Building Fire Evacuation: An IoT-Aided Perspective in the 5G Era

: Complex and tall buildings have been constructed in many cities recently. Fire safety should be a major concern of building designers, engineers, and governments. Previous ﬁre experience has made us understand the importance of acquiring ﬁre-ground information to facilitate ﬁreﬁghting operations, evacuation processes, rescues, etc. Recently, the rapid advancement in Information Technology, Data Analytics, and other detection and monitoring systems has provided the basis for ﬁre safety researchers to re-think ﬁre safety strategies in the built environment. Amongst all ﬁre safety studies, evacuation in tall buildings, including elevator evacuations, has attracted much attention. IoT-aided building ﬁre evacuation is a new concept of the building evacuation mode, which improves the building evacuation process by making decisions of escape based on the real-time ﬁre-ground information, such as the ﬁre environment and occupant situations. Focusing on IoT applications in building ﬁre evacuation, this paper explores the advantages and insufﬁciencies of current smart building ﬁre evacuation systems. A conceptual design of an IoT-aided building ﬁre evacuation control system is described. The system is introduced in the sequence of information needs, information sources and data transmission, and potential services and applications. Finally, new insights into promising 5G technologies for future building ﬁre evacuations are discussed.


Introduction
Rapid urbanization and increases in the urban population have resulted in highdensity development and erection of tall buildings in cities. Large complex tall buildings can accommodate over ten thousand occupants. Their safety should be a great concern of building designers, engineers, and governments.
In June 2017, the world was all astounded by the Grenfell Tower fire in London, UK. The fire caused the entire building to burn out, causing 80 deaths and over 70 injuries [1]. Delayed building evacuation was considered to be one of the factors leading to the great loss of life in that fire [2]. In the Grenfell Tower fire, a "stay-put" policy was initially employed [3]. The "stay-put" policy suggests that residents not directly affected by the fire should remain within their flats and wait for further notice [2]. This policy was made because a fire in such a building would usually be contained within the floor of origin, and it is expected that occupants other than those living or staying near the fire origin may not need to evacuate. However, the Grenfell Tower fire spread dramatically and was soon out of control, making the "stay-put" policy totally lose its functions. The abandonment of the "stay-put" policy was made very late, when the thick toxic smoke had already spread to the single narrow stairwell in the building. As the only escape route was blocked, only a few residents successfully escaped after relinquishing the "stay-put" policy. There were indeed many people trapped in their rooms on high-level floors.
In the Grenfell Tower fire, due to a lack of feedback on fire-ground information about fire development, trapped occupants, and evacuation progress, the incident commander (IC) may not have had sufficient information to judge if the evacuation tactics should be altered, so that the occupants may not have been alerted to escape immediately. Like the Grenfell Tower fire, currently, collections of fire-ground information of most of the other building fires can only depend on the reports by firefighters who have entered the building. This sort of information collection mechanism may delay and make it hard for ICs to obtain the full picture of the fire development situation immediately and affect the IC's decision-making process.
(2) Lack of effective communication means to alert building occupants.
The Grenfell Tower may not have been equipped with the public address (PA) system or other effective means of communication to advise the occupants of the fire situation and escape arrangement [4]. The only two ways that the IC could alert the occupants inside the building were through the physical deployment of firefighters to each floor or to notify the occupants directly from the control room using phone calls. Both approaches were time-consuming and difficult to operate. Although conditions in the stairs did not present an insurmountable hurdle at first, full evacuation of the building was not considered timely. The same situation can be suffered by any type of buildings around the world. Due to the lack of effective means of communication to alert and assist building occupants in these buildings, evacuation of the occupants can be seriously delayed.
(3) Difficulty of organizing an effective evacuation.
Even if all the occupants in the building were successfully notified to evacuate, a sudden evacuation would cause a mass gathering of people in stairwells and reduce the evacuation efficiency. For tall building evacuation, it is necessary to control the sequence and means of evacuation for each building floor in order to make the evacuation remain orderly and more efficient. However, it is not easy to carry out such an organized evacuation strategy. It requires reliable information on the building plan, fire information, and occupants' locations to help plan the evacuation strategy. It needs effective means of communication to guide the occupants during the evacuation. Therefore, it is difficult to organize a safe and effective total building evacuation in tall buildings at current stages.
The delayed evacuation in the Grenfell Tower fire indicates that building fire evacuation still faces some common difficulties and challenges. The major deficiencies include inadequate situational awareness of interior fire environments, less effective communication between occupants and rescuers, and a lack of systematic analyses of evacuation strategies. If these insufficiencies cannot be addressed, the tragedy of the Grenfell Tower fire is likely to happen again.
To successfully overcome these insufficiencies, we must have innovative approaches and new technologies for managing the facilities, such as effective detection, monitoring, tracking system, data analytics, communication systems, etc., to provide valuable support for developing an innovative building fire evacuation system. The 21st century is undergoing a fast-paced trend of digitalization with the emergence of the Internet of Things (IoT). The IoT is an integration of technologies that connects ubiquitous devices and facilities with various networks to provide efficient and secure services for all applications anytime and anywhere [5]. By facilitating the collection and exchange of data among virtually everyone and everything, the IoT is enabling the cyber and physical environments to become unprecedentedly entangled, which promotes efficiency in performance and economic benefits and minimizes the need for human involvement. At present, tall building fire evacuation insufficiencies come from the inadequate perception of fire-ground information and the lack of effective communication means between the interior and exterior of the economic benefits and minimizes the need for human involvement. At present, tall building fire evacuation insufficiencies come from the inadequate perception of fire-ground information and the lack of effective communication means between the interior and exterior of the incident premise. These insufficiencies eventually cause inappropriate decisions in evacuations. The use of IoT can provide a possible solution for building evacuation control in fire emergencies. By using the IoT to build a connection with a wide variety of sensors, actuators, and devices pre-installed or post-deployed on the fire-ground, the IC and fire control room can obtain fire-ground information in real-time. Moreover, embracing the strong analytical power of IoT to analyze a huge volume of data generated from various connected IoT sensors and devices, the IoT system can tell more available information to help ICs make more appropriate decisions during the commanding.
Another factor that makes the IoT a welcome solution for building fire evacuation control is the incentive of the fifth generation of cellular technology (5G). 5G is a new global wireless standard after 1G, 2G, 3G, and 4G networks [6]. It was proposed and started its commercial deployment in 2019 [7]. Compared to the previous 2G, 3G, and 4G Long Term Evolution (LTE), 5G operates in an additional millimeter-wave (mmWave) spectrum from 28 GHz up to 95 GHz to provide a larger band of frequencies [8]. As a result, it enables a peak throughput of 10-20 Gbps, 100 times faster than 4G LTE networks. More than increasing the speed, 5G also contains technologies enabling approximately 1 ms latency in data delivery, increased energy efficiency consumption, massive device communication, etc. [9]. These attributes make new experiences and services in IoT connectivity and applications possible [10]. Three new service areas were categorized, which are enhance mobile broadband (eMBB), massive machine type communication (mMTC), and ultra-reliable and low latency communication (uRLLC). The overall vision for these three broad 5G usage families and their basic service requirements are illustrated in Figure 1. With the support of 5G in communication, the utilization of IoT to support building fire evacuation will become a reality. A wide variety of IoT applications can be utilized for improving evacuation in high-rise buildings. For instance, many new mobile IoT applications that can only be supported under 5G environments, such as drones with HD cameras and augmented reality/virtual reality (AR/VR)-based evacuation guidance, can be used for the first time on the fire scene. Furthermore, due to the 5G characteristics of massive capacity and connectivity, the data collection from a wide range of sensor sources can also become possible. These data sources include temperature, smoke, states of various fire protection systems, and many other sensor sources, providing a solid basis for understanding and decision making of the building evacuation under fire emergencies. With the support of 5G in communication, the utilization of IoT to support building fire evacuation will become a reality. A wide variety of IoT applications can be utilized for improving evacuation in high-rise buildings. For instance, many new mobile IoT applications that can only be supported under 5G environments, such as drones with HD cameras and augmented reality/virtual reality (AR/VR)-based evacuation guidance, can be used for the first time on the fire scene. Furthermore, due to the 5G characteristics of massive capacity and connectivity, the data collection from a wide range of sensor sources can also become possible. These data sources include temperature, smoke, states of various fire protection systems, and many other sensor sources, providing a solid basis for understanding and decision making of the building evacuation under fire emergencies. Therefore, there is no doubt that the IoT-aided building fire evacuation will truly play a role in the upcoming 5G era.
To facilitate the understanding and research of IoT-aided building fire evacuation, we conducted a literature review of the state-of-the-art smart building fire evacuation system. After that, based on the benefits and insufficiencies, the system architecture of an IoT-aided building fire evacuation control system was designed. Finally, new insights that 5G technologies can bring to the building fire evacuation were discussed. The article is organized as follows: • Section 2 introduces the concept of IoT in smart building applications. • Section 3 describes a review of the state-of-the-art smart building fire evacuation system. • Section 4 suggests a preliminary design of an IoT-aided building fire evacuation control system. • Section 5 discusses the opportunities in the 5G era. • Section 6 provides a discussion. • Section 7 provides a concluding remark.

