# COALA: A Protocol for the Avoidance and Alleviation of Congestion in Wireless Sensor Networks
## 4 Proposed Protocol Evaluation
### Presentation and Appraisal of Simulation Results  
The first set of simulation tests performed, evaluate how the increase in the rate of data transmission influences the average time it takes for an event notification, sent by a network node that senses the specific event, to reach the sink. The corresponding simulation results are illustrated in Figure 3. In these tests, all weighting factors are considered to be equal to 1.  
As it can be seen in Figure 3, in COALA protocol the average packet transmission time not only is less than that in DAlPaS Hard, but it is also more robust against the gradual growth of data transmission rate.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Topology size & 1000 m \(\times\) 1000 m \\ Number of nodes & 100 \\ Number of different topologies & 100 \\ Number of simulation runs for the same topology & 30 \\ Node transmission range & 50 m \\ Node sensing range & 50 m \\ Node buffer size & 10 packets \\ Initial node energy & 1.5 J–2.5 J \\ Event range & 100 m–400 m \\ Event range increase step & 100 m \\ \hline \hline \end{tabular}
\end{table}
Table 2: Simulation Parameters.  
Next, the performance of COALA protocol is investigated relatively with how the data transmission is affected by the progressive rise of the of data traffic. This correlation is depicted in Figures 4 and 5, through the graphical representation of the overall number of the data packets received and the percentage of the data packets lost respectively.  
The examination of both Figures 4 and 5 makes evident that COALA protocol not only achieves lower data losses but also resists more against the progressive increase of the traffic load.  
The next performance metric evaluated is the overall energy consumption of the network. Figure 6, illustrates in what way the rise of traffic load increases the network energy dissipation.  
Figure 4: Number of Packets Received vs. Traffic Load Rate.  
Figure 3: Average Packet Transmission Time vs. Traffic Load Rate.  
The examination of Figure 6 demonstrates that COALA protocol outperforms DAIPaS Hard in energy efficiency increasingly as the traffic load raises.  
Finally, a set of simulation tests were performed in order to examine how the variation of the weighting factors of the accumulated cost of nodes deviates the results of COALA protocol utilization. As an example, the way by which the variation of two weighting factors, i.e., \(w_{\widetilde{D}}\) and \(w_{\widetilde{B}\widetilde{O}}\) influences the average packet transmission time and the number of lost packages was investigated.  
Specifically, the influence of the \(w_{\widetilde{D}}\) variation in the two aforementioned metrics is depicted in Figures 7 and 8 correspondingly.  
Figure 5: Average Lost Packets Ratio vs. Traffic Load Rate.  
Figure 6: Network Energy Dissipation vs. Traffic Load Rate.  
The increase of \(w_{\widetilde{D}}\) makes the distance criterion have key priority within the accumulated cost function. The examination of Figures 7 and 8 validates that this increase accelerates the data transmission but deteriorates its quality. This is because data are preferred to be relayed through the closest neighboring nodes although these nodes may have more traffic load than other more distant neighbors.  
Similarly, the influence of the \(w_{\widetilde{B}\widetilde{O}}\) variation in the two aforementioned metrics is depicted in Figures 9 and 10 correspondingly.  
Figure 8: Number of Packets Lost vs. Traffic Load Rate for variable \(w_{\widetilde{D}}\).  
Figure 7: Average Packet Transmission Time vs. Traffic Load Rate for variable \(w_{\widetilde{D}}\).  
The increase of \(w_{\widetilde{BO}}\) makes the buffer occupancy criterion have primary priority within the accumulated cost function. The examination of Figures 9 and 10 confirms that this increase improves the reliability of the data transmission at the expense of the throughput. This is because data are preferred to be relayed through nodes that have less traffic load although this may involve longer paths.