## 4 Platform Design
### Communication Protocol Design  
The communication protocol can affect the performance of a WSN platform in several important ways. First, the protocol is mostly responsible for propagating the data across the network according to application requirements. Consequently, it controls the most part of the radio activity of the nodes, which is generally recognized as a major energy consumer. Thus, the protocol can significantly influence the selection of energy supply type and size, which relate closely to its cost and to the scheduled node maintenance. Moreover, the characteristics of the communication protocol may also affect the complexity and quality of the field deployment of the network, thus its duration, cost and the need for subsequent corrective network maintenance [6]. Most WSN deployments tend to use simple communication protocols that are enough to satisfy the requirements of the applications [87]. However, very few appear to consider during network design the effects of the protocol on the complexity, quality, maintenance requirements and cost of network deployment [43]. Deployment and deployment-related maintenance can add up to significant costs for pervasive environmental monitoring applications. The analysis in Section 3.1.2 presents some of the most significant trade-offs to consider when selecting the protocol for a specific application. For instance, using a radio receiver can increase the cost of the sensor node and of the application by adding to the cost of components, of the microcontroller (may require more resources), increases node energy consumption, thus adding to the cost of energy supply and/or the scheduled maintenance to change node batteries. However, a receiver also allows a better management of the communication channel, can increase the reliability of message delivery, can reduce the transmission power per node through multihop message forwarding and can support bidirectional network communication that allows one to remotely query or reconfigure the nodes in the field. To define the specifications of the communication protocol, we started from the analysis of application requirements. The communication needs of sensor nodes by state can be summarized as follows:  
normal state: the node periodically communicates its health status and, if possible, some background environmental measurements; fault state: the node communicates the nature of the fault (if the fault does not affect the radio); alert state: the node detected a fire and reports the event in the shortest time, with the highest priority. If the nodes periodically communicate their health status and some background environmental measurements, remote node interrogation becomes less important or even impractical for a large number of nodes, which are typical for pervasive monitoring methods (from hundreds to tens of thousands of nodes per application). Thus, for such applications, a bidirectional communication may be less important than the optimization of node cost. Multihop message propagation through the network using a mesh topology can reduce the transmit power of the nodes, which may lead to lower energy consumption at the node level. However, two other factors should be considered. First, operating a radio receiver, even with very low duty cycles, can still significantly raise the energy consumption of the node. Second, lower radio frequencies, which propagate better in forest environments [88, 89], have also a considerably longer communication range for the same transmit power, thus making the use of a star topology more practical. Moreover, a network with a star topology can be functional, even with sensor nodes with no receive capabilities, which can lead to multiple cost reductions, as we have shown. Thus, on the one hand, we decided to use the 433-MHz UHF industrial, scientific and medical (ISM)band for its better propagation in forests. On the other hand, we analyzed if a communication protocol based on unscheduled channel access can satisfy the requirements of the application.