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.
Therefore, there is no doubt that the IoT-aided building fire evacuation will truly play a role in the upcoming 5G era.
To facilitate the understanding and research of IoT-aided building fire evacuation, we conducted a literature review of the state-of-the-art smart building fire evacuation system. After that, based on the benefits and insufficiencies, the system architecture of an IoT-aided building fire evacuation control system was designed. Finally, new insights that 5G technologies can bring to the building fire evacuation were discussed. The article is organized as follows: • Section 2 introduces the concept of IoT in smart building applications. • Section 3 describes a review of the state-of-the-art smart building fire evacuation system. • Section 4 suggests a preliminary design of an IoT-aided building fire evacuation control system. • Section 5 discusses the opportunities in the 5G era. • Section 6 provides a discussion. • Section 7 provides a concluding remark.

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 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]. Moreover, the penetration losses of 4G LTE signals in indoor building areas were also demonstrated by Zulkefly et al. [23] and Soliman et al. [24]. Therefore, networking devices such as access points, wireless repeaters, and routers should be applied to ensure the reliability of wireless communication in indoor environments. Furthermore, middleware is followed to build the connection between physical and application layers. Middleware is usually described as a software system designed to be the intermediary between IoT devices and applications, so that seamless connectivity and data management for distributed applications can be enabled [12]. To realize IoT application in smart buildings, middleware based on events with database storage is gaining popularity due to the easiness of deployment and lightness of resource utilization. Message Oriented Telemetry Transport (MQTT), Constrained Application Protocol (CoAP), and BLE (Bluetooth Low Energy) are some popular connectivity protocols explicitly designed for this aim [25]. The service layer lies at the upper level of the IoT architecture. Owing to the strong analytical power of IoT, the service layer can strongly interact with the application layer to cater to the design and demands of the application layer for providing effective utilization of the collected data. The service layer is established in smart building applications to provide various services, including device management, information analyses, and security control. Finally, with the development of user interfaces in the application layer to access various IoT-based services, monitoring and control of the smart building facilities can be enabled.

Review of Current Developments
To help the evacuation of occupants, many studies have tried using IoT technologies to develop smart evacuation systems in buildings. In general, a smart building fire evacuation system is composed of a detecting (or measurement) unit, a route planning solver (e.g., an algorithm), and a guiding device (e.g., exit signs, personal devices) [26]. Innovations and new technologies are fully applied in each part. Therefore, in this section, we conduct a review of the state-of-the-art smart building fire evacuation systems and try to find out what advantages and insufficiencies exist in current smart building fire evacuation systems.

