## 2 Related Work  
Industrial wireless technology is a revolutionary technology that can reduce the cost of industrial measurement, its control systems, and improve upon the application scope of the system. At present, three international standards have been formed in the field of industrial wireless technology, namely, the WirelessHART standard, the ISA100.11a standard, and the WIA standard. [PERSON] et al. first published a report on building the WirelessHART protocol stack [29]. In their research, the WirelessHART standard and its overall architecture was introduced, and several challenges that must be addressed in the prototype implementation were described and solved, including reliable mesh network, communication security, whole network synchronization, and timer design. So far, many studies have proposed improvement protocols for reliability and real-time assurance [45; 46; 47; 48; 49]. [PERSON] et al. summarized future research challenges of wireless sensor networks applied to industrial automation and pointed out that safety, reliability, and availability are the main issues to be solved [23]. Among them, safety is the most important feature that has been considered so far, while other important features include the interoperability, availability, and assurance of real-time performance, in which delay is an important parameter that requires specific attention. The end to end delay is larger because of the easy loss of data transmission in wireless sensor networks, multi hop transmission, and wireless conflict [50; 51; 52; 53; 54]. In order to reduce data transmission delay, researchers have put forward many effective methods [13; 14; 18; 24; 30; 31; 41]. These methods are summed up in the following categories in this paper. (1) The delay minimization at the network routing level. This method of delay optimization on the network routing level is one of the most common research areas for researchers [55; 56]. A variety of routing methods have been proposed. The shortest routing algorithm was first proposed. The principle of this method is to reduce delay. Because the delay in the arrival of the packet to the sink is the sum of the delay for each hop in the routing process, reducing the number of hops on the routing path can reduce the end to end delay accordingly. The most effective way to reduce the number of hops in the routing path is to choose the shortest path from source node to sink for data transmission. At this time, the number of hops is the least and so the delay is also the minimum [55]. However, choosing the shortest route does not always reduce the delay. In some cases, avoiding congested areas will increase the route length, but can oftentimes reduce the delay. In some cases, the transmission path will often bypass energy consumption hotspots to avoid energy consumption of nodes in this area that are too fast to die [56]. Thus, the method of reducing the length of the transmission path to reduce the delay is limited by some restrictions. And in many cases, the effect of the path length on the delay is far less than the other factors. Therefore, some quality of service (QoS) based routing methods are then proposed [42]. These QoS methods mainly include data transmission reliability, security, transmission rate, and many other indicators. Delay is only one of a variety of QoS indicators. Because multiple QoS indicators are interrelated and interact with each other, the next hop selection of routing becomes a multi-objective optimization problem, which makes routing more complex. For example, the reliability of data transmission refers to the probability that a packet is transmitted to the sink successfully [14; 36]. Obviously, if the high reliability is guaranteed, more data retransmission may be needed, thus increasing the delay [41]. In the multipath routing strategy, which guarantees data packets arriving to the sink safely, this strategy copies data packets into multiple copies, and each copy is transmitted to the sink through different paths.