# Energy-Aware Control of Error Correction Rate for Solar-Powered Wireless Sensor Networks
## 2 Related Work
### ARQ and FEC in WSN Environments  
The sensor nodes comprising WSNs have limited resources. Moreover, many nodes share a frequency bandwidth with other nodes. In terms of inter-node communication under the ZigBee protocol (IEEE 802.15.4), the resistance to noise is weaker than in heterogeneous wireless communication devices. For this reason, errors are highly likely to occur during inter-node communications in a WSN [2; 3]. Moreover, as a WSN continually enlarges, inter-node interruptions and the probability of errors also increase, thereby reducing the entire throughput of the WSN. In this regard, the data-link layer, which ensures reliability, is crucial for inter-node communication in WSNs. Among many protocols for the data-link layer, the ARQ and FEC methods are primarily employed for inter-node communication.  
In the case of successful data transmission, the ARQ method provides information through ACK that the data were successfully received, and it then requests subsequent data. However, when errors or data losses occur during data transmission, it requests data transmission again through a negative acknowledgment (NACK). Then, the data that failed in the original transmission are retransmitted instead of subsequent data. In contrast, the FEC method generates parity by encoding the data using an appropriate coding scheme (e.g., RS code) and transmits it along with the data. When errors occur, the broken data can be restored by decoding the parity using the same coding scheme.  
The graph in Figure 1 compares the total amount of data that should be transferred in a network in order to enable the sink node to gather all 80 KB of sensory data from each leaf node. This is performed through both the ARQ and FEC methods [14]. A balanced binary tree topology with eight leaf nodes was adopted in this experiment. Each leaf node contained 10 KB of sensory data. In Figure 1, the \(x\)-axis represents the spacing between nodes and the \(y\)-axis refers to the total amount of data generated (including retransmission data) in the entire WSN in order to reach 80 KB of sink-collection data. The spacing represents the physical distance between nodes, but it can also be thought of as an error rate, because the greater the distance, the higher the error rate. The results verify that the difference is insignificant when the spacing value ranges from one to four. A significant difference is evident when the spacing value is five or higher. Furthermore, as the spacing value increases, the values in the ARQ method increase in the form of an exponential graph, whereas those of the FEC method increase in the form of a linear graph. When the spacing value is six, the two methods exhibit a difference of more than twice that. Note that the increased amount of generated data has a negative influence on not only the energy consumption, as it uses much energy for data transmission, but also on the network performance, such as the throughput, because there is a higher chance of channels becoming occupied and congested. The results in Figure 1 confirm that the FEC method is significantly more efficient than the ARQ method for WSN environments in which errors are prone to occur. Consequently, most WSNs choose the FEC method as their link-layer protocol.