Review of the State-of-the-Art Smart Building Fire Evacuation Systems
The most common design of the detecting unit in smart building fire evacuation systems is to apply sensors to perceive various environmental parameters during the fire emergency, including smoke [27][28][29], temperature [30,31], and other sensor sources [32,33]. The data communications of these sensors are usually built by Wi-Fi [28,29], Zigbee [32], and Bluetooth [29][30][31]. A comprehensive IoT-based building fire evacuation system was exemplified by Park et al. [34]. The system enabled the detection of fire occurrence places by monitoring the real-time environment data from multiple smart sensors, including gas leak detector sensors, temperature sensors, electric leak detector sensors, and multi-gas detector sensors. In addition to this, the system was also connected with various actuators, including electric breakers and gas valve breakers, to enable remote cut-off of further fire hazards in buildings. The data transmission of all these sensors was built under the ZigBee environment.
Moreover, some solutions consider detecting occupants' situations (e.g., position, flow rates, presence). These states of occupants are also important factors in building evacuations. In order to dynamically direct people to the safest exits and speed up the evacuation process, many researchers have developed smart systems tracking locations of the building occupants. With received signal strength indication (RSSI) through Bluetooth Low Energy (BLE) beacons, many researchers enabled localization of occupants via occupants' mobile phones [28][29][30]. In addition, a mobile RFID-based system for localization was proposed by Chittaro and Nadalutti [35]. A ZigBee-based localization module that identifies people's positions and understands whether they are evacuating was proposed by D'Orazio et al. [36]. The systems described by Jiang [37] and Atila et al. [38] both employed purposely deployed RFID beacons to collect personnel localization in buildings. Furthermore, personal localization using Global Positioning System (GPS) and Wi-Fi access points were also mentioned by Dominicis et al. [39]. In addition to tracking the locations of building occupants, some studies also consider optimizing the evacuation routes according to the personnel density or speed in the evacuation route. The crowd evacuation speed and flow rates are often detected via surveillance cameras [40,41]. The detection of occupants' presence and/or other behaviors in building areas during the evacuation process was also introduced by several authors [42][43][44][45]. The most typical approach used to count the number of people in a target area is through visual-based cameras. Finally, some studies combine BIM technologies to manage and visualize the collected fire and occupant information in a 3D model [29,30,[46][47][48][49]. This BIM innovation successfully makes it easier for building managers and firefighters to be aware of the real-time fire environment and evacuation on-site. Furthermore, focusing on intelligent route planning, researchers also propose various intelligent building route planning algorithms considering real-time environmental factors and occupants' situations during the evacuation. By giving information about fire origin, nodes of trapped occupants, and nodes of firefighters, Chou et al. [31] proposed a fire rescue path planning approach based on Dijkstra's algorithm. The approach enabled finding the optimal path of escape in a short computational time. In the work of Jiang [37], an intelligent route planning algorithm was proposed by considering the intensity of the fire. In this way, fire hazards can be considered when planning the evacuation route. In order to suggest evacuees with less crowded paths toward safety exits, a situation-aware route planning system considering evacuees' behaviors during the evacuation (e.g., running, waiting in a queue, herding) was proposed by Billhardt et al. [43]. With beacon sensors to recognize the location and movement of every occupant inside the building during the evacuation, this system successfully enables evacuees to bypass the congestion and dangerous area in advance. Moreover, Ma and Wu [49] developed a system that optimizes the evacuation route for evacuees and rescued people. By monitoring the number of people using the stairs, strategies for stair evacuation can be adjusted. However, the research on intelligent route planning focuses more on individual path-finding during the evacuation. It is rarely considered how the smart system can coordinate the total building evacuation containing a large number of occupants.
Finally, many researchers also developed various smart devices regarding evacuation guidance to make the evacuation signal and planned evacuation route successfully delivered to building occupants. One possible approach is to send signals and instructions to users' mobile phones via a pre-installed application [29,31,34,47], frequently applying Wi-Fi and cellular networks for communication. However, these networks are infrastructurebased networks that are prone to fail in critical environments. Therefore, a MANET (Mobile Ad Hoc Network)-based evacuation guidance system was developed by Ohta and Dunkel [50]. The MANET is a wireless network without a centralized server or network infrastructure, so it can be used to provide more reliable evacuation guidance to occupants. The same sort of communication approach was also used by Zualkernan et al. [28]. In addition, by considering that not everyone is necessarily equipped with a smartphone or has downloaded the application, which means that some people may not be able to receive the guidance, another approach for evacuation guidance using "smart exit signs" was proposed by several authors [28,40,[51][52][53]. These "smart exit signs" are usually equipped with dynamic signage, which can dynamically change their directions to inform evacuees of the directions towards the shortest safe evacuation paths.

Summary of Advantages and Insufficiencies
From the state-of-the-art smart building fire evacuation systems review, some of the advantages and inefficiencies are summarized.
First, by using various devices and sensors, the smart building fire evacuation systems can successfully detect fire situations and evacuation processes on the scene. The parameters they collected include not only environmental factors but also occupants' situations. By monitoring the environmental factors, such as flame, heat, and smoke, ICs and building managers can become more aware of the current fire situation and promptly initiate the evacuation. As for the occupants' situations, monitoring of the occupants' presence or positions during the evacuation process can help understand the bottlenecks of the evacuation. The ICs and building managers can then adjust the evacuation strategy and guide the occupants to escape smoothly. Moreover, the position information also offers help to the search of trapped occupants during the rescue mission.
Furthermore, the development of various smart systems helping evacuation guidance also provides a valuable contribution. Lacking effective means of communication to guide occupants to escape is one of the major difficulties for building fire evacuation. Therefore, a "smart exit sign" can be useful when responders hardly reach the incident floor and guide the occupants to leave. In addition, the "smart exit sign" is a stationary device pre-installed inside buildings, so no other devices or software need to be used by occupants, which satisfies building evacuation scenarios with mass populations.
The major insufficiency of current smart building fire evacuation systems is that most of the systems only achieve the perception of either the information of fire environment or occupants' situations. The interpretation of these data and how they will influence the total building evacuation have been less considered. Even though some previous research has enabled smart route planning based on the information collection regarding fires or occupants' situations, the focus was more on evacuees' path finding within single floors. The systematic understating and prediction of the total building evacuation considering dynamic fire development and occupants' situations was insufficient. As a result, the current smart building fire evacuation system does not seem to be able to coordinate and organize the prompt total building evacuation under fire emergencies, which is the major difficulty faced by tall building evacuations.
With more extensive use of various wireless communications in the 5G era, the connectivity and analytical power of the IoT will increase a lot. More connectivity of various IoT sensors and fast computation of the collected data can be enabled. Therefore, a preliminary design of an IoT-aided building fire evacuation control system is proposed in view of the advantages and insufficiencies in current smart building fire evacuation systems.

Preliminary Design of an IoT-Aided Building Fire Evacuation Control System
In the upcoming 5G era, various sensors and devices will become available to monitor various environmental parameters and occupants' situations in buildings. Taking into full consideration the available information on facilitating a more effective and efficient fire evacuation, we analyze the information sources, means of data transmission, and potential services and applications that should be provided for building fire evacuation control. After that, an IoT architecture for an IoT-aided building fire evacuation control system is proposed.

Information Needs
Generally, two aspects of safety issues are the major concerns during the fire evacuation. At first, there is the need to ensure that occupants stay away from the thermal hazard caused by the fire. Enclosure building fires create a large volume of heat and toxic smoke inside the building, and the spread of these hazards causes a major threat to evacuees [54]. Thus, it is essential to monitor the indoor fire environment and know where the hazardous areas are in a fire. In addition, fire safety equipment, such as firewater systems and fire doors, is the active design to stop the fire spread. The condition of these facilities in fires plays an important role in building evacuation. Therefore, information of the state of various fire safety equipment should also be monitored.
Furthermore, in terms of occupants' situations during the evacuation process, chaos and disorder are also a concern [55]. Tall buildings encompass a large number of occupants working and living together. In fire emergencies, the convergence of the crowd in the stairwells may cause mass blockage and panic, so it is rather challenging to evacuate all these occupants simultaneously. Phased evacuation control has been proven more suitable for tall building fire evacuation [56]. The phased evacuation strategy conducts a controlled, area-by-area evacuation to decrease the queuing time and density of people. Still, its effectiveness must be strongly built on the timely information feedback of the overall evacuation progress [57]. Therefore, real-time occupant count data of different building areas should be collected to optimize the evacuation strategy and assist the building evacuation.
Finally, evacuation signals and the implemented evacuation strategy need to be delivered to occupants in time in a fire emergency. The existing fire emergency systems, such as fire alarm systems, fire emergency lighting systems, and PA systems serve this purpose. Therefore, it is necessary to monitor the state of these systems to avoid a malfunction in emergencies. Moreover, due to the importance of providing evacuation guidance in tall building fire evacuation, smart devices that can promptly guide the occupants to escape should be used. The most direct way to deliver clear evacuation guidance in tall buildings is through voice and visual indicators. Therefore, remote control and operation of the PA system and the "smart exit signs" need to be built.
According to the analysis above, the overall information needed for building fire evacuation control is listed in Table 2. Table 2. Information needs for building fire evacuation control.

