# An Untraceable Data Sharing Scheme in Wireless Sensor Networks
## 6 Performance Evaluation
### Simulation  
We compared our scheme with the shortest path routing algorithm, phantom routing algorithm and the cloud-based scheme in terms of privacy protection and energy-efficiency. In the simulation, 4000 sensor nodes were randomly scattered in a 3500 m \(\times\) 3500 m square region. The communication range, \(R_{c}\), of the sensor nodes was set as 50 m and we set the parameter \(\rho=2\). The sink node was located in the center of the area. Initially, the location of the panda was randomly selected in the network and it slowly moved in the network with a speed 1 m/s. The wearable devices collected the information of the panda every 1 s and then the data were transmitted to the sink node. Moreover, we assumed that the packages in the cloud can be transmitted for 8 steps.  
We employed the probability that the adversary can locate the source node in the simulation period to estimate our scheme. The probability was calculated by the number of times that the adversary can locate the source node to the total time of runs. As shown in Figure 8, with the increasing of the hunters' number, the source-location detection probability increases for all the four schemes. The shortest path routing algorithm cannot protect the source location properly. Phantom routing algorithm performs better because of the random walk phase. However, the adversary can easily locate the source node by employing several hunters together. Though the cloud-based scheme can defend against the adversary if the number of hunters is small, it cannot defend the smart adversary model if the number of hunters is large. Our scheme performs the best because it is extremely hard for the adversary to find the cloud let alone the source node.  
In Section 6.1, we theoretically analyzed the data transmission amount of our scheme. Though most energy of the network was consumed in delivering the data, some extra energy was consumed in processing these data. The total energy consumption of the network is presented in Figure 9. It can be observed that the average energy consumption of our scheme is slightly larger than that of the shortest path routing algorithm and phantom algorithm, and it is much more energy-efficient compared with the cloud-based scheme.  
Figure 7: Total data transmission amount with different parameters.  
Our scheme can protect the source-location-privacy in an energy-efficient manner and this is the main effect of our scheme. An interesting side-effect of our scheme is that the success rate of message delivery increases. This can be explained by the fact that we integrated the secret sharing scheme into our scheme. The simulation result is presented in Figure 10.  
It can be observed that with the expending of the network's size, the success rates of all the schemes decrease. In a larger network, the packages need to be relayed by more steps and hence the rate of package drop increases. The shortest path routing algorithm has the best performance because the packages can be delivered to the sink node with the least hops. The phantom routing algorithm also has a similar performance. Considering some extra walk steps in the random walk phase, the successful rate of the phantom routing algorithm is lower than that of the shortest routing algorithm. The cloud-based scheme has the worst performance. This can be explained by the fact that  
Figure 8: The probability of successfully locating the source node.  
Figure 9: Energy consumption with different sizes of the cloud.  
the packages walk in the cloud for a long distance and hence the hops greatly increase. Though the packages in our scheme also walk in the cloud, the secret sharing scheme improves the performance of our scheme. The sink node can recover the original message based on a set of shares rather than all the shares and this improves the tolerance of node failure.