## 1 Retrospective: The Development of Intelligent Ventilation Systems in Underground Mines
Generally speaking, ZigBee WSN technology is functionally similar to 5G technology, while the energy consumption and operating costs of ZigBee WSN technology are lower. Meanwhile, in the research of [PERSON] et al. [16, 17], ZigBee WSN technology is often considered an economical, efficient and applicable option compared to other WSN in underground mines. ZigBee WSN technology, they argued, could improve the workplace health and safety for workers, cost-effectiveness, management of technical issues, energy conservation and real-time response to events. Therefore, ZigBee WSN technology is more suitable for underground mines in the monitoring and control. However, in recent years, there has been a lot of research on underground communication. Around 2020, [PERSON] et al. [31] and [PERSON] [32] carried out the research on the advanced technology for the underground monitoring. Furthermore, around 2021, some experts [33, 34, 35, 36] have applied 5G technology, the GIS (Geographic Information System), the WAMC (Wide Area Monitoring and Control) system, the CBTC (Communication-based Train Control) system and other advanced technologies to underground mine monitoring. Although there has also been a lot of research on wireless sensor networks and other technologies for underground monitoring in recent years, we believe that the ZigBee WSN technology and sensors are currently the most suitable for the underground monitoring and intelligent ventilation systems in most underground mines. The reasons are explained as followed:  
* Although some advanced technologies can better solve the problem of mine fire monitoring, such advanced technologies are obviously not suitable for all mines, especially some backward mines in China. Furthermore, most of these advanced technologies have high operating costs, such as the operation of 5G technology, which requires the establishment of a large number of base stations. * On the contrary, the operation of ZigBee WSN technology in underground mines does not require complex conditions, and the operating costs and energy consumption of ZigBee WSN technology are lower compared to other communications technologies, according to the related research [37, 38]. * What's more, as a short-distance information transmission tool, ZigBee WSN technology is considered suitable for underground narrow environments. Both [PERSON] [16, 17] and [PERSON] [39] thought the ZigBee WSN technology was very suitable for underground monitoring [26], and [PERSON] and [PERSON] [40] revealed that most underground mines in China were equipped with ZigBee WSN technology as monitoring networks by 2015. As researchers and engineers, we should not only pursue the research of frontier technology, but also choose the appropriate underground monitoring system for the suitable mines. Therefore, we believe that at this stage, ZigBee WSN technology still has the superiority in underground mine monitoring, and we believe that ZigBee WSN technology still has the potential for deep development. Therefore, in our previous work, we have emphasized the superiority of ZigBee WSN technology in environmental monitoring of underground mines. Discussion: The Weakness and Future Work of the Application of ZigBee WSN Technology in the Underground Mine  
Although ZigBee WSN technology has been used well in many underground mines, it still has many shortcomings. In fact, the security of ZigBee protocol is a concern of many experts [41; 42; 43; 44]. As [PERSON], [PERSON] and [PERSON] [41] thought, although ZigBee WSN Technology has a lower power consumption, the ZigBee protocol has some security weaknesses for newly added devices, while [PERSON] et al. [42] argued that ZigBee, as a communications technology for building IoT systems, has weaknesses in its coverage and security. However, at present, relevant experts, including us, are also actively improving ZigBee WSN technology in order to overcome its shortcomings.