## 7 Conclusions and Future Work
For instance, the antenna may end up too close to large ferromagnetic bodies, like the battery side seal or the case of the quartz crystal resonator in our experiment. While we could use a different case for the crystal resonator to avoid changing the node geometry, we had to increase node length to leave enough distance between the antenna and the battery seal. Furthermore, protecting the antenna inside the node package reduced its efficiency by 2 dB, but we found no cost-effective way to avoid it. However, component cost is less important for gateways, and we plan to install their antennas clear of the package or solar panel shielding, which have markedly reduced their performance during the field test. Changes in gateway behavior in the case of failures (e.g., GPRS link or field radio receiver failures) need to be detected at the gateway or network level and reported to network operators to schedule corrective actions. Remote configuration or reprogramming should be available, at least for the gateways, in order to be able to remotely add functions, correct errors or protect the network against occasional faulty node behavior. As we have seen during the field experiment, this feature would have spared us an unscheduled maintenance, whose cost can be important for remote fields or large networks. Nodes that store field messages before forwarding (e.g., gateway nodes) should implement suitable in-field redundancy to avoid loss in case of self-reset or failure. Additionally, we plan to implement a save-restore function on gateways to preserve node data across watchdog-triggered resets. Gateway nodes handle higher energies, which can generate significant heat, both during harvesting and consumption peaks (e.g., during GPRS data connections with the server). Thus, as we have seen during the field experiment, an adequate thermal dimensioning considering the climate and the internal and external heat sources is important. For some applications, we may consider other energy accumulation techniques instead of the widely-used lithium-ion batteries, which have limited charge-discharge cycles and temperature restrictions for charging and operation. We also consider redesigning the power supply of the gateways to automatically switch to a primary cell whenever the harvested energy reserves are too low, to increase the node QoS. Unlike the sensor nodes, which are reliably supplied by the battery soldered on-board, the energy reserves of the gateways can fluctuate due to unpredictable harvesting and consumption patterns. Moreover, the rechargeable batteries can be changed when they loose storage capacity. Hence, the use of the embedded brown-out detector can considerably improve gateway stability. The field experiment showed us that the deployment of the network can be very time-consuming, even for networks with just a few dozens of nodes. Thus, the procedure should be simple and automated as much as possible to be accessible to domain operators, to reduce human errors and cost. Nevertheless, it should allow one to thoroughly check the quality of node installations, since RF propagation in open nature can be unpredictable, not the least due to small and difficult to notice terrain features. We plan to improve the user interface of the handheld installation device, automate position collection and data reporting and extend it to the installation of the gateways. For the latter, it should assess cellular coverage for the GPRS modem, light exposure for energy harvesting and the coverage of the sensor nodes in the field. We also decided to develop a repeater node that can improve gateway coverage of the field in difficult propagation conditions. WSN platform design, development and maintenance has a high cost, which can be recovered faster if the platform can be reused for several related application domains. While the sensor nodes do not have sufficient resources to support an IP-based protocol, the gateways are IP-enabled, albeit they are sporadically connected.