## 4 Performance
### Variation of Number of Sending Nodes  
#### 4.2.1 Reception ratio  
The probability of network traffic congestion can increase in proportion to the number of sending nodes in a network. Each added increment of network congestion would increase the probability of packet loss, which would lead to the loss of images. As a result, the number of sending nodes can have an influence on the received images. In this simulation, the number of image-sending nodes was set at 1, 2, 4, 8 and 20. The sending queue size was fixed at 32 and the sensor nodes were synchronized within 40 s. With this setup, we evaluated the total packet reception ratio and total image packet reception ratio.  
Figure 10 shows the number and ratio of received packets and images with variations in the number of sending nodes. In Figure 10 (A), bar graphs show the number of received packets with variations in the number of image-sending nodes, and the line graphs show the ratio computed by dividing the number of total received packets by the number of total sent packets. When only one sensor node sent images, MintRoute showed a 100% ratio of packet reception, but the packet reception ratio decreased even with two sensor nodes in MintRoute and SRI. As the number of sending nodes was increased, the packet reception ratio in MintRoute and SRI decreased drastically. When 20 sensor nodes were sending images, about 3,000 packets were lost, which was 33% of the sent packets. This result demonstrates that with MintRoute and SRI, many packets were lost due to network congestion caused by images. On the other hand, with RAIT, all sent packets were received by a sink node, showing a 100% reception ratio, even though the number of sending nodes was increased.  
Figure 10 (B) shows this difference in performance even more distinctly. When we classified images with one or more lost packets, the image reception ratio in MintRoute decreased more drastically. Although more than 90% of the sent packets were received by a sink node, the image reception ratios were about 85% with MintRoute. Given 20 sending nodes, only 70 out of 200 images were received (35%), even though about 67% of the packets were successfully received. 30% of the packets were lost during transmission and even worse, 30% of the packets were dropped at the sink node. Lost or dropped packets are directly related to the wastage of energy in sensor nodes because packet transmission consumes a great amount of energy, which is a very limited resource. With MintRoute, the reliability of image transmission was not only guaranteed, the energy efficiency was also decreased. On the other hand, RAIT and SRI showed a 100% image reception ratio regardless of the number of sending nodes. With SRI, the image reception ratio was 100% even if the packet reception ratio was decreased. An image that was transformed with wavelet can be reconstructed by wavelet transform with lower resolution even if some packets were lost. Therefore, the image reception ratio is high even if the packet reception ratio is low. With RAIT, the image source nodes or relaying nodes could exclusively possess the parent nodes during transmission of all the packets of an image, so that there could be no network congestion, ensuring that all sent images were received.  
Figure 10: Number of received packets (A) and number of received images (B) with variations in number of sending nodes. RAIT collected whole packets of transferred images while the reception ratio with MintRoute and SRI was decreased in an inverse proportion to the number of sending nodes.