## 2 System Definition
### System Specifications
The gateways may further process and aggregate the data and can communicate with the application server over long-range data channels. The server has many functions, such as reliably storing the field data, providing suitable user interfaces for observation and analysis, data post-processing, configurable alerts and interfacing with other IoT applications. From the comparative analysis in Section 2.1, the requirements of a WSN platform well suited for reliable and cost-effective long-term pervasive environmental monitoring can be defined as follows:  
* low-cost, small-sized nodes (sensors and gateways) that can process the transducer data on board, perform self-tests and recover from errors (preserving field data if possible);
* receive software updates over the air with minimum service disruption and have one or more long-range communication channels (at least the gateway nodes);
* the field communication protocol should efficiently support:
* network sizes from a few sparse nodes up to a very large number of nodes;
* low data traffic per node, in small packets;
* resource-constrained reliable operation, with small memory and energy overheads;
* reliable operation in case of node failures, especially if several nodes are destroyed by fire;
* transmit events instead of raw data to reduce communication energy by processing the data close to the source (on sensor nodes and gateways);
* fast and reliable field deployment procedure for network nodes;
* adequate gateway hardware and software resources and interfaces for application-specific extensions (e.g., local transducers, data storage, communication channels, other peripherals);
* node ruggedization to withstand long-term exposure to environmental conditions;
* long maintenance-free service (the beneficiary requirements were 5-10 years for the sensor nodes);
* high availability of service for nodes and servers;
* multimodal access to server data for human interfaces and automated processing;
* programmable multichannel alerts on field events;
* automatic detection and reporting of platform faults;  
Figure 1: Typical tiered structure of WSN platforms. * extensible server architecture for integration in IoT applications. These specifications would also allow the reuse of the platform for several event-based long-term applications besides wildfire monitoring, such as:  
* water level of lakes, streams, sewage. Sensor nodes can detect and alert about extreme levels (high or low) and periodically report level statistics;
* gas concentration in air for laboratories, deposits, ship holds, cities. Sensor nodes can detect potentially harmful concentrations of one or more gases;
* soil properties for precision agriculture;
* static or dynamic parameters for structures, e.g., the inclination of dams;
* position changes, e.g., land slides;
* lighting level, e.g., for intrusion detection in dark spaces, like cable channels;
* infrared radiation from combustion (fire) or human or animal presence, for security or habitat monitoring. On-node analysis using thresholds and speed of variation can screen alerts from natural-occurring patterns. Platform reuse can improve the economic sustainability of its development and maintenance. This is important, since cost can be a major factor that prevents commercial adoption of _in situ_ pervasive monitoring solutions. The overall cost of the platform has several components that we consider in detail during platform design in Section 3:  
* Node cost, which is roughly proportional to the monitored area and can significantly weigh on the total cost. Generally, a WSN has fewer gateways than sensor nodes; thus, platform design needs to carefully optimize at least the cost of the latter. * Exploitation cost basically made up of the cost of the analysts to validate the alerts and the cost of node and network maintenance.