## 2 Original Star-Based Configuration  
### Node Description  
Figure 1 shows our leaf node inside a \(2.5\times 5\) cm cylinder embedded in wood. When installed, the node is powered and the hole sealed using a wood cap to maintain internal ambient conditions. This node, which uses a star wireless protocol, can last for more than 13 years, as shown in Section 2.3. Since  
this type of nodes only transmits measurements and it does not retransmit messages from other nodes, the energy required for RF is negligible when compared to other energy requirements, as proven in Section 2.2. The original configuration is based on a set of these small nodes inserted in wood. These nodes collect ambient information and send a digest to a sink. Figure 2 shows an image of the node elements. A Silabs C8051F930 low power microcontroller is the heart of the node. This relatively new 8-bit 8051 derivative performs really well and the available low power modes are very flexible. Power requirements specified on the datasheet were verified in this case as well. The node computes equilibrium moisture content of the wood based on the readings of ambient temperature and humidity using a Sensirion SHTx sensor. The node also has an attractor for insects that are detected using light reflection variations produced with a high-efficiency LED from Avago and a high sensitivity sensor from Taos. This sensor is activated every 2 s. The node is powered using a high energy density 1,100 mAh, 3.6 V. lithium-thionyl battery. The RF section of the node uses a Texas Instruments/Chipcon CC1101 ISM band transceiver. The chip has been configured for the 868 MHz European ISM band. This choice is a trade-off between small antenna size and range. The 2.4 GHz band has smaller antennas but it has shown poor  
Figure 1: Node inserted in wood. Figure 2: PCB component-side of the node and installed battery. performance with water moisture and crossing objects. The 433 MHz European ISM band is the best choice for our environment conditions, but the antennas are too large. In the star configuration, nodes only transmit data, and the receiver part is not used. Data is sent three times per day. The time instant of transmission is calculated using a random number generator seeded with the node's ID. Payload of the data packets can be reconstructed if some packets are lost, so such losses are not critical. The sink was built by assembling evaluation kits and commercial modules. Figure 3 shows an image of its internal arrangement. In this case, the microcontroller that coordinates the entire system is a Silabs C8051F120 high performance microcontroller evaluation kit. This microcontroller can reach a peak performance of 100 MIPS. The RF radio modem section is based on a TI/Chipcon CC1101 EMK868 evaluation kit. To this kit we attached a lambda/2 inverted dipole antenna from Antenna Factor. The collected data was aggregated and sent using a GSM/GPRS Telit GM-862 to a remote location. A RS-232 serial link allows the configuration of the sink using any terminal emulation software on serial port such as Microsoft Windows HyperTerminal or Unix minicom. The collected data allows for continuous monitoring of the EMC of the wood and insect movement. As an example, Figures 4, 5 and 6 shows the results of a laboratory-controlled experiment using a colony of termites (_Reticulitermes lucidigus_), using nodes inserted in blocks of pine wood. As a result of this experiment, we know that there is an exploratory phase and, sometimes, an abrupt increment in termite traffic until the node is affected by the termite activity. This forced us to develop an extra algorithm to detect the initialization of the insect sensing portion of the node and the resulting \"silence\" of the detector. Figure 5 shows an example of this situation. To detect termites, we use an analog light detector and a LED. Every 2 s, the LED is turned on and the analog light detector if powered-up and read through an ADC converter of the microcontroller.