## 6 Conclusions  
This work comprehensively describes the new generation of OGC's Sensor Web Enablement (SWE) framework of specifications by particularly focusing on the performed changes compared to the first generation of SWE. The gained experiences from applying and deploying SWE in several projects during the last years were used to develop this new generation of SWE. This evolvement of SWE has taken into account limitations, difficulties and suggestions for enhancement that arose from its practical application. Within SWE's information model, several significant changes were made. The SWE Common data model has been extracted from the SensorML specification and is now defined in its own standard document, which emphasizes its gained importance. On the other hand, the development of TML can be considered as discontinued. The model design of the second version of the O&M specification has been brought into the ISO standardization process to strengthen its reliability and relevance. Profiles for certain aspects of the information model are emerging. An example is WaterML 2.0, a hydrology profile for O&M, as well as the SensorML profile for discovery. An important addition to the SWE architecture is the introduction of the SWE Service Model specification which defines a common model consisting of basic types for requests and responses as well as the Capabilities document of SWE services. Also, well-established IT standards such as SOAP and WS-Notification have been incorporated. SOS 2.0 and SPS 2.0 are now based on this common model which reduces redundancy and shall facilitate the implementation of those standards. Further, SOS and SPS have been evolutionary updated. The specifications are now modularized and consist of a core, extensions and profiles. Significant enhancements have been made in the field of eventing and alerting. Whereas filtering and alerting functionality was previously covered by the SAS specification, the new concepts of SES and EML promise a more powerful solution. While the event filtering functionalities of the SAS were rather limited, SES and EML allow the usage of techniques such as Complex Event Processing and Event Stream Processing to achieve a new level of filter capabilities including the consideration of time, the logical conjunction of filter rules and more advanced geographical filtering. Finally, the SWE framework is completed by first approaches to integrate SWE and Catalogues in order to make sensing resources discoverable. Despite these advancements, there are still research challenges in the field of Sensor Web Enablement to be tackled in future, as identified in this article. An important topic is to increase the interoperability of SWE components and to facilitate the utilization of the specifications by developing profiles that reflect the requirements of certain user groups or thematic domains. Further, the gap between low level sensor interfaces and the interfaces of Sensor Web services needs to be closed, in order to integrate sensors more easily into Sensor Web infrastructures. Of a broader scope than the SWE framework are the evening concepts defined by the SES and EML specifications. In future, they will be extended to an OGC wide event architecture. Also, the application of SWE to build a Human Sensor Web and the integration of Sensor Webs with online social networks are challenges needed to be addressed. Finally, the ongoing work on the enabling of a Semantic Sensor Web and applying the Linked Data principle to sensors and sensor data are promising and involve interesting research challenges. In summary, the new generation of SWE specifications provides a significant step forward. As the development of the new specifications has been strongly driven by the experiences gained from applying the first generation of SWE, it is expected that the acceptance of the SWE architecture in practice and the number of SWE applications will increase further. The new generation of SWE can be considered as an evolutionary advancement of the existing standards baseline.