## 2 Related Work
### Research on Delay Optimization  
Generally, the delay in WSNs refers to the difference between the time when the data is generated and the time data reaches the sink. There is various research which aims to reduce delay. Based on the layer the optimization methods work in, the research can be categorized to the optimization methods in the MAC layer, network layer, transport layer and application layer. Moreover, cross-layer optimization methods have been studied and proposed. In the methods which work in the MAC layer, a critical performance indicator which is related to delay is data reception ratio [16]. In WSNs, the distance the data can be transmitted and the reception ratio is positively related to the node's transmission power. It is a feasible method to optimize delay by optimizing the transmission power. In [16], the relationship between transmission power and data reception ratio is shown. There is quite a bit of research on optimizing delay in the network layer. For example, in the shortest routing scheme, each node chooses the node closest node to the target as the next hop each time. It is a delay-efficient routing algorithm. However, in some WSNs, it may cause excessive energy consumption to some key nodes on the shortest routing path. The delay and energy consumption should be taken into account in routing algorithms to balance the energy and improve the overall network performance. This comprehensive performance optimization strategy can be seen in [20; 36; 38; 39; 57]. Besides, network attacks exist in wireless networks, delay routing optimizations including security factors become more complicated. To prevent the attacks, the data packets are transmitted on multiple routing paths to the sink. In this policy, the sink may receive multiple identical data packets [36]. Although the energy consumption in the strategy is more than that of single data packet routing strategy, the delay can be reduced as the delay is the time difference which is determined by the first data packet reach the sink. In Ref. [63], SDER scheme is proposed. Each data packet splits into \(m\) slices, all slices are routed along the different paths. A data packet can be restored by \(n\) slices. It's difficult for attackers to catch \(n\) slices, so the security can be improved. This is a compromise between energy consumption and security. Compared to multiple data packets transmitted to the sink, this scheme consumes less energy and performs better regarding security. However, there is no advantage for the delay as \(n\) slices received by the sink is considered as success transmission. In duty cycle based WSNs, nodes periodically switch between the sleep state and the awake state. When the sender has packets to send, the node closest to the sink in the forwarding node set may sleep and cannot receive packets. The senders has two options, waiting for that closest node or selecting the closet awake node. Which option causes more delay can't be judged. A dynamic routing algorithm is proposed in [64] to optimize the delay in duty cycle WSNs. Additionally, an optimization method is proposed in [39]. The main idea is that, the sender sends data to multiple receivers. Data received by a receiver is regarded as the successful transmission. This method is able to decrease the probability of retransmission, so delay decreases. Because the reliability of data transmission in the wireless transmission environment is much lower than that of the wired network, the strategy to guarantee the transmission reliability of data also has excellent optimization effect on delay. The most commonly used is the send-wait retransmission mechanism. In this mechanism, after sender sends a data packet, it waits for the receiver to reply an ACK confirming the reception of the data packet. The receiver is considered not to receive the packet when the sender does not receive the ACK after a stipulated period. If so, the sender resends the packet. The process of sending a packet continues until the sender receives the ACK, or the number of