Information Needs Purposes
Fire hazard monitoring (A) Measure the hot gas layer (HGL) temperature at corridors to avoid thermal hazards on the evacuation routes.
Smoke (A12) Measure carbon monoxide level at corridors to avoid toxic smoke spreading on the evacuation routes.
Fire protection system (A2) Firewater system (A21) Monitor the state of firewater systems, such as sprinkler systems, indoor fire hydrant and hose reel installation, etc., to avoid malfunction. Building areas (B3) Corridors and common areas (B31) Collect the occupancy information in buildings.
Rooms and private use areas (B32) Collect the occupancy information in buildings.
Evacuation guiding (C) Fire emergency system (C1) Fire alarm system (C11) Provide visual alarms and sound alarms to evacuees. Emergency lighting (C12) Provide light sources to evacuees. Evacuation guidance system (C2) Exit and directional sign (C21) Inform evacuees of the directions towards the safe evacuation paths.
Public address system (C22) Provide voice-based evacuation guidance to evacuees.

Information Sources and Data Transmission
To enable the collection of the information needs as mentioned above in building fire evacuation, there is the need to determine the perception devices and their means of communication, which formulate the perception and the network layers in the IoT architecture.

Fire Hazard Monitoring
To monitor the environmental fire information in a building, various physical sensors are mainly applied. Thermocouples are the most widely used sensor devices to enable the measurement of temperatures in an enclosure space. The range of the measurements varies from 0 to 1700 • C by type [58]. To measure the indoor carbon monoxide (CO) concentration levels, CO gas detectors can be used [59]. By installing these sensors in spaces and corridors throughout the whole building, identification of the fire origin and its affecting areas can be achieved. In addition, the state of various fire protection systems can be monitored by additional sensor sources. For example, door lock sensors can be installed to monitor fire doors' open or closed status in buildings. The state condition of firewater systems can be monitored by water flow or pressure sensors.
Due to the fact that there is a large amount of environmental and equipment state sensor sources and they are installed everywhere inside buildings, flexibility and simplicity of connectivity may be one of the major concerns. Data communication via wireless networks shows its advantages in this aspect. The transmission of these sensor data runs in a small volume of data and has low refresh rate requirements, so narrowband wireless networks for communication, such as ZigBee, NB-IoT, LTE-M, and Lora, can fit their needs. At the same time, the narrowband wireless networks also support communications among massive low throughput IoT devices at a relatively low cost and low device power consumption. However, the prerequisite of using these wireless communication networks for sensor data transmission is that they should be resistant to high temperatures and dense smoke in fire environments. Former tests carried out by Schubert and Scholz [60] showed that sensor data transmitted under 2.4 GHz signals (e.g., Wi-Fi, ZigBee, Bluetooth) might not be affected much by the dense smoke and high fire temperature within their test regime. A similar conclusion was obtained by Hofmann et al. [61], also showing that smoke and fire do not influence much the communication in the 2.4 GHz frequency band. Therefore, working in a lower frequency band with stronger penetration, these proposed narrowband networks are believed to support sensor data communication under fire conditions.

People Counting
To obtain the occupant count and localization information inside buildings, various occupant counting techniques can be employed. The mainly used technologies are radio frequency (RF), infrared, video cameras, or network connecting signals such as GPS, cellular data, wireless local area network (WLAN), and Bluetooth [62]. The benefits and drawbacks of these approaches were reviewed by Yang et al. [62]. In order to cause minimal influence on occupants' normal lives and privacy, multiple applicable techniques are applied to count occupants in building fire evacuation control.
First, a turning gate and swipe card system can be used at the building entrances to count the total entering and leaving occupants. The data count can be directly sent to the local server and connected to the IoT platform with a wired connection. Since the turn gate system only allows the authorized occupants to gain entry, manual recording should be kept for additional visitor entry. The building manager can record visitors' information with a web-based spreadsheet through the Wi-Fi network in buildings.
Additionally, it is considered that a turn gate system may not be suitable for people counting in residential or commercial buildings, which much affects occupants' normal life, and it hardly tells the specific number of occupants on each floor. Therefore, it is suggested to use visual-based video counters in these areas. The visual-based video counter is a video camera combined with computer vision-based algorithms to count people [63]. It can be used by direct mounting above the building/floor entrances and conducting the counting. This approach causes minimal influence on building occupants and has high recognition accuracy. Due to the high bandwidth and high reliability needs for streaming video data transmission, it is better to use wired connections for local data transmission. By connecting with the existing backbone cable of the building, the real-time local video can be directly sent to the local server and connected to the IoT service platform. An alternative approach is to link these visual-based video counters to a gateway (placed elsewhere) with cables and then connect to the IoT service platform with broadband cellular networks. The connectivity in this way not only makes it easier for the service platform to be placed on the cloud but also can ensure the reliability of streaming video data transmission in local fire environments. For the broadband cellular networks, 4G LTE and the latest 5G NR can be employed.
However, it is worth noting that people counting using visual-based video counters may fail when the building floor is full of smoke, so a thermal counter can be used at the exits of each floor as an alternative option in case the thick smoke obscures the visual-based video counters. The thermal counter is a low-resolution infrared camera combined with the people counting algorithm to enable the counting of people in low light conditions [45]. In practical usage, the thermal counter can be put inside a protective box to isolate it from the effects of fire temperature and smoke. For the data transmission of the thermal imaging data of the thermal counter, wired connections to the service platform of this IoT system are better applied.
Finally, occupant counting needs to be carried out at the room level. For corridors and public areas in buildings, the visual-based approach is applicable. However, the visualbased approach seems inappropriate for individual rooms and private use areas because privacy protection is the major concern in these places. As a result, it is recommended to use infrared beam counters. The infrared beam counter mainly consists of Passive Infra-Red (PIR) sensors and their communication modules. The PIR sensors can detect general movement by identifying infrared radiation emitted by or reflected from occupants without collecting any other personal information. They have been widely used in occupancy counting in buildings [64][65][66][67]. The major disadvantage for infrared beam counters is that when two or more persons walk side by side into the area of PIR sensor detection, the infrared beam counter may count only one, so it is not suitable for rooms with large front entrances. Fortunately, the width of the doors for most individual rooms in buildings only allows one or two persons to pass at a time. At the same time, the test by Schubert [60] also showed that fire conditions have no direct influence on infrared sensor uses. Therefore, it is applicable to use the PIR sensor for counting occupants at room level. Since the infrared beam counter transmits only the counted number of occupants, narrowband wireless networks in low power consumption can be applied for data communication between the sensors and the IoT platform, the same as those sensors and devices for fire hazard monitoring.
To better understand the solutions for people counting in building fire evacuation control, the major proposed people counting solutions are summarized in Table 3.  The evacuation guiding system needs to maintain functionality at all times. Therefore, monitoring of the state of various emergency evacuation systems and remote control of the public address system and the "smart exit signs" system is needed. Like the fire protection systems, the state monitoring of the emergency evacuation systems can be achieved by detecting the states and transmitting the data through narrowband wireless networks. For the exit and directional sign systems, narrowband wireless communications can be used to directly build the connection between the IoT devices and the system platform due to its low transmission of data. For public address and voice alarm systems, considering that voice messages or later video streaming may even be used to aid the occupants in building fire evacuation, it is suggested to use cables to build the local connectivity and then link these systems to the IoT system platform on the cloud through a route or gateway with wired or wireless broadband networks. In this way, the voice or video guidance of the evacuation can be easily delivered by ICs from the exterior of the incident premises.
As a summary, the overall information sources and means of data transmission for

