# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 2 Related Work  
### Loss Sensitive Works in WSN  
This section focuses on the reliability and delay of data transmission in wireless sensor networks [11; 29; 35; 37; 47]. Due to interference, low power transmission, conflicts and other factors, wireless communication is error-prone and unreliable [50; 51]. Aiming at maintaining the reliability of communication, there are many studies on this topic, which mainly concentrate on the network routing layer, so we first introduce the transmission reliability guarantee mechanism of the wireless sensor network routing layer:  
(1) An opportunistic routing scheme [49] serves as a basis for this paper. According to previous research [47], one-hop transmission reliability cannot be expected in wireless sensor networks. Sometimes the packet error rates (PER) of the communication links between two sensor nodes are more than 10% [47]. In this case, without adopted of a PER 10% (i.e., reliability = 90%) reliability transmission mechanism in the network, the probability of successfully reaching a sink at 10 hops is only 34.86%. Worse still, if the route hops are greater, the receiving success rate will be too low to meet the requirements of the application. Therefore, a reliable one-hop transmission is indispensable in the improvement of the end-to-end reliability. As for opportunistic routing, it improves the reliability of data transmission by the following methods [49]: first, the sender determines a candidate set which comes from the Forwarding Nodes Set (FNS). The forwarding node set is a collection of nodes that are within the sender's transmission distance, but closer to the sink than the sender. The candidate set consists of those nodes in the FNS that are close to the sink and have high reliability. The sender transmits information forward by broadcasting to all nodes in its candidate set. The transmission succeeds as long as any node in the candidate set receives the message correctly. If there is more than one node accomplishing this, the most favorable node to transmit to the sink will continue routing, while the others will simply drop their received messages. At the same time, in the transmission process, the data only needs to be sent once, and received by multiple nodes. Compared with unicast transmission methods, in which many receiving nodes imply multiple sending times, the energy consumption of opportunistic routing is rather economical. In addition, its delay is relatively small, mainly because it takes less time to transfer data, though, determining which is the most favorable node to continue forward routing is requires communication between candidate nodes, and this will increase the delay slightly. (2) Retransmission mechanism, one of the most widely used reliability assurance mechanisms [47], is characterized by simple requirements and wide application. Automatic Repeat-reQuest (ARQ) is the most basic protocol of this mechanism, and its principle is as follows: when a sender node finishes sending a packet, it starts a timer and waits for the receiver to confirm the information. Once the packet is received correctly, the receiver returns an ACK message. Then, if the sender node receives the ACK message in time, it will consider that the previous data packet has been received successfully, so the next packet can be sent, but if the sender has still not received the ACK message after waiting beyond the scheduled time, the packet is considered to have been lost and needs to be sent again. The retransmission will be repeated until the sender receives the ACK message or reaches a threshold time. At that point, the transmission of the current packet is considered to be aborted, and the next packet is sent instead. Thus, the ARQ protocol can effectively maintain reliable data transmissions without high energy consumption. In detail, the additional energy consumption only comes from ACK messages, which cost is rather limited due to its small length. However, there is a drawback of high delay.