## 2 Related studies  
There has been little research on the design of a reliable transfer protocol for wireless sensor networks. Although there has been considerable research into reliable transport in wireless networks, such as the Ad-Hoc network, the current approaches of reliable transport protocols are regarded as being suitable for a sensor network, especially for images. PSFQ, MintRoute, and RMST are known reliable transport protocols for wireless sensor networks. These protocols are based on NACK and utilize ARQ for reliable data transport. The cost metric in MintRoute consists of a hop count to the sink node and link quality. The main purpose of the cost metric is to maximize data delivery by choosing the shortest and most reliable path to the sink node. Hop count increases reliability by reducing the number of nodes toward the sink node. Link quality also increases reliability by favoring more reliable links. These protocols can prevent packet loss caused by collision or error during node-to-node transfer, but packet drops due to congestion inside the node can still take place. Even when packets are received properly, the packet will be dropped if the sending queue is full. Error protection in image transport is well understood in conventional networks such as the Internet. Several methods have been proposed to improve the error robustness of packet transfer. Forward error correction (FEC) allows for recovery from errors by incorporating controlled redundant data [18-22].  
Multipath transport has been studied in the past, in both wired and wireless networks. It has mainly been used to increase the aggregate capacity and improve the load balancing and fault-tolerance. Recently, a number of interesting proposals regarding the delivery of image and video over wireless networks using multiple paths have been introduced. The problem of allocating packets to multiple paths has been investigated in order to minimize power consumption and end-to-end image distortion. However, it requires continuous monitoring of path quality at each hop and the reporting of this information to the source node, which makes it difficult to utilize in sensor networks. It is necessary to discuss why multipath transport schemes are not suitable for wireless sensor networks. First, as presented in previous studies, multipath transport is mainly used to combat wireless link errors through path diversity. Network congestion, which may be common in sensor networks, is not considered. Second, multipath transport schemes, which incorporate end-to-end error control, typically split data at the source and combine data from different paths at the destination. However, the intermediate nodes are unaware of the errors inside packets. Thus, the errors accumulate as the packet travels toward its destination (_i.e.,_ error propagation). Above all, sending the same packets from an image through multiple paths increases energy consumption in proportion to the number of paths; a very limited critical resource in wireless sensor networks [23-27].  
Obviously, the use of FEC coding alone cannot also address the problem of network congestion in which the corrected packet is dropped. Furthermore, FEC requires redundant data to be transferred as an error check and correction, which increases the number of packets required, given the large number of packets that images require [28,29].