the corresponding beacons plus additional contention due to the transmission of these packets. Due to the same reason, the number of dead nodes in the 2 nd and 3 rd interval in the case of 75 and 100 nodes goes higher in PH as compared to NPH. The increase in the requests, response beacons and higher contention created due to bigger number of nodes, increases energy consumption and thus increases the no. of dead nodes. How can PH still produce better packet delivery ration despite its slightly higher death rate in case of 75 and 100 nodes. The answer lies in the functioning of NPH. In case of NPH, the nodes which get partitioned, do not have any mechanism to receive beacons. Therefore, despite having enough energy, these nodes can neither transmit their own data packets, nor can they act as relay. Therefore, their energy remains intact which results in a lower death rate. However, as the partitioned nodes cannot transmit their packets, the overall number of delivered packets is reduced at least by the number of partitioned nodes. On the other hand, PH incurs some additional cost, in the form of request and beacon response packet, in order to access the partitioned nodes thus enabling them to transmit their data, thereby improving the packet delivery ratio.  
In case of 25 nodes, PH has a lower number of dead nodes during routing cycle number 21 to 40 and 41 to 50, respectively. Due to the smaller number of nodes, the effect of traffic generated due to our partition handling mechanism is minimal. Thus, even though the proposed scheme improves the packet delivery ratio, the volume of overhead for partition handling is small and therefore does not  
Figure 15: No. of delivered packets (Partitioned Network).  
affect the death rate considerably. Moreover, as explained before, the early death of nodes is avoided by the introduction of residual energy threshold.  
In case of 50 nodes, a similar affect can be observed. However, the number of dead nodes in PH during the same intervals has increased compared to 25 nodes. Moreover, the difference between the number of dead nodes of PH and No-PH has also decreased. This is due to the increasing overhead of PH to carryout partition handling routine.  
In PH, in case of 75 nodes, a higher number of partitioned nodes generates higher communication overhead in the form of localization requests and response beacons. Besides, overhead due to contention also increases, thus resulting in higher number of dead nodes in case of PH compared to NPH. The smaller number of dead nodes in case of NPH is also due to the fact that the partitioned nodes do not transmit at all, thus reducing the contention domain. With a smaller contention domain, fewer packets are lost which reduces death rate of NPH in the later routing cycles. However, this reduction in death rate is achieved at the cost of eliminating partitioned nodes and thus reducing the packet delivery ratio. PH, on the other hand, keeps the network connected by allowing the partitioned nodes to send some extra packets in the form of beacon request.