# IoT-Enabled Smart Cities: Evolution and Outlook
## 3 Key Technologies Developed
### Sensor and Actuator Technologies  
The IoT is formed by multiple technology tiers. They permit sharing of information among things over the Internet to eventually deliver smart, self-sufficient actions, the value of which heavily relies on the quality of the information itself. Therefore, sensors and actuators are a crucial slice of the IoT technology stack and a fundamental aspect in the deployment of every IoT system. Sensors are the devices dedicated to detecting events or changes in their surroundings and transform these physical phenomena (e.g., temperature, noise, air humidity, pressure, presence of substances, etc.) into voltage and/or current signals that can be correspondingly understood. On the other hand, an actuator, is a device that converts the commands that it receives in the form of electrical impulses into any kind of mechanical motion which alters its close environment. This is done through a range of simple actions typically including, but not limited, to opening and closing valves, activating or deactivating relays, changing their behaviour (i.e., colour, position, etc.) or emitting some kind of alarm. Such devices have been around for many years already. They are tightly bound to industrial applications of any kind. However, their ability to be miniaturized and wirelessly connected through the Internet has created an unprecedented explosion of new opportunities and applications of this kind of devices in a huge variety of application domains, including smart cities as can be seen from the examples described in Section 2. The IoT has simply transformed the way in which they are used as it enables that the data that they generate can be consumed by cloud-based data analytics to develop intelligent solutions for machines, people and environment alike. They form the baseline of IoT and it is critical to have efficient and low-cost sensing and actuating solutions as they are ultimately responsible for monitoring and controlling the environment, and streamline operations in almost every kind of sector, from smart cities to environment protection. There are many different types of sensors and actuators in an IoT system. Flow sensors, temperature sensors, voltage sensors, humidity sensors, and the list goes on as the application scenarios do (cf. Section 3.4). In addition, there are multiple ways to measure the same thing. However, there is one fundamental issue that can broadly characterize the design of smart city monitoring systems, namely, the way sensors and actuators are deployed. In this respect, it is possible to differentiate between stationary sensors, mobile sensors and crowd-sourced sensors. * **Stationary sensors:** In the static deployment of sensors, all the nodes are stationary. In this case, the most important aspect is the design of the network, i.e., specifying beforehand where the sensors should be deployed for an optimal monitoring, paying special attention to providing a good coverage of the monitored area and to guaranteeing connectivity of the wireless nodes. Static WSNs are heavily used in smart city scenarios as they are well-suited for smart metering of public services networks such as water and sewage [33; 34], public lighting [35], parking [36] or traffic monitoring [37; 38; 39], as well as for generating the baseline monitoring of the city environment [38; 39]. * **Mobile sensors:** In the case of mobile deployments, nodes are installed in vehicles that move around to collect data. These vehicles could be specially designed robots [40], or more recently drones [41], or public vehicles that are equipped with specific sensors [42; 43]. Mobile sensors are a sensible solution to enhance the performance of city monitoring in terms of coverage, cost or resiliency, to mention a few. In this sense, they are mainly used to complement the sensing of existing static WSNs.