# Building Fire Evacuation: An IoT-Aided Perspective in the 5G Era
## 2 Overview of IoT Technology for Smart Buildings  
The IoT system is an integration of technologies to provide end-to-end IoT solutions with a mix of IoT devices, IoT data, IoT platforms, and IoT applications [11]. Therefore, the design of its system architecture is the core that enables the function of an IoT system. Generally, the architecture of IoT systems can be divided into layers. An intelligent IoT system can be represented as a 5-layered architecture, which contains (1) perception layer, (2) network layer, (3) middleware, (4) service layer, and (5) application layer [12]. Figure 2 illustrates the schematic diagram of such an IoT architecture for smart buildings. The perception layer is located at the bottom of the IoT system architecture. It consists of various sensing and actuating devices to provide data and information collected from the physical world. In smart buildings, a large number of sensors, actuators, and devices have been used to measure the built environment conditions such as ambient status, state of facilities, occupancy rate, etc. [13; 14; 15]. The collection of this sort of information helps  
Figure 2: A five-layer IoT architecture for smart buildings. various smart building applications, including smart facility management, lighting control, electrical usage, HVAC control, etc. After capturing the sensor data in the perception layer, the collected information needs to be sent to a receiver for further processing and control. As a result, the network layer is established. In building systems, structured cabling is one of the commonest methods for telecommunication [16]. With cables installed within a raised floor void or suspended ceiling reaching everywhere inside buildings, data communication among sensors can be enabled with wired connections. However, with greater demands of using various data sources to collect information needs in building systems, wired connections may not be as flexible and convenient. Therefore, a variety of wireless personal area networks (WPAN), such as Bluetooth, Zigbee, Wi-Fi, etc., is being used to connect IoT sensors and devices in buildings [17]. By enabling these WPAN to connect to the backbone network of the building through a route or gateway, wireless communication between data sources and the IoT platform can be set up. In addition to the communication based on structured cabling, cellular-based wireless networks are also becoming popular for indoor wireless communications, whether with low throughput sensors or bandwidth-hungry devices. The endpoint devices can directly connect to the Internet via cellular-based wireless networks due to their extensive indoor and outdoor coverage, so simpler local set-up and mobility can be achieved under this connection method. The most frequently used cellular-based communication technologies include NB-IoT for low-power wide-area communication [18] and LTE for high-speed communication [19]. Table 1 summarizes the main characteristics of several wireless communication technologies for indoor telecommunication. Even though wireless communication technologies provide a more flexible and accessible means of data communication for IoT sensors or devices, the range and strength of radio signals may be strongly influenced in some specific indoor spaces (e.g., multiple rooms, long corridors, elevator shafts, and basements) because of the attenuation of these structures to radio signals [20]. The specifications of mobile communication technology given in Table 1 only present the performance measured in the most favorable signal propagation conditions. The performance of these mobile communication technologies may all get weakened to some extent in indoor environments. For example, through the testing of signal quality of ZigBee in rooms and corridors, it was found that the recommended distance for indoor environments is a distance not exceeding 25 m [21]. As for Wi-Fi, accuracy locating with signal strength could only be enabled within a range of 25 to 50 m for indoor environments [22].