# Smart Parking: A Literature Review from the Technological Perspective
## 4 Discussion
### Networking  
Regarding sensor networks, 40% of the selected research papers did not specify using a certain protocol or networking solution, or their scope was focused on presenting a survey or some algorithm. From the remaining papers, 85% proposed a wireless implementation using WiFi, Bluetooth, mobile communication technologies, or wireless IoT protocols; sometimes implemented as ZigBee, LoRa, or NB-IoT. Most of the solutions using wireless technologies justify this because of the reduced cost of deploying this network architecture compared to wired solutions [13; 121]. With the aforementioned, we can say that wireless IoT protocols and WiFi technologies are the preferred ones for sensor communication, taking 51% and 31% of the classified solutions, respectively, in contrast to 15% corresponding to 3G and 4G technologies. Whereas, on the user side technologies there are several, but all of them are focused on the web. Anyhow, network technologies used to reach end users can be 3G/4G, WiFi, cable, etc., as long as they can provide access to the Internet. Analyzing wireless IoT protocols usage per year (Figure 9), we can see an increase in the usage of these technologies from 2013 to 2014 with a slight decrease in 2017 and a final increase for 2018. A stable trend in the usage of wireless IoT technologies for smart parking applications exists. On wireless protocol usage, the most used technology is ZigBee, followed by LoRa and NB-IoT, sharing second place. Of all the classified articles in the wireless IoT protocols category, 25% did  
Figure 9: Wireless IoT protocol usage per year. not specify the technology used and only mentioned the standard IEEE 802.15.4 which is the base of several protocols of such a type. This trends appear in Figure 10. From the research performed it can be seen that Zigbee is the most used protocol for connecting sensors with 60%. In second place, IEEE 802.15.4 reaches 25%. There are others, like LoRa and NB-IoT, that reach 5% for every one. The increase in using wireless IoT protocols over WiFi and Cellular technologies occurs because wireless IoT protocols are designed with IoT applications in mind. Most of them have less power consumption compared to cellular technologies and a bigger range compared to WiFi, but less bandwidth overall according to a survey by [PERSON] [3]. Another advantage of some wireless IoT technologies is using nonlicensed bands, making operations costs cheaper. Network design was more focused on LR-WPAN (i.e., ZigBee), with 71% technologies, compared to LPWAN (i.e., LoRaWan), with 29% as shown in Figure 11. Although both LPWAN and LR-WPAN are based on the IEEE 802.15.4 standard, the key differences between them are the operation range. This is a deciding factor at the moment of choosing one or the other depending on the application to be deployed according to [122]. In terms of user networks, most of the implementations are focused on web-enabled devices like smartphones and tables, so the most common networking solution for this part was WiFi and  
Figure 11: Network design distribution. Figure 10: Wireless Internet of Things (IoT) protocol usage distribution. mobile technologies. This also includes other devices that can be connected by wire, taking 67% of the classified solutions. Only 25% of the solutions focus on mobile devices only. This is not surprising because of the common use of mobile and web applications nowadays. For most of the analyzed papers, security topics were not taken into account in terms of network technologies, despite security vulnerabilities on wireless IoT protocols, such as LoRaWAN [123]. In some research papers, this was unnecessary because some proposed solutions were done via wired technologies or were focused on algorithms and architectures for smart parking implementations. There are many cases where researchers did not implement additional security for the use of secure enough technologies like WiFi or data was not even meant to be protected.