*3.1. Physical/Perception Layer*

This layer works with various types of sensors to generate and collect the data from different sources from across urban systems and domains based on sensor-centric and human-centric sensing mode. A sensor is a device that converts signals from one energy domain to an electrical domain.

Urban sensing includes mobile sensing, participatory sensing, crowd sensing, satellite remote-sensing, and light-detection-and-ranging (LIDAR) sensing. For example, these pertain to mobility, traffic, energy, environment, road networks, transport systems, and built objects. Accordingly, the billions of connected devices forming the IoT infrastructure across a city are equipped with sensors to collect data about the way they are used as well as about the environment surrounding them, with built-in wireless connectivity and communication capabilities enabling them to exchange the generated data. One of the key features of sensors, once deployed, is their ability to interpret the data received from the surrounding environment and generate an output. Sensors measure physical input and send signals to the processor, converting it into data that can be interpreted by a machine or a human. With respect to the former, sensors send their readings to a backend system with humans being left out of the loop. In essence, sensors have the ability to convert data obtained from the outside world into a format that can be preprocessed and further processed and analysed. Sensors are the core enabling technology of IoT, providing an automated approach to urban data generation and thus serve as the main source for big data managemen<sup>t</sup> and analytics as a form of large-scale computation through middleware. However, among the challenges of urban sensing are resource deployment, implicit and noisy data, skewed sample data, and data sparsity and missing data [35–37].

Raw data are generated about a city in terms of its urban environments, human activities, and physical movements by means of a broad network of sensors spread across the city, including radio frequency identification (RFID) tags, near-field communication (NFC), accelerometers, surveillance cameras, LIDAR, transponders, smart metres, global positioning system (GPS), transduction loops, smartphones, and a number of other digital platforms generating ranges of real-time data. In terms of sensors associated with human mobility and activity, sensors leverage humans as data agents to investigate urban phenomena and dynamics during their movements in urban areas for the purposes of solving and servicing urban problems collectively.
