## 7 System Prototype
### Experimental Results  
In order to validate the implementation of the MAC protocol, a test tool was created. This tool runs in a separate CC2430 module connected to a PC, continuously collecting received signal strength indicator (RSSI) samples at the selected channel. Each sample is obtained every 128 \(\upmu\)s, by reading the RSSI register of the radio transceiver, along with the first byte of the sleep timer counter, which represents the axis of time. These values are sent through the serial connection to a PC application, which plots a graph with the RSSI as a function of time. Figure 13 shows the RSSI values with four allocations in the NTP. All transmissions are made with an output power of 0 dBm. Figure 14 exemplifies the operation of the contention-free retransmission mechanism of the eLPRT protocol. The first superframe shows the transmission of a data packet (message) in the allocated slots in the NTP. In the second superframe, the sensor node does not transmit its associated data packet on purpose, to simulate the loss of the packet; consequently, the base station automatically assigns a RP allocation to the node in the next superframe. The packet is then retransmitted before the transmission of respective data packet of that superframe. Figure 13: Transmissions of four sensor nodes in the NTP. The next results show the effectiveness of the frequency hopping mechanism implemented by the eLPRT protocol. The experimental tests were performed with one base station and one sensor node inside a RF shielded anechoic chamber with 2.91 m (length) by 2.06 m (width) by 2.06 m (height). The base station and the node were placed in the center of the chamber at a distance of 1.67 m from each other and at a height of 1.1 m from the ground. The output power of both the base station and the sensor node was set to 0 dBm and the level of the signal received from the sensor node by the base station was \(-\)47 dBm. The IEEE 802.15.4 channel 22 was used in the test without frequency hopping. The interference source consisted of a file transfer between two laptop computers using the IEEE 802.11g standard [19] at channel 11, the DCF (Distributed Coordination Function), ad-hoc mode and bit rate of 54 Mbps. Figure 15 shows the spectrum of the IEEE 802.11g interference (the numbers displayed in the horizontal axis correspond to the IEEE 802.15.4 channels) measured using the Wi-Spy 2.4x [20] spectrum analyzer. The transmitter laptop was placed 40 cm to the side of the sensor node and the receiver laptop was placed 40 cm to the same side of the base station. Both laptops were placed at a height of 20 cm from the ground. The strength of the 802.11 interference measured at the base station was in the range between \(-\)35 dBm and \(-\)40 dBm. The generated interference was almost continuous, except for the backoff periods and interframe spaces associated to the IEEE 802.11 DCF protocol. The experimental tests used the traffic and network parameters indicated in Section 4, as well as all features of the eLPRT protocol. Each test was executed during 30 minutes, which corresponds to 18,000 superframes. The results are summarized in Table 4. The hopping sequence is given by Equation (1) and depends on the channel jump (\(n_{j}\)) parameter. The delivery ratio (DR) without retransmissions corresponds to the percentage of data packets transmitted in the NTP that are  
Figure 14: Example of operation of the contention-free retransmission mechanism. successfully delivered to the base station. The DR with retransmissions includes also the packets successfully retransmitted in the RP of the next superframe using the contention-free retransmission mechanism. The last row of the table shows the percentage of lost data packets that the retransmission mechanism was able to recover. In the first test (\(Ch=22\)), the frequency hopping mechanism was not used and the eLPRT network remained in channel 22 of IEEE 802.15.4 all the time.