# Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests
## 6 Related Work  
The link performance is always a major concern in wireless network applications. In particular, in wireless sensor networks, the low-power link is very easily affected by surrounding environments and rendered difficulty with respect to prediction. Investigation about how the low-power radio links behave has attracted more attention in the literature. In general, the characteristics of the radio link concerned by researches mainly include the link reliability, the link symmetry, the link regularity, the link performance estimator designs and their relationship with the environments, as well as the patterns of on-site deployment. The salient work [34] investigates the link performance of wireless IEEE 802.11 networks, especially analyzing the link loss rate and providing some insights into the MAC and routing designs; the authors find that the wireless network as a whole is unreliable, and the link quality is not strongly correlated with the link distance and the signal-noise ratio. The key contribution in [34] is to emphasize the significance of carefully considering the link performance in wireless network applications. [PERSON] et al. [23] empirically find that the realistic wireless link performance does not agree with the widely-used simulation models, and they statistically study the distribution of lossyradio links and evaluate their models through testbeds, which are deployed in indoor and outdoor environments. In [28], the authors initially evaluate the link RSSI that serves as a metric to profile link quality in wireless sensor networks. Through a precisely-controlled experiment (the environment is not explicitly given), the authors achieve different results: sometimes, the RSSI is a more desirable link metric than the pure LQI. [PERSON] et al. [29] model the irregularity presented by low-power radios and analyze the effect on the MAC and routing protocols. [PERSON] et al. [15] theoretically study the transitional region (grey zone) issue in wireless sensor networks; and in their later work [30], they deeply explore the unreliability and the asymmetry of 915-MHz low-plow wireless links that are deployed in a building aisle and a football field, respectively, and then propose several simple numeric link-layer models. [PERSON] et al. [24] deploy two wireless sensor networks both in the ceilings of the indoor scenario and show their experimental observations that support and dispute the ordinary assumptions on wireless communications. Particularly, the authors discuss the significance of real low-power link performance for the communication protocol designs. Their research suggests that there exists a clear gap between algorithm studies and real deployment. [PERSON] et al. [17] overview the literature and compare the observations and analyses of low-power links in prior works. The authors conclude that different wireless platforms and experimental environments will lead to different or even disputed outcomes, indicating the deficiency of some laboratory results and the necessity of investigating low-power links under certain real-world scenarios. As mentioned in [17], the above results are often achieved in the case with the open sky view or the indoor environments with less obstacles and simple reflection surfaces. However, different environmental conditions might lead to different link behaviors even for the same wireless platform. In the recent few years, therefore, researchers have begun to investigate the low-power link of wireless sensor networks in several practical scenarios. [PERSON] et al. [31] analyze the characteristics of IEEE 802.15.4 links in an indoor factory environment; they find that the path loss rate with factory-like surroundings is more significant and hard to model, in comparison with the counterpart under the open field. [PERSON] et al.