## 7 Recommendations for WSN Designers  
Because WSN at ground level have a lot of application including traffic monitoring, smart parking or bridges monitoring, we believe that engineering recommendations can be drawn from this study. Figure 16: RSSI Validation: Empirical and Theoretical CDF. Figure 15: PRR Validation: empirical and theoretical CDF. Leading this experimental campaign allowed us to gain a significant insight regarding the deployment of wireless sensor nodes at ground level, as well as regarding the properties of WSN links in general. Based on this experience, we formulate five recommendations for researchers and engineers interested in similar deployments. _Recommendation #1: Account for asymmetric links._  
A classical and simple assumption when evaluating the communication protocols in WSN is that the links are symmetric: if node \(\mathbf{S_{1}}\) hears node \(\mathbf{S_{2}}\), \(\mathbf{S_{2}}\) also hears \(\mathbf{S_{1}}\). However, our results indicate that this property is not always true, especially when nodes are deployed at ground level. Indeed, depending on the settings, up to 40% of the links in the network are unidirectional. This raises several important protocol problems. First, this means that solutions based on acknowledgment (ACK) messages might be inefficient, since the ACKs would be lost despite a successful message reception. Second, routing protocols need to take into account that ingress and egress routes might have very different properties. _Recommendation #2: Remember that the correlation between link quality and distance is weak._  
The physics of radio wave propagation predict a monotonic decrease of the received power strength with the distance. This is also a common assumption in analytical models and simulation tools. Nevertheless, our results confirm the findings of previous experimental campaigns, which demonstrate that this property is not true in practice. Moreover, the deviations from this predicted behavior seem to be exacerbated by a deployment at ground level. This also indicates the need for a denser deployment, which would allow coping with the poor quality of certain links. At a protocol level, opportunistic links are not uncommon and could be exploited. _Recommendation #3: The communication channel 26 must be preferable and frequency hopping is better._  
Many studies on WSN do not consider the communication channel as a parameter, assuming the same quality on all transmission frequencies. In most field tests, designers evaluate their protocols on their testbed using only one channel. The results presented in Figures 10 and 11 show that communication performance varies depending on the channel used by the nodes for communication. In the particular case of IEEE 802.15.4, our results indicate that communication on channel 26, which is not shared with other technologies, is probably a good idea. However, the WSN performance should be tested on several communication channels in order to find the one with the most reduced interference. Frequency hopping on several channels will lead to a better robustness. _Recommendation #4: At ground level, high density or redundant deployment will be preferable._  
The results presented in this paper, and particularly those presented Section 5.1, show that the link quality is worse at ground level when compared to a more classical deployment at height. Thus, in a WSN with nodes deployed at ground level, special measures must be taken not only during the design of the communication protocols, but also during the network deployment. If we assume that a link is usable only when its PRR is greater than or equal to 90% (a good link in Figure 4), then only 70% of the links of 6 m length will be valid in a WSN with nodes deployed at the ground level, compared to 100% for a deployment at height (Figure 6b).