## 5 Performance Evaluation
### Results and Discussion  
Concerning the main results, we focus on the energy consumption of sensor nodes to test the performances of the system on it from two aspects: the hardware and the topology. In order to evaluate the performances of our designed hardware, we compare its performances with a famous commercial sensor node: the MicaZ sensor node of Memsic (presented in Tables 1 and 2). The MicaZ sensor node (Figure 9) is a sensor node based on a Texas Instruments microcontroller and respects ZigBee specifications. We choose to compare the performances of our hardware with MicaZ nodes because of the nearly similar features of both nodes in term of frequency, communication range and transmitting power (see Tables 1 and 2). The first result regards the discharge process. We performed the test of this metric using our designed sensor node. As illustrated in Figure 10, we compare the different topologies in terms of the battery's voltage discharge in router nodes according to time. The initial voltage for all router nodes is approximately 2 V. It is clear that the discharge process using MSCT is longer than the mesh and cluster-tree topologies. We notice also that this process is accentuated for the mesh topology when  
Figure 8: Architecture of the coordinator area. Figure 9: Memsic MicaZ sensor node. the voltage level is below 1.5 V. In this case, the node is considered out of resources. Note that this value is lower in the other topologies: 1, 3 V for the cluster-tree and 0, 9 V for the MSCT. This first result shows that the energy consumption is minimal and the discharge process is slower by using the MSCT topology. Figure 11 presents the second result where we compare the average energy consumptions for the two nodes. This metric includes the average consumption of all components in the \(Tx\) (transmitting) and \(Rx\) (receiving) modes, and values are measured in milliamperes (mA). We plot this result for all types of devices; coordinator, routers and end-devices. Logically, the coordinator and the routers consume more energy than end-devices. We can deduce that the MSCT topology shows better results in terms of energy consumption compared with the other typical topologies of ZigBee. Note that in Figure 11, the acronyms \"C-T\" and \"M\" represent respectively cluster-tree and mesh topology. This result is very important since it shows also how energy consumption is reduced by our hardware device compared with the Memsic nodes. This can be explained by the fact that the devices used in our prototype have low power consumption. Furthermore, the size of packets has been reduced at the physical layer, which has positively impacted the computational costs, as well as the energy consumption of the nodes. Figure 11: Average of the power consumption of nodes according to hardware and topology. MSCT, minimum spanning cluster-tree; C-T, cluster-tree; M, mesh. Figure 10: Average voltage of the designed sensor node batteries according to time and topology. The next result shown in Figure 12 concerns the network's lifetime, considered as the main result. We represent the lifetime with two metrics. The first one is the time until the first node dies (FND). The FND time is considered as a stability period for the network since there is a node that leaves the network during it. The second one is the total network's lifetime that represents the time until there is no sensor node in the transmission range of the coordinator; this time is called the network's lifetime (NL). We can clearly deduce from this result that our designed sensor node, enhanced by the MSCT topology, considerably improve the network's lifetime compared with Memsic nodes by reducing the energy consumption. Finally, in order to evaluate the communication performances of the hardware, we measure the delivery of packets to the coordinator. We illustrate in Figure 13 the packet loss ratio according to hardware and topology. We first deduce that the packet loss ratio is reduced by our designed sensor nodes compared with the Memsic nodes.