# Cooperative Data Collection Mechanism Using Multiple Mobile Sinks in Wireless Sensor Networks
## 5 Performance Evaluations  
This section aims to investigate the performance comparison of the proposed CDCA against the existing HiCoDG [20] and balanced regional partitioning algorithm BRPA [22] in terms of the rendezvous time, network lifetime, fairness index, the SD energy consumption and efficiency index. Four scenarios are considered in the experiments. Herein, the BRPA used a single mobile sink to collect the data from all CPs. To compared with the BRPA, the whole region is partitioned into several subregions and then each subregion applies the BRPA to select the CPs and allocate a mobile sink to collect data from the selected CPs. Table 2 lists the parameters of the simulation environment. In the experiments, the sensor nodes are placed uniformly in an 800 m \(\times\) 800 m rectangular region. The sensing range of each sensor is set by 20 m while the communication range is set by 30 m. The initial energy of each sensor is 100 J. Each sensor creates one data packet in each round which is a predefined time period for mobile sinks cooperatively passing through all rendezvous points and collecting data from all local mobile sinks. Assume that every mobile sink \(m_{i}\) is aware of the time point that can meet the global mobile sink \(m_{0}\). To further investigate the performance of the compared mechanisms in different distributions of the sensors, four scenarios are considered in the experiments, as shown in Figure 3. The first scenario, called balanced deployment scenario or BD-Scenario in short, adopts randomly deployment. All sensor nodes can communicate with each other. The other three scenarios are unbalanced deployment scenarios, called UD1-Scenario, UD2-Scen and UD3-Scenario. The big holes with circle-shape and X-shape are existed in the central region in UD1-Scenario and UD2-Scenario, respectively. In the UD3-Scenario, the sensors are connected to form an X-shape, which is partitioned by the holes. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Node deployment & Uniform random distribution \\ Given region & 800 m \(\times\) 800 m \\ The number of sensor node & 600–800 \\ Mobile sink speed & 3 m/s \\ Sensor node transmission range & 30 m \\ Consumed energy in transmitter circuit & 0.18 J \\ Consumed energy at the receiver circuit & 0.1 J \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation setting. Figure 3b-d give instances of scenarios 2, 3 and 4, respectively, where the number of deployed sensors is 800. Figure 4 shows the rendezvous time points between each of the eight local mobile sinks and the global mobile sink in four scenarios. In Figure 4, the four scenarios show the similar trend that the performance of BD Scenario is best and the rendezvous times are increased with the number of mobile sinks. In average, the BD Scenario saves 57% time cost, as compared with UD3-Scenario. Moreover, the UD2-Scenario and UD1-Scenario save 18% and 23% time costs, respectively, as compared with UD3-Scenario. The main reason is that BD Scenario has balanced deployment of sensors and contains no hole which might cause mobile sink moving from one side to another without any data collection operation, only for the purpose of travelling. As a result, the BD Scenario leads to a shorter path length. For each group, the proposed CDCA selects more CPs and creates shorter path length for each local mobile sink. Thus the average number of hops from each sensor to the CP and the average length between CPs is shorter. Consequently, the delay time of each local mobile sink is reduced. This also reduces the rendezvous time since the global mobile sink can rendezvous with each local mobile sink earlier. Figure 3: Four scenarios considered in the experiments. (**a**) BD-Scenario; (**b**) UD1-Scenario; (**c**) UD2-Scenario; (**d**) UD3-Scenario. Figure 5a,b investigate the network lifetimes of the four scenarios by varying the number of sensor nodes, ranging from 300 to 800 using seven and nine mobile sinks, respectively.