# Metrics-Based Comparison of OWL and XML for Representing and Querying Cognitive Radio Capabilities
## 7 Conclusions  
This paper proposes a method to compare the OWL-based and XML-based approaches to represent and query cognitive radio capabilities using quantitative metrics. In order to prove the feasibility and the correctness of the method, we developed a proof-of-concept system for the method. Two types of metrics, matching quality metrics and performance metrics, were collected by the system with progressively less complete background knowledge representations and different sized sets of devices and queries. The evaluation results clearly demonstrate the advantages of the OWL-based approach in terms of the quality of matching. The results also demonstrate the benefits of using a more specific ontology to improve the quality of the matching results at the cost of sacrificing some level of performance. To be specific, the quality metrics evaluation results show that (i) the quality of matching in the OWL-based approach is always no worse than the XML-based approach, regardless of the richness of the selected ontology and sizes of the devices, and (ii) in the OWL-based approach, a more specific ontology results in better matching results. (iii) A derived consequence of the above conclusions is that shallow use of OWL does not buy much in terms of the quality of matching.  
The performance metrics evaluation results show that (i) the registration process in the XML-based approach takes less time, regardless of the number of devices; (ii) in the OWL-based approach, a more specific ontology results in a longer device registration time; (iii) the query processing time in the XML-based approach increases exponentially with the number of devices and is significantly longer than the query time in the OWL-based approach. However, we cannot decisively conclude that the XQuery query is less efficient than the SPARQL query, since the query processing time also depends on other factors, such as the query processor, query optimization, and dataset structure; and (iv) in the OWL-based approach, the query processing time depends on the richness of the ontology and thus it takes more time to process the same queries against the same devices with richer ontologies. Although we have not performed any investigations of the use of the proposed method in other domains, we suggest that the method is also applicable to areas other than just RF devices.  
Following the main conclusion of this paper, the continuation of this research should focus on the OWL-based approach, which provides \"more semantics\" than XML. This can be achieved in two ways--by using richer ontologies and extensions to OWL using rules. The standardization of ontologies for the communications domain began with the development of the Cognitive Radio Ontology (CRO) by the Wireless Innovation Forum [51]. The CRO was then extended to CRO2 [52](available at [53]). Work on the standardization and extension of the expressive power of OWL is continuing at the IEEE [54].  
Conceptualization, Y.C., M.M.K. and J.M.; methodology, Y.C., M.M.K. and J.M.; software, Y.C. and J.M.; validation, Y.C. and M.M.K.; formal analysis, Y.C. and M.M.K.; investigation, Y.C. and M.M.K.; resources, Y.C. and M.M.K.; data curation, Y.C. and M.M.K.; writing--original draft preparation, Y.C. and M.M.K.; writing--review and editing, J.M., K.R.C.; visualization, Y.C. and M.M.K.; supervision, M.M.K.; project administration, Y.C. and M.M.K.; funding acquisition, M.M.K. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Defense Advanced Research Projects Agency (Grant Number W911 NF-14-C-0065).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data are contained within the article. The data presented in this study can be requested from the authors.  
**Conflicts of Interest:** The authors declare no conflict of interest.