# Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests
## 4 Performance of In-Network Links  
### Deployment and Experiment Designs  
Besides the evaluation of the dynamics of a single link, we deploy a small-scale wireless sensor network in the Jiufeng National Forest Park, Beijing, China, to achieve a more comprehensive understanding of low-power links in the forest environment. Figure 13 shows the deployment involving ten TelosB sensor nodes, as well as a sink node; and Figure 1c shows the sensor placement and the in-forest terrain characteristics. The ten sensor nodes, labeled with S2\(\sim\)S12 (excluding S3, due to its damage), are not deployed randomly; they are placed at most 0.2 m away from ten grid-powered hydrology-monitoring devices that have been mounted and maintained by the Water and Soil Conservation Department of our university according to specific standards for ecology monitoring. As Figure 1c shows, such an experimental deployment with designated locations of sensor nodes can better simulate real-world wireless hydrology-monitoring sensor network applications. For all of the pairs of sensor nodes with available links, the maximum link distance is 8.85 m, and the minimum one is 2.2 m. Any pair of nodes is almost non-line-of-sight; between both of which there exists irregular tree leaves, branches, shrubs and even tree trunks. Each sensor node is driven by a pair of AA batteries and sends data with the maximum transmit power (1 mW). The sink node, labeled with S1, is placed at the rim of the sensor network area, and it collects and stores all of the link information returned by the sensor nodes. Figure 12: RSSI distance fitting in the Bajia forest environment. (**a**) tx = 31; (**b**) tx = 11. In the experiment, each sensor node periodically discovers its neighborhood every two minutes by broadcasting a set of successive probing messages. If a probing message is successfully received, an acknowledgment message will be immediately replied by the receiver to the sender. By doing so, each sensor node can know its neighborhood: who its neighbors are and the link qualities in terms of the PRR, the RSSI and the LQI. In detail, every two minutes, the sender broadcasts ten successive probes with the interval of 50 ms, by which the PRR can be deduced at the receiver side by calculating the ratio between the number of successfully received probes and the total number of probes sent by the sender. Noticeably, we do not impose a globally-synchronized clock for all nodes, because doing so will consume extra network resources and make it complicated to control the neighbor discovering. To collect the link information for the analysis purposes, we use the CTP [7] (Collection Tree Protocol, a de facto routing protocol for wireless sensor networks) to deliver the link information recorded by each sensor node to the sink node. Every two minutes, the sensor node first determines its neighbors and the corresponding link qualities by back-and-forth message exchanges and then pushes the link information of relevance into a single timestamped packet, which will be transported to the sink node over the CTP path. We carried out a two-hour experiment in the middle of August 2015. Note here that if a node cannot determine its forwarders, CTP will first stop the transportation service at this node, drop the corresponding application traffic and then go to re-discover the neighborhood; such a handling process of CTP results in a few packets not being able to be delivered to the sink in our experiment. That is the major reason that several later figures involve a small quantity of missing values. In fact, CTP involves a link estimation component, called the _4-bit link estimator_. However, it provides the link quality estimation from the point of view of routing, i.e., it is designed mainly to facilitate the forwarding decision; therefore, such a link estimator cannot reliably reflect the actual link status.