## 3 Experimental Results  
Once the scenario has been characterized in terms of the topological dependence of the wireless channel performance, experimental results are obtained in order to validate these previous estimations. We have considered the basement of the \"Las Encinas\" building sited at the Arrosadia campus of the UPNA. This building is made of concrete and aluminum metalwork with double glazing. Offices (D01 to D020) are walled using plasterboard modules and aluminum metalwork. The inner walls of the central rooms are concrete, while doors are chipboard. Figure 14 depicts the experimental scenario where the WSN is deployed. We consider eight WSN nodes, where seven of them act as aggregators and the last one as gateway in charge of data collection. Those nodes communicate among them using the IEEE 802.15.4 standard. Due to the complexity of the environment, where several wireless signals could be propagating within it (e.g., WiFi), a spectrogram has been measured in order to gain knowledge about the radiofrequency pollution within the scenario, leading to an adequate and interference free wireless channel choice. Figure 15 shows the measured spectrogram for the scenario under analysis. Figure 14: Map of the experimental scenario. As can be seen, the ISM 2.4 GHz band, which is the frequency band used by the Waspmotes, contains a variety of external signals. Trying to avoid those interferences, the IEEE 802.15.4 channel 11 (2.400-2.405 GHz) has been chosen. In Figure 16 a new spectrogram is depicted, this time with the operating Wireless Sensor Network (WSN) nodes. The spectrum of the emitting nodes can be clearly seen, with a noticeable higher power level than the interferences. In fact, the SNR level produced by inter-system interferences in transmitter 3 (Figure 9) get values far away from the critical limit when the system is working in its highest bit rate. Nodes have been placed next to the concrete walls at locations 1, 4, 5 and 7 (see Figure 8), next to plasterboard at locations 2 and 6, over a PC computer at location 3, over a plastic chair at location GW, and over a wooden table at location Office. Figure 16: Measured spectrogram with the operating WSN network. Figure 15: Measured spectrogram without operating WSN nodes. The wireless sensor network is built by following a chain where each node receives a message from its predecessor, aggregates its information, and sends a message to its successor without any acknowledgement or retransmission. That ensures a minimal message exchange and avoids collisions. Limitations may concern latency and packet fragmentation due to large payloads caused by data aggregation, but the reduced number of nodes (eight) involved in this WSN grants no packet fragmentation and an invaluable latency. Nodes are deployed following a chain where node #1 is the initiator and sends a message to its successor (#2) each 100 milliseconds. As node #1 does not receive any confirmation message, and no retransmission occurs, it does not experience any loss of messages. Each node aggregates three values to the message received: its node ID, the last RSSI value obtained and its battery voltage; and resends the message to its successor. At the end of the chain, node gateway (GW) collects all the information and stores it. The gateway is connected to a laptop through a USB cable. Information collected is processed and analyzed obtaining the results described below. Messages are exchanged between nodes without acknowledging, since information is sent frequently (100 milliseconds period) and acknowledgement messages introduce a high consumption with a null benefit. Fifty thousand (50,000) iterations distributed in five experiments of 10,000 iterations have been performed and the messages at two different locations have been collected: GW and Office (see Figure 14). Node #7 communicates directly (straight line) with node gateway, while it communicates by multipath with the office.