.3. Evacuation Guiding
The evacuation guiding system needs to maintain functionality at all times. Therefore, monitoring of the state of various emergency evacuation systems and remote control of the public address system and the "smart exit signs" system is needed. Like the fire protection systems, the state monitoring of the emergency evacuation systems can be achieved by detecting the states and transmitting the data through narrowband wireless networks. For the exit and directional sign systems, narrowband wireless communications can be used to directly build the connection between the IoT devices and the system platform due to its low transmission of data. For public address and voice alarm systems, considering that voice messages or later video streaming may even be used to aid the occupants in building fire evacuation, it is suggested to use cables to build the local connectivity and then link these systems to the IoT system platform on the cloud through a route or gateway with wired or wireless broadband networks. In this way, the voice or video guidance of the evacuation can be easily delivered by ICs from the exterior of the incident premises.
As a summary, the overall information sources and means of data transmission for

.3. Evacuation Guiding
The evacuation guiding system needs to maintain functionality at all times. Therefore, monitoring of the state of various emergency evacuation systems and remote control of the public address system and the "smart exit signs" system is needed. Like the fire protection systems, the state monitoring of the emergency evacuation systems can be achieved by detecting the states and transmitting the data through narrowband wireless networks. For the exit and directional sign systems, narrowband wireless communications can be used to directly build the connection between the IoT devices and the system platform due to its low transmission of data. For public address and voice alarm systems, considering that voice messages or later video streaming may even be used to aid the occupants in building fire evacuation, it is suggested to use cables to build the local connectivity and then link these systems to the IoT system platform on the cloud through a route or gateway with wired or wireless broadband networks. In this way, the voice or video guidance of the evacuation can be easily delivered by ICs from the exterior of the incident premises.
As a summary, the overall information sources and means of data transmission for

.3. Evacuation Guiding
The evacuation guiding system needs to maintain functionality at all times. Therefore, monitoring of the state of various emergency evacuation systems and remote control of the public address system and the "smart exit signs" system is needed. Like the fire protection systems, the state monitoring of the emergency evacuation systems can be achieved by detecting the states and transmitting the data through narrowband wireless networks. For the exit and directional sign systems, narrowband wireless communications can be used to directly build the connection between the IoT devices and the system platform due to its low transmission of data. For public address and voice alarm systems, considering that voice messages or later video streaming may even be used to aid the occupants in building fire evacuation, it is suggested to use cables to build the local connectivity and then link these systems to the IoT system platform on the cloud through a route or gateway with wired or wireless broadband networks. In this way, the voice or video guidance of the evacuation can be easily delivered by ICs from the exterior of the incident premises.

Evacuation Guiding
The evacuation guiding system needs to maintain functionality at all times. Therefore, monitoring of the state of various emergency evacuation systems and remote control of the public address system and the "smart exit signs" system is needed. Like the fire protection systems, the state monitoring of the emergency evacuation systems can be achieved by detecting the states and transmitting the data through narrowband wireless networks. For the exit and directional sign systems, narrowband wireless communications can be used to directly build the connection between the IoT devices and the system platform due to its low transmission of data. For public address and voice alarm systems, considering that voice messages or later video streaming may even be used to aid the occupants in building fire evacuation, it is suggested to use cables to build the local connectivity and then link these systems to the IoT system platform on the cloud through a route or gateway with wired or wireless broadband networks. In this way, the voice or video guidance of the evacuation can be easily delivered by ICs from the exterior of the incident premises.
As a summary, the overall information sources and means of data transmission for information needs of building fire evacuation control are given in Table 4.

Potential Services and Applications
The proposed information sources and data transmission provide a practical means of gathering on-site information for IoT-aided building fire evacuation control. However, an IoT system is more than data collection. The utilization of the wealth of collected data for service and application is the main purpose of an IoT system. Therefore, several potential services and applications for building fire evacuation control have been designed.

BIM-Based Monitoring Platform
In recent years, Building Information Modeling (BIM), as well as Virtual Design and construction (VDC), has gained increasing impact in the Architecture, Engineering, and Construction (AEC) Industry. The development of BIM/VDC provides a virtual design and construction platform to facilitate the design and construction as well as the project management process. The tool permits the display of building plans in three dimensions; people can easily understand the building geometries and the added information right into the building components. Recently, there have been some research works concerning the use of BIM for building fire safety management and evacuations [29,47,49,68,69].
For the proposed IoT-aided building fire evacuation control system, it is also important to integrate the collected sensor data into a BIM model and establishing visualization for the users. Visualized information can make it easier for building managers and ICs to guide the evacuation and manage various emergency evacuation facilities and equipment. The visualized information includes both static and dynamic information. The static information contains geometries of the building and compartments, locations of various fire protection and evacuation equipment, and maintenance information. The dynamic information is data collected from the local sensors, which can contain people counts, states of various facilities, and detections of the indoor environment. By attaching the dynamic information to the corresponding equipment or spaces inside the BIM model, browsing the real-time sensor data from a 3D interface is able to be achieved. Apart from visualizing these monitoring data, the BIM model also creates a platform for exchanging data with the fire risk assessment module and evacuation decision supporting module to provide more useful services. The approaches and functions of these two modules are introduced next.

Fire Risk Assessment
The major function of the fire risk assessment module is to help identify the hazardous areas of the fire event so that occupants can bypass the area and evacuate smoothly. By collecting the ambient status from various environmental sensors, monitoring of the indoor fire environment is enabled. Afterwards, by comparing these monitoring data with the criteria for occupants to safely evacuate the building, such as at 2.0 m height of the enclosure space, air temperature below 100°C, concentrations of carbon monoxide below 2800 ppm, etc. [70], the areas at risk can be identified. Finally, by linking these identified hazardous areas to the BIM platform, hazardous areas can be directly viewed on BIM models.
In addition to identifying the hazardous areas in a fire, the collected data can also be used to mine more necessary information regarding fire characteristics, such as fire origins, fire sizes, stages of fire development, etc. This sort of problem is called a fire inverse problem, which involves using the observed sensor data to infer the causal factors that produced it. Many studies have explored the inference of fire origins, fire sizes, and stages of fire development from temperature observations [71][72][73][74]. With the machine learning approach to find out the relationship between the fire characteristics and a large volume of simulated fire data, dynamic inference of the various fire characteristics from a series of on-site sensor data collection can be enabled. Understanding the characteristics of the ongoing fire provides the necessary information basis for building managers and ICs to predict the risks in the evacuation process, so it is very important for evacuation decisions.

