# A Cross-Layer Optimized Opportunistic Routing Scheme for Loss-and-Delay Sensitive WSNs
## 2 Related Work
### Delay Sensitive Works in WSN  
In contrast, ensuring reliable data transmission is the main research content, and delay mainly depends on the reliability guarantee mechanism. Some delay-sensitive works of wireless sensor network routing layer are expressed as follows:  
(1) Coding-based reliability guarantee mechanism [53; 54]. According to the mechanism of redundancy encoding, the data packet is encoded before being sent. With the help of redundant code, a receiver can still decode the received data packet to get the correct information, even if there losses in wireless communication due to some errors.  
Reed-Solomon coding is an effective redundancy method [53; 54]. There have been some studies on the application of Reed-Solomon coding to reliable data transmission in wireless sensor networks. In [53], a Reed-Solomon codec algorithm for WSNs is proposed to reduce energy consumption. In [54] packets are encoded the by Reed-Solomon code and routed to the sink node along multiple paths. Network coding is regard as an effective data transmission scheme. [PERSON] and others [55] first proposed the concept of network coding in 2000. The basic idea is that the network nodes can encode multiple packets from different links (for example, through XOR or linear coding, etc.) into one packet, and then send it out. As a result, the amount of information in a single transmission is increased exponentially.  
The advantage of reliable data transmission based on coding techniques is that sender can transmit data to receiver at a high probability of success with only one attempt. Therefore, it has a small delay for saving the cost caused by multiple retransmission. The disadvantage, however, is that the coding scheme requires the data packets append extra information for correction, but the length of the additional encoded data is not negligible. As a consequence, the amount of data is increased, and the nodes need to encode and decode, both of which require a certain processing time and will increase the load of the nodes. Simultaneously, the communication capability of the network is actually reduced as the probability of data transmission conflict is increased, and the network lifetime is reduced.  
(2) Data fusion can also be considered as a coding mechanism. For example, in the application of querying the average value, the maximum value, and the minimum value in the network, multiple data packets pass through one node. After the calculation (equivalent to encoding) of all received packets, a node will fuse this information into one data packet and continue routing. Hence, this method can effectively reduce the amount of data and the transmission conflicts, and prolong the network life, so it is widely used in all kinds of applications [56]. Certainly, in wireless sensor networks data fusion is most often used when multiple data packets can be gathered and generalized to be smaller. Related studies can be found in [56].  
Therefore, the general research on reliable data transmission will also analyze the delay of the method adopted. In the same way as previous researches, this paper first proposes a data reliability transmission scheme, and then analyzes the delay performance. Similarly, this paper only considers the transmission reliability and delay [57] caused by the quality of the communication link, but irrespective of the impact of network security problems [58; 59; 60].