**6. System architecture**

Various WSN architectures, including mesh and cluster three, have been investigated as potential candidates, each exhibiting advantages and drawbacks; in this application, as it will be explained below, the VOC detectors have to be continuously powered-on and the wireless node has to transmit data at minute data-rate. VOC detector cost, in terms of power consumption, is thus predominant with respect to transmission cost; in that context, the mesh configuration turned-out to be unnecessarily complicate in terms of protocols and less efficient in terms of energy budget; consequently, the much simpler and effective cluster three configuration represented in Fig. 3 was selected. The basic elements of the network are, the SN, the EN and the Router Node (RN). In this application only SNs and ENs were used. The GPRS unit is always connected to the GSM base station and transmits the gathered meteo-climate data down to 1 second rate (e.g wind). The ENs are normally in the low-power sleeping mode; they wake-up for a short time at 1 minute time interval, perform read-out of the VOC sensor and forward the gathered concentration data to the SN unit, along with other climatic and diagnostic information.
