## Experimental Results
### The Impact of Communication Channel  
Because the ISM band used in IEEE 802.15.4 is shared by other technologies such as IEEE 802.11 and Bluetooth [37; 38], interference from other networks influence the link properties in a WSN. To test the impact of the communication frequency on the link quality, we conducted a set of experiments using 5 of the 16 channels (numbered from 11 to 26) defined by the IEEE 802.15.4 standard. For these experiments, four sensors were linearly deployed on the ground surface with a distance of 3 m between two consecutive sensors. Nodes were configured to use a transmission power of 0 dBm. For each channel, experiments were conducted for various application payloads, but results presented in this section consider an application payload of 82 bytes (since, in the previous section, we obtained better links with this application payload).  
Figure 10 presents the distribution of the PRR and RSSI on communication channels 11, 18 and 26. In this figure, we consider the cases of links of distance 3 m only and 6 m only, as well as aggregated results for all the links in the network.  
These results show that the quality of links was better on channels 18 and 26 compared to channel 11. Figure 11 also shows that, on channel 11, we observed more poor quality links, while more good  
\begin{table}
\begin{tabular}{l c c c} \hline \hline  & **2 bytes** & **42 bytes** & **82 bytes** \\ \hline \(AVG_{|PRR_{8a}-PRR_{8a}|}\) & **0.2165** & 0.05 & 0.0505 \\ \hline \(MAX_{|PRR_{8a}-PRR_{8a}|}\) & **0.9** & 0.5 & 0.55 \\ \hline \(AVG_{|\overline{RSSI}_{8a}-\overline{RSSI}_{8a}|}\) & **1.94** & 1.68 & 1.67 \\ \hline \(MAX_{|\overline{RSSI}_{8a}-\overline{RSSI}_{8a}|}\) & 4.47 & **5.0** & 4.7 \\ \hline \(\%\)_Undirectional_ & **18** & 6 & 8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Link asymmetry for different packet sizes.  
Figure 9: Link quality classification for different packet sizes.  
quality links were observed on other channels. The poor performance of links on channel 11 can also be observed in Table 3, which presents the impact of communication channel on links asymmetry. Even if the PRR measured on forward and backward links were almost equal on channels 11 and 18, the values of measured RSSI were very different in both links. This table shows that almost half of the links in the network were unidirectional when communication channel 11 was used. This lower quality of the links on channel 11 could be related to interference caused by IEEE 802.11 networks deployed on the university campus.  
We also observed that the links were better on channel 26 compared to other channels. This was because channel 26 is the only one that does not overlap with the spectrum used by IEEE 802.11 networks [37]. In this section, it is important to note that, for each experiment, all sensors in the  
Figure 11: Link quality classification for different communication channels.  
Figure 10: CDF of PRR and RSSI for distances 3 m and 6 m and for all links in the network, for different communication channels.  
network communicated on the same channel. Better performance could be obtained if more advanced MAC layer techniques, such as Time-Slotted Channel Hopping, were used [39; 40].