## 3 Experimental Results
Since node #7 and the data collector located in the office do not have straight line communication, but several thick concrete walls, the only effective mode of communication is multipath propagation. This implies a high rate of losses as depicted in Figure 17. Experiments 1 to 4 are performed with empty corridors, without people. The fifth experiment has been conducted with the presence of several people moving freely through the halls without any movement pattern. The higher packet error rate (PER) values obtained at the base station located on the office are due to the fact that office's door was closed during measurements while in the other three experiments the door remained open. Figure 18 shows the number of iterations successfully accomplished by nodes. We obtain a PER value of 5.612%, and we can observe that node #2 acts as a traffic stopper and then, the rest of nodes have similar  
Figure 17: Packet error rate (PER) comparison between the hall and office base stations. PER values. A greater stopping effect and a higher PER value are observed as the initiator's transmission frequency increases. Table 2 summarizes the packet error rate of the WSN. We can observe a very small PER, lower than 6%. Given its condition of chain initiator, node #1 does not receive any message and hence it's PER is zero. The rest of nodes have a non-zero PER, where higher values correspond to nodes #2, #4 and gateway. Node #2 acts as a filter/stopper, ensuring a transmission rate that can be absorbed by the other nodes. Node #4, due to its low SNR, suffers a greater loss of packets, but it is not too relevant. Finally, the gateway has three times the PER of the node #2 mainly due the saturation of the receiver's buffer (and then the loss of some messages) caused by multipath propagation. While multipath propagation makes possible the reception at the office location of messages sent by node #7, multipath propagation also causes a slight increase of the PER of the gateway located at the hall. However, the total amount of messages lost (2806) is considerably low with respect to the total number of cycles initiated (50,000). Figure 19 shows the distribution of messages lost by node (left), and also the number of messages lost by node. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Messages Lost by Node** & **\#1** & **\#2** & **\#3** & **\#4** & **\#5** & **\#6** & **\#7** & **Gateway** \\ \hline
**Average** & 0.00\% & 1.21\% & 0.16\% & 0.42\% & 0.03\% & 0.08\% & 0.12\% & 3.58\% \\ \hline \end{tabular}
\end{table} 
Table 2: Packet error rate (PER) by node. Figure 19: Distribution of messages lost by node. Figure 18: Iterations accomplished by nodes. The analysis of the RSSI values obtained for the nodes of the network shows a great difference between nodes #4 and #7, while the difference is not so great when considering the rest of nodes (#2, #3, #5 and #6). Figure 20 illustrates the RSSI obtained for each node and for the whole system. RSSI values for nodes #3, #6 and #7 are closer among them than those corresponding to nodes #2, #4 and #5. The higher RSSI average value corresponds to node #6 (\(-\)57.838 dB), while the lower average value corresponds to node #4 (\(-\)79.275 dB). Standard deviation of the RSSI values range from 0.605 dB for the node #7 to 2.150 dB for the node #2. Mode and median values of RSSI match on all nodes, showing the considerable stability of the system, which is very important for the implementation of applications over the network. The maximum variation between maximum and minimum RSSI values corresponds to node #2 with 36 dB, while the minimum variation corresponds to node #7 with 11 dB. Table 3 summarizes these results. The RSSI diagram depicted in Figure 21 shows that node #4 is the most conflictive of the network according to its RSSI values. However, the average values are far from the sensitivity threshold of the