## 2 Related Work
So, even if some transmission paths are attacked, there is still a path to successfully reach the sink, thus ensuring the security of the network. In such a routing strategy, delay is the time required to arrive at the sink first. Therefore, in this case, the delay can be effectively reduced. Therefore this way of routing is often used to improve the reliability of data transmission [56; 57; 58]. (2) The effect of the communication link quality and data transmission mechanism on delay [14; 36]. Because of the unreliability of network transmission, data transmission is often not successful for the first time, and some reliability systems need to be adopted to ensure the reliability of data transmission. These reliable data transmission mechanisms have an important impact on the delay. The simplest data transmission mechanism is the send-wait mechanism. Under this mechanism, the sender sends data packets and waits for the receiver to return the ACK message to the sender. Once the sender receives the ACK, it indicates the receiver has successfully received the packet, then the sender starts sending the next packet [41]. Otherwise, the sender waits for a longer timeout time and resends the current packet. The above process goes on until the packet is sent successfully, or the number of retransmission times exceeds the predetermined threshold and then the transmission of this packet will be abandoned. It can be seen that in this way, the time of a packet retransmission is greater than round trip time (RTT). Therefore, in a network with poor communication link quality, its delay is often several times the delay in the ideal communication link [41]. The reason why the delay of a simple send-wait protocol is large is that the sender sends a packet and must wait for the receiver to return the confirmation information. If the packet is a loss, the sender needs to wait for a longer timeout time, which results in greater delay. The main idea of the related improved method is to send multiple data packets continuously while the receiver also continuously returns the confirmation information, then the sender reissues the packets that did not have confirmation information [41]. It can be said that this method uses a parallel working mechanism to reduce the delay. The protocols for this kind of improvement are: Go back-N (GBN) protocol and Selective repeat (SR) [41] algorithm. (3) The effect of transmission power on delay [14; 36]. It can be seen from the above that if the quality of the communication link is not good, in order to ensure a certain reliability of data transmission, a large maximum retransmission number of data must be set. However, in this case, not only is the delay very large, the energy consumption is larger. Therefore, improving the quality of communication link can effectively reduce delay and so on. The way to improve the communication quality is to improve the signal-to-noise ratio of the communication link. The higher the signal-to-noise ratio of the communication link is, the higher the probability of the correct data reception is, the smaller the number of data retransmission is, and the smaller the delay is. The signal-to-noise ratio of the communication link can be improved by increasing the transmission power of the sender, which can effectively reduce the delay. However, this method improves the energy consumption of data transmission, so researchers have provided a method to determine the transmission power of the optimized sender [14; 36]. (4) The reduction of delay based on broadcast based routing. The common routing method is the way that a sender corresponds to a receiver. Thus, in this way, if the receiver does not receive the packets correctly, it is necessary to carry out the retransmission work described above, which leads to a larger delay. The opportunity routing method described below can effectively alter this situation [36]. Opportunistic routing [59] is based on the following method to reduce the delay on the basis of ensuring the reliability of data transmission.