## 6 Discussion
This may be because personalized security requirements in these projects increase the project's budget and the level of difficulty for different platforms. Nevertheless, of the 24 primary studies analyzed in this SMS, 11 provided support for at least one non-functional requirement. The most frequently investigated non-functional requirements in the primary studies were security, privacy, maintainability, performance, scalability, interoperability, and safety. A bibliometric analysis was carried out, considering the terms detailed in Table 1. These terms were used in the search strings applied in each data source in this SMS. The analysis was performed using Vosviewer ([[https://www.vosviewer.com](https://www.vosviewer.com)]([https://www.vosviewer.com](https://www.vosviewer.com)) accessed on 13 January 2022), which is a software designed to analyze scientific productivity during a period of time. Moreover, in order to identify trends in the literature regarding RE and IoT, an initial analysis of co-occurrences of keywords was performed, considering publications with a minimum of five incidences. This resulted in six clusters (see Figure 3), involving 68 keywords. The clusters were (1) internet, (2) thing, (3) network, (4) software, (5) design, and (6) challenges. Our analysis showed a weak relationship among the clusters regarding the terms internet, thing, network, and software, with the red zone corresponding to the clusters for design, evaluation, industry, and practice. As IoT and RE both represent relatively new research fields, it is interesting to observe when articles on these topics were published. Furthermore, this is useful to identify when such themes were first studied and when the research on this direction expanded. Figure 4 illustrates the growth of the selected primary studies from this systematic mapping study over the years. It is possible to note that practically all studies (16 articles) were published in the last four years (no study was published before 2013), thus confirming that the integration of IoT and requirements engineering is a focus of current interest from the scientific community. This is despite the fact the COVID-19 pandemic affected different sectors of society, including academia, from 2020 to 2021. Furthermore, it is possible to note the increasing trend regarding the number of articles available in data sources through the years, indicating that more significant publications on this theme can be predicted for the coming years. Figure 3: Bibliometric visualization of the primary studies. The effect of not providing support for RE in developing IoT software systems lies in the lack of knowledge regarding the challenges involved in its implementation. The proliferation of IoT in daily life is growing, and nowadays aspects of machine learning have also been incorporated into IoT systems.This implies the need for support in the form of tools and frameworks that can assist in the development process, considering the fundamentally heterogeneous audience of this type of system, that is, users with diverse profiles. The effect of not having new generations of methodologies that consider RE with sufficient weight at the beginning of the development process will result in industrial IoT solutions (with different types of devices) that do not fully meet user expectations. Therefore, the quality of these solutions in the industry will be affected. In this regard, NFRs have been barely considered, with research mostly focused on security, whereas quality aspects such as interoperability and responsiveness have been left aside. These quality attributes must be considered more closely since, due to the development of machine learning, IoT systems include autonomously intelligent devices.