## 4 Performance  
### Evaluation Environment  
To evaluate the performance of the proposed RAIT, we simulated MintRoute [13], SRI [23] and RAIT together in TOSSIM, which is a simulator for TinyOS. TinyOS is an operating system developed for event-based sensor networks at UC Berkeley. TOSSIM provides a simulation environment that simulates a real sensor network with TinyOS. Because an application running on TOSSIM can be run on real sensor nodes such as a micaz or a telos, it can be said that our implemented simulation truly reflects the real world. In the simulation, 144 sensor nodes with 20 image sensor nodes were set to send a total of 10 frames of an image with a size of 54 packets. Each sensor node had a receiving queue with a size of 1 packet and a sending queue with a size of 32 packets [30].  
Within TinyOS, MintRoute is the standard routing protocol. MintRoute is a proactive routing protocol in which sensor nodes send routing messages periodically to notify their local states. MintRoute does the best effort in transmitting packets by selecting a parent node with highest link quality. However, MintRoute does not guarantee the transmission in case of congestion. SRI is an image transmission protocol based on the wavelet transform. SRI assigns the priority to the packets based on the wavelet stage and drops packets with lower priority packets first in case of the congestion. With SRI, an image can be reconstructed with lower resolution even if some packets of the image are lost in the network.  
We selected three performance evaluation matrices: total reception ratio of image, total reception number of packets, and average transmission duration of an image. \"Total reception ratio of image\" means how many images out of 10 images sent by sensor nodes arrive at a sink node. If one or more packets were lost out of 54 packets for an image, that image was regarded as lost. \"Total reception amount of packets\" means the total number of packets received by the sink node. \"Average transmission duration of an image\" means the average time it takes for all the packets of an image to be transmitted from the source sensor node to the sink node.  
We varied the number of image-sending nodes from 1 to 20 and the time synchronization range from 0 to 40 s. If there is only one sending sensor node then there is no competition or congestion, and all the packets of an image will arrive at a sink node. On the other hand, if two or more sensor nodes try to send images, there will be competition and congestion, and as a consequence, some sent packets will be lost. The more sensor nodes there are trying to send images, the more packets will be lost. In addition, we also changed the time synchronization range among the sensor nodes. Time synchronization refers to the relative difference in the clocks among sensor nodes in a sensor network. If the clocks of the sensor nodes are synchronized very precisely, every sensor node tries to send an image simultaneously. In the simulation, 0 s synchronization meant that the sensor nodes were perfectly synchronized. As the synchronization range was increased, the sensor nodes' transmission of an image became more random. We varied the time synchronization range to see the effects of competition and congestion. The more sensor nodes there were trying to simultaneously send images, the more competition and congestion took place and thus the more packets that were lost during transmission.