# Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests
## 5 Summary of Observations  
By the comprehensive real-world experiments and comparative analyses, we summarize our observations and remarks as follows. _Observation 1_. The link performances are different, even under slightly different forest environments: the distributions of trees and understory shrubs, as well as vegetation will lead to different effects. Interestingly, a handful of shrubs (in the Campus forest) possibly constructively affect the link quality to some extent, in comparison with the forest with a \"clean\" understory (e.g., the Bajia forest of this paper). The wireless sensor deployment, therefore, does not have to circumvent the short and sparse shrubs if the line-of-sight links are hard to achieve in practice. _Observation 2_. In forests, the link quality usually degrades as the communication distance increases, but this degradation is nonlinear, indicating that sometimes, longer links may have better link quality than shorter ones. It is therefore unreasonable to estimate or compare the link qualities only by the link distance, as theoretical studies often do. According to the results of Figure 4, in particular, our experiments do not show obvious transitional regions (also called the grey region), which however, are found under open areas in [23; 24]. Additionally, higher transmit power levels do not always imply higher link quality, and then, it may be undesirable for topology controls to improve the wireless network connectivity just by simply increasing the transmit power. _Observation 3_. For the time-constrained application [27], the efficient prediction of the link quality pattern is very important. The work [28] acclaims that RSSI is a good link estimator for low-power links. By comparing the variation patterns of the PRR, the RSSI and the LQI, however, we find that in forests, the LQI has a higher temporal correlation with the PRR, compared to the RSSI. Therefore, the LQI may be a better link estimator that can be employed by the upper layer protocols (such as MAC and routing) for in-forest deployment. In fact, the weak signal strength will not possibly lead to the failed packet reception; the variation of RSSI measurements mainly caused by the irregular obstacles in forests, therefore, cannot well reflect the real link performance, sometimes. Figure 19: Correlation of all incident link pairs. (**a**) Correlation distribution; (**b**) CDF of the absolute value of correlation. Figure 20: Correlation coefficient of any two links from each sending node. Observation 4 In the experiment, the link in two directions is rendered significantly asymmetric in terms of the PRR, the RSSI and the LQI. In our experiments, the link symmetry does not obviously correlate with the link distance, which agrees with the previous empirical evaluations in clean environments. For a given communication distance, higher transmit powers of the two-way link usually lead to good link symmetry due to the good packet receptions. Different from prior results [28; 29; 30; 31], however, no good link symmetry can be always observed either for the good link or for the poor link; the reason behind this may be that no significant transitional regions exist in the forest environment. We also find out the temporal variation of link asymmetry and different varying patterns for links with different PRRs. The link asymmetry is persistent and difficult to accurately predict. In total, for good or poor links, the asymmetry can be kept stable with light fluctuations, while for middle links, the asymmetry varies significantly within the relatively large range. Thus, careful considerations are needed in evaluating the low-power link reliability of the one-hop communication in the forest scenario; in practice, the data sent out over a link need an acknowledgment from the receiver for the purposes of confirmation, so the product of two-way PRRs of a link is often used to weigh the actual link quality. Observation 5 The temporal correlation between low-power wireless links with the common sender is less common, but exists.