Evacuation Decision Support
The evacuation decision support module works for the major purpose of evacuation control, including the planning of evacuation strategies and subsequently the evacuation guidance. For the planning of evacuation strategies, evacuation simulation provides a possible approach. Evacuation simulation is a computer-based approach to determine the total egress time based on the simulation of crowd dynamics and pedestrian motion. The existing software or modeling approach include FDS + Evac [75], SGEM [76], agent-based modeling (ABM) [77], and cellular automata (CA) [78]. By determining the input parameters, such as building geometries, initial occupants' locations, evacuation strategies, and individual behaviors and interactions, evacuation simulation can predict the evacuation process and evaluate the evacuation with some indicators, including total egress time, travel distance, door flow rate, etc. From these indicators, whether or not the evacuation strategy is acceptable can be determined. In the currently proposed system, the data collection of people counting for each floor is dynamically linked with the simulation model. In addition, the hazardous areas and characteristics of the ongoing fire analyzed in the fire risk assessment module are also connected to the evacuation model. The evacuation simulation may become more adaptable to the real fire situation on-site by considering these dynamic parameter inputs. With such dynamic evacuation modeling to assess the effectiveness of different evacuation strategies, such as evacuating occupants with different sequences, whether evacuating occupants with the help of elevators, etc., safety and effectiveness of implementing the building evacuation can be significantly improved.
As for the evacuation guidance part, it is meant to make the planned evacuation strategies come into action. With the control of the exit and directional signs, the evacuation decision support module can deliver the planned evacuation route to evacuees. Moreover, the state and condition of each exit and directional sign can be monitored and viewed in the BIM-based platform. Furthermore, considering that it may need to control the sequence of evacuation for each floor, the evacuation decision support module also enables sending voice alarms and instructions to the designated areas or floors via PA systems.

Advantages of the Proposed System
By summarizing the information sources, means of data transmission, and potential services and applications in building fire evacuation control, a preliminary design of an IoT-aided building fire evacuation control system is proposed. The system architecture is demonstrated in Figure 3. With the sensors and devices to collect various information on the fire-ground, a variety of means of data transmission are applied to build the communication between sensors and the IoT platform. In order to provide more diversified services and applications to help the building evacuation, promising technologies including BIM, fire inverse, and evacuation simulation are embraced in the system. Furthermore, considering the importance of data protection and system security, the system is also equipped with an authentication and encryption layer to protect the endpoint devices so that malicious attacks and data breaches can be avoided. In addition to protecting the security of endpoint devices, data security in the IoT platform can be enabled by firewalls and strong user authentication processes. With these measures, the proposed system can be secure and only used for its intended uses (i.e., fire-ground situational awareness, evacuation planning, or evacuation guidance).
The advantages of the proposed system are summarized as follows: (1) Monitoring the fire environment and occupants' situations in buildings.
IoT technology provides a possible approach to link the cyber and physical environment together in building fire evacuation control. By establishing the perception layer and network layer of the proposed IoT system, the ubiquitous sensors and various communication networks can be integrated to enable the monitoring of the fire environment and occupants' situations in buildings. Compared with traditional fire evacuation, the IoT-aided fire evacuation highly improves the information collection and update on-site, ensuring safety in evacuations. Moreover, the collected people count information is also useful for building energy management in regular time. Therefore, it is applicable to implement such a system in buildings.
By exchanging the monitoring data from various sensors and devices through the Internet, IoT enables the aggregation of all the on-site information into one place. Thus, combined with BIM, the visualization and management of various evacuation equipment and facilities within the BIM model can be achieved. With the BIM model to show the locations of various evacuation facilities and hazardous areas, a direct understanding of the relationship between the building geometry and the fire situation can be built by building managers and ICs, making it easier to guide occupants' evacuations. As for the information regarding the state of various firefighting equipment and evacuation facilities, it consists of its historical data and real-time monitoring data. By attaching the state information to the specific equipment and facilities in the BIM model, browsing and control of these installations can be achieved. With the integration of IoT devices, IoT data, and IoT platforms, the IoT system enables data transfer and interoperation to support more advanced services and applications. Therefore, the fire risk assessment module and the evacuation decision support module are proposed, respectively. These modules can help the planning of evacuation strategies from a systematic level. Compared with the traditional pre-defined fire evacuation plan, the currently proposed system enables dynamic adjustment of the evacuation strategy according to the fire development and the pattern of occupants' location. As a result, more effective and efficient fire evacuation control in tall buildings can be achieved. The advantages of the proposed system are summarized as follows: (1) Monitoring the fire environment and occupants' situations in buildings.
IoT technology provides a possible approach to link the cyber and physical environment together in building fire evacuation control. By establishing the perception layer and network layer of the proposed IoT system, the ubiquitous sensors and various communication networks can be integrated to enable the monitoring of the fire environment and occupants' situations in buildings. Compared with traditional fire evacuation, the IoTaided fire evacuation highly improves the information collection and update on-site, ensuring safety in evacuations. Moreover, the collected people count information is also useful for building energy management in regular time. Therefore, it is applicable to implement such a system in buildings.
By exchanging the monitoring data from various sensors and devices through the Internet, IoT enables the aggregation of all the on-site information into one place. Thus, combined with BIM, the visualization and management of various evacuation equipment and facilities within the BIM model can be achieved. With the BIM model to show the locations of various evacuation facilities and hazardous areas, a direct understanding of the relationship between the building geometry and the fire situation can be built by building managers and ICs, making it easier to guide occupants' evacuations. As for the

Opportunities in the 5G Era
With the commercial deployment of 5G starting in 2019, we are stepping into the new era of mobile communications. More than improving the mobile broadband speed, 5G also fits the needs of a diverse range of speed, bandwidth, and quality of service requirements, making the nature of the 5G communications more suitable for future IoT system design requirements [79]. The major technological breakthroughs in 5G, including millimeterwave (mmWave) spectrum, massive MIMO, network ultra-densification, network function virtualization (NFV), software-defined networking (SDN), device-to-device (D2D) communications, etc., ensure such a more diverse and reliable network connection [80]. With the continued deployment of the 5G infrastructure, there are certainly more opportunities to apply 5G technologies in building fire evacuation control in the future. Therefore, opportunities for building fire evacuation in the 5G era are introduced in this section.

