## 6 Simulation Evaluation
The number of objects that lose the query messages increases significantly as the total number of object nodes increases. The largest number of lost  
object nodes is up to 9 and 6 depending on the broadcast window size. On the other hand, the proposed approach allows only few query losses at object nodes and maintains a low number of failed object nodes as the total number of object nodes increases, leading to good scalability in network performance. When the total number of object nodes increases to 100, the number of failed objects in the proposed approach is only about 11 % and 17 % to that of the conventional approach. This is because the proposed approach utilizes local detection and recovery, and is only little affected by the network size. A small time window improves the broadcast speed and decreases the delay and energy consumption, but may cause more message losses at object nodes. In the evaluation of the proposed approach and related conventional approaches, the sink's position affects the performance in the recovery of query losses. We set up two types of sink node positions: defined position and random position. As for the defined position of the sink node, there are two kinds of setup corresponding to two positions, which are in the top-left corner and on the middle top edge. As for the random sink position, the position of the sink node is selected randomly in the network area in each query. The operation of the proposed approach utilizes the local recovery until all detectors fail the recovery. A sink based recovery will be initiated if all detectors fail the recovery. Figure 10 shows the results of recovery route length with predefined positions of the sink node. The route length to sink-based recovery increases as the number of nodes increases. On the other hand, the route length of the proposed approach shows hardly any increase even when the number of nodes becomes large. Although the location of the sink node varies, there are only minor differences between the route lengths. The difference is because the sink position affects the length of route between the object and sink. The route length of the proposed approach varies little regardless of the positions of  
Figure 9: The average number of object nodes that have query losses with broadcast windows of 5 and 10 time units. The plots illustrate the scalability performance of successful query with regard to the number of object nodes. the sink node. The results reveal that the proposed approach achieves good scalability with regard to the network size, and the positions of the sink node have little impact on the performance of recovery. We further examine the impact of the number of detectors on the successful recoveries in a shared wireless communication media in which each detector contends for media access in a random manner. Contention-based media access has been widely used in wireless networks due to its simplicity, easy setup, and low requirements especially on small sensor nodes [21, 22]. Two types of media contention windows are set up, _i.e._, 15 time units and 3 time units. As shown in Figure 11, the successful recoveries increase as the number of detectors increases. The result of successful recoveries 1 corresponds to the larger contention window of media access and the result of successful recoveries 2 corresponds to the smaller contention window. The result of successful recoveries 1 reveals that the larger contention window of media access results in and maintains a higher successful recovery rate when the number of detectors is larger than 3. On the other hand, the result of successful recoveries 2 reveals that the network with smaller contention window has a large successful recovery rate only when the detector number is between 3 and 9, and there is a sharp decrease of successful recovery rate when the detector number is larger than 9. This is because the smaller contention window causes more packet conflicts among detectors, leading to packet losses in the recovery operation especially when there are a large number of detectors.