2.2.1. Sensing layer
The sensing layer is primarily based on the SOS introduced by the SWE framework, which provides a popular interoperability interface to store and distribute sensor data from heterogeneous sensors. The potential value of SOS for the interoperability in PA
was explored in this study. Moreover, SOS also supports spatial queries, temporal queries, property queries, and combination queries, thus benefiting different user-defined data requests. Before standardizing data through SOS, a gateway service was deployed to transform sensor data to SWE-enabled information.First, the IP address and port of the gateway service were configured in a field-data hub. A TCP/IP connection was then established once the gateway service detected the connection request from the field-data hub. The gateway service could then send a
sensor data request to the field-data hub according to different 80 N. Chen et al. / Computers and Electronics in Agriculture 111 (2015) 78–91 data communication protocols. After receiving a data response from the field-data hub, the gateway service parsed it from binary format to plaintext. Finally, the gateway service encoded raw sensor data into O&M-compliant XML format. Therefore, the O&M is used as the final standard data format in the sensing layer. O&M is an international standard (ISO 19156) that defines a conceptual schema for different observations to enhance data semantics and accessibility. Although O&M was proposed in the context of geography, the model is not limited to it. Collected agricultural data, as well as meta-data in PA can also be modeled by O&M, which are stored in SOS. SOAP and HTTP transmission protocols are supported.