Massive Machine Type Communications
From the proposed IoT-aided building fire evacuation control system, it is obvious that the major communication in the fire evacuation control system is Machine to Machine (M2M) communication. M2M transmits a small volume of data but connects a great number of devices, so it is mainly used for automated data transmission and measurement between mechanical or electronic devices, such as temperature measurement, state monitoring, infrared people counting, etc. In the 5G era, with the connection of various building systems and components to implement smart buildings, traditional IoT-enabled communication technologies, such as Zigbee, Bluetooth, GSM, etc., may become insufficient to support massive M2M communications in buildings because of their limitations in coverage and connected devices. Therefore, the low-power wide-area networks (LPWAN) are worth using. Currently, the most commonly used LPWAN in IoT is Narrow Band Internet of Things (NB-IoT).
The NB-IoT is a cellular-based LPWAN proposed in 2016, supporting a high volume of low complexity connected devices intended to transmit small data packages. Thus, it fits the need of M2M communications in IoT applications by transmitting tiny amounts of information in a long-range [18]. Due to the fact that NB-IoT meets most 5G mMTC requirements, NB-IoT will continue to play an important role in communication in the 5G era and eventually evolve into a part of 5G technologies [81]. The benefits of using the NB-IoT in fire evacuation control is summarized as follows: (1) Extensive indoor and outdoor coverage.
NB-IoT provides improved indoor and outdoor coverage-approximately 1 km in urban areas and 10 km for rural areas [82]. Therefore, it is convenient to install the IoT sensors wirelessly in any place in a large complex building, not only the deep underground spaces but also floor gardens on top of the building.
NB-IoT allows for deployments of up to 200,000 devices in theory per cell [83]. With such great multiplexing, more sensors and devices in buildings are available to connect to the network and send their states to the building managers and/or fire services. This promotes the situational awareness abilities and management of resources on the fire-ground.
Devices with NB-IoT usually consume less power than other enabling technologies. The battery life of NB-IoT-based devices is over 10 years. This is highly valued for IoT applications because a large number of sensor nodes spread everywhere. Frequent battery replacement not only consumes a large amount of power but also increases the cost of labor.
NB-IoT uses a licensed spectrum as other cellular technologies do, so the quality of service (QoS) and security can be guaranteed. Communicating in a licensed spectrum, NB-IoT avoids interference from other users located nearby. Furthermore, carefully specified by standards body 3GPP, NB-IoT also benefits from the security features of cellular networks.
The NB-IoT scores with low module costs as well as low operation and service prices. At present, the cost of NB-IoT modules is as little as USD 6-12 each [84]. The services charge is around USD 3-6 per year per device for China Mobile in China. In addition, the extensive coverage of NB-IoT eliminates the use of nodes and gateways in buildings, so the cost of deploying the network is also reduced.

AR-Enabled Fire Evacuation
Augmented Reality (AR) is becoming one of the most promising technologies to facilitate evacuation safer, quicker, and easier. Unlike Virtual Reality (VR), AR overlays digital or computer-generated information over a real-time environment. Thus, AR supplements reality rather than completely replacing it. In building fire evacuation, with the use of a smartphone or head-up display (HUD), the virtual indicators can be superimposed onto the real objects in buildings, such as exits, stairwells, and hazardous areas. In this way, occupants can be alerted and find evacuation routes immediately.
The application of AR facilitating fire evacuation has been studied a lot. In the study of Tsai and Yau [85], AR technologies, route planning, and three-dimensional graphics technologies were integrated to provide evacuation route guidance by giving virtual indicators on the escape route in the context of a radioactive accident. An AR-based building fire evacuation system was developed by Sharma S et al. [86]. The system applied a Microsoft HoloLens augmented/mixed reality device to provide a visual representation of a building in 3D space and allowed the occupants to see where exits were in the building. Moreover, AR technologies were applied to help firefighters search and rescue in fire emergencies. The C-Thru helmet from Qwake Technologies was developed to help firefighters' fire-ground operations by showing the silhouettes of otherwise indiscernible objects and people on the fire-ground [87]. However, limited by the mobile communications previously used that possess low speed and high latency, the application of AR is not to scale. Using AR in fire evacuation is still being explored.
In the 5G era, this situation will change. The high bandwidth and ultra-low latency that the 5G network provides will bring more new experiences to AR-enabled fire evacuation. In the future, occupants can use AR to identify the risks in surroundings during the evacuation, such as falling ceilings, thick gas layers, or even collapsing walls. In this case, it is necessary to enable real-time video streaming via the 5G network because the captured video must be transmitted to a local or cloud processor for visual-based data processing in real-time due to the limitation of the computing power on the end device. Moreover, with the edge computing work with 5G, the network latency will be highly reduced by processing data closer to the end-user. Low wireless latency improves the immersion of the users, helping evacuees receive feedback from the processor more quickly and receive better information acquisition from the interaction with the environment.

Network Slicing Technology in Emergency Communication
Network slicing has emerged as one of the essential components in 5G technologies [88]. Compared with previous mobile network communications, the 5G network allows numerous users with varying requirements to operate within one network. Therefore, it is imperative to apply a technique to subdivide one network into various virtual networks and accommodate various priorities and use cases. By employing network function virtualization (NFV) and software-defined networking (SDN) to manage the dynamically changing capacity requirements, the network slicing technology will fit the needs by unlocking and orchestrating the specific resources to create network slices fitting different application scenarios [89].
In the 5G era, one of the most useful applications for network slicing is mission-critical communications [90]. Mission-critical communication is mainly used for emergency response (ER) operations where the communication needs to be reliable, available, and secure in any circumstance. In the past, mission-critical communication was mainly supported by public safety networks (PSNs), such as Terrestrial Trunked Radio (TETRA), Digital Mobile Radio (DMR), and Project 25 (P25), all of which are private networks only serving the ER services [19]. To some extent, these networks provide dedicated and reliable communication for mission-critical operations, but they are developed by narrowband communication technologies, so only voice services are supported. With the wide distribution of public cellular base stations and the demand of using faster and rich information communication to support public safety services, such as voice and video calling, internet/intranet access, high-quality imaging, and real-time video recording [91], it is inevitable to evolve PSNs from traditional radio-based networks to commercial cellular networks. However, it is dangerous to directly use commercial cellular networks for ER communication because the reliability, availability, and security of commercial networks may not be guaranteed in some circumstances. For example, in large public events, thousands of mobile users may connect to the network at once, causing the network to collapse easily. Emergency communication under the same network can be severely disturbed in such a situation. The network slicing technology provides a possible approach for solving this. Through the 5G network slicing, we can bypass the commercial consumer networks, which may already be congested or at full capacity, and create additional network capacity to support the ER information that involves transmitting significant amounts of real-time data in a reliable and uninterrupted manner.
In the building fire evacuation situation, a sharp increase of network connection by occupants and citizens may emerge at the surrounding area of the building after the fire occurrence. Therefore, it is necessary to create two slices, one of which is dedicated to emergency responders and the others to serve public users. Communicating under the network slice for ER, the IC can enable liaising with responders in the fire building without interference. As a result, the real-time evacuation progress can be reported to IC clearly and on time. Moreover, the broadband network slice will support many bandwidth-hungry applications to facilitate fire-ground situational awareness. These applications include using drones with high-resolution cameras to monitor fire development on building exteriors and setting up video channels to collect real-time fire situations from the firefighters entering the fire-ground. With the help of the 5G network slicing technology, more effective and efficient fire evacuation and ER operations can be conducted.

D2D in On-Site Information Delivering
Device-to-Device (D2D) communication, which refers to a radio technology that enables direct communication between devices (i.e., users) without data traffic going through any infrastructure node such as an access point or base stations, is recognized as one of the important technical components of the evolving 5G architecture [92]. By establishing the connection with each other directly, this type of communication can work in the absence of network operators or any central entity such as a base station [93]. Therefore, it is very useful for emergency situations where the cellular infrastructure is completely or partially dysfunctional. To help the build of emergency communication under such emergencies, many D2D applications have been proposed. One of the most important application scenarios is to help rescue missions by maintaining proximate communication among rescuers, communication between rescuers and victims, etc. [94][95][96]. The D2D technology can provide increased data rates and better QoS in emergency communication by building direct connections among users. In addition, the D2D communication can be used in a multi-hop manner to relay signals by connecting to other devices that reach operational base stations [97][98][99]. This sort of application can enable the extension of the network coverage or alternatively boost the existing network's capacity in critical conditions.
There is also a large likelihood of encountering partial or total network failure in building interiors due to fire damage or network congestion in urban tall building fire events. The lack of prompt on-site communication may cause the IC to make wrong commands and delay the escape of the building occupants. Furthermore, due to the increased reliance on wireless sensors or devices to facilitate various operations before or during the ER in the future, the collapse of the network in buildings may cause even more severe impacts to ER operations. Therefore, it is essential to maintain the robustness of the communication network in tall buildings. One possible approach is to use the network slicing technology to create additional slices for emergency communication. However, this approach is only helpful when the network infrastructure is intact. Fire incidents in tall buildings may damage the cellular infrastructure in buildings causing total network malfunction. The D2D communication successfully solves this issue by providing direct communications among users without going through any infrastructure nodes. Thus, it will surely play an important role in future building fire evacuation in tall buildings. Moreover, another application of D2D communication is to directly deliver the live information of the fire event and evacuation guidance to occupants' mobile devices. This allows the occupants to be aware of the fire event in a timely fashion and take prompt actions.

Discussion
The currently proposed IoT-aided building fire evacuation provided a possible approach for turning the building fire evacuation smart. By integrating the promising technologies of IoT, BIM, and 5G into fire emergency response and building evacuation, it is believed that a more effective and efficient building fire evacuation mode can be achieved, and more lives can be saved. However, to deploy such a system in practice, there may still be some difficulties. One of the important aspects is that its effectiveness and reliability may be a great concern to fire authorities. Even though 5G and other existing wireless communication networks provide a much simpler and flexible way to support various IoT use cases in built environments, there still needs to be more evidence to convince that the effectiveness and reliability of wireless sensors (e.g., fire detector, fire alarm) can be ensured and meet the requirements of current regulations of fire detection and fire alarm systems in terms of various fire situations. In addition, the interoperation of IoT devices between different vendors is also a problem. Currently, the IoT industry has no widely adopted data standards that IoT vendors can conform to, which makes it difficult to integrate the diversified sources of data in buildings into one platform. Therefore, in terms of the legislation control for IoT-aided building fire evacuation in the future, specific information needs, means of data transmission, and data interoperability should be stipulated by authorities. Furthermore, the stipulation of the construction of IoT-aided building fire evacuation systems should run through its whole life cycle, including the system design along with the building design in the design phase, implementation of the system in the building construction stage, and specific system operation and data management requirements in the operation phase.
In addition to acquiring the fire hazard information and occupants' situations on the fire-ground inside buildings, the currently proposed IoT-aided building fire evacuation also expects to employ the collected information to assist occupant evacuation with a dynamic adjustment of evacuation strategies via various modern information and communication technologies (ICT). Therefore, people's responses in such critical situations in connection to modern ICT or their adaptability to real-time evacuation guidance may play important roles in the successful operation of the system. Previous research conducted under an international survey and a full-scale evacuation trial [100,101] demonstrated that the active dynamic direction-changing signage systems indeed have their effectiveness in real evacuation scenarios. In addition, the effectiveness of public displays providing dynamic directions for crowd evacuation was also convinced in a large-scale football stadium scenario with simulations [102]. As for the usability of smartphones in providing dynamic evacuation strategy notification to evacuees, a study by Amores et al. [103] showed that people might have limited cognitive capacity when using mobile phones under stress. However, most of the experimental participants believed that the mobile phone is somewhat useful to their evacuations. From the previous research, it is believed that occupants can be adapted to the evacuation mode proposed by the IoT-aided building fire evacuation system, and the IoT-aided building fire evacuation has the potential to become a game-changer in future building fire emergency response and evacuation.

Concluding Remarks
From the tragedy of the Grenfell Tower fire, we can see room for improving the current emergency response and building fire evacuation, with the ongoing trend that all engineering fields are actively improving their efficiency and effectiveness with the aid of IoT-based systems. As one of the important aspects of building and fire safety, building fire evacuation should also not fall behind this progress. The IoT-aided building fire evacuation is a new concept of building evacuation mode which enhances the building fire evacuation, including the elevator evacuation process, by making the most appropriate evacuation strategies based on the real-time fire-ground information, such as fire environment and occupants' situations. With the maturity of IoT technologies and the appearance of more IoT application scenarios in the 5G era, the IoT-aided building fire evacuation will no longer be only a concept. It has great potential to be used in practice and play more important roles in future emergency response. Therefore, by focusing on IoT applications in building fire evacuation, this paper investigated the advantages and insufficiencies of current smart building fire evacuation systems. After that, a preliminary design of an IoT-aided building fire evacuation control system was proposed. The proposed IoT system was designed in the sequence of information needs, information sources and data transmission, and potential services and applications, which corresponds to the architecture of an IoT system. In addition, the advantages of the proposed system were also concluded. The proposed system could enable (1) monitoring of the fire environment and occupants' situations in buildings; (2) 3D-visualization of the building information; and (3) system-level-based evacuation strategy planning. With the help of the proposed IoT system, safe and more efficient building fire evacuation will surely be enabled.
The 5G era will be characterized as the age of boundless connectivity for all and intelligent automation, which enriches people's lives and improves the efficiency of all engineering fields. In building fire evacuation, 5G combined with IoT, BIM, and various data analytics technologies also presents vast amounts of possibilities and opportunities to overcome the challenges ahead of information perception, data management, and intelligent decision making. The wonderful things that 5G brings to building fire evacuation are not only allowing for more IoT use cases to transmit their data in a wireless approach, but also bring various communication services with different performance requirements, whether they are massive machine type communications, bandwidth-hungry applications (e.g., HD video streaming, AR applications), or critical emergency using scenarios, so that information needs related to indoor environments, states of various building facilities, and occupants' status can be systematically integrated and freely worked together in actual fire emergency scenes. However, to deploy such a system in practice, there may still be difficulties. Aspects include system reliability, legislation control, and acceptance of the public, which still need to be improved. Only in this way can IoT-aided building fire evacuation truly fulfill its functions in the 5G era.