# Querying on Federated Sensor Networks
## 4 Results
### Evaluation of L-FSN According to the Island Size  
For this experiment, we applied protocols described in Section 3.3 as intra-island querying protocols of L-FSN. We sent a single query from a random node within all islands in order to measure consumed energy and query resolving latency. When we applied the same querying protocol into each island as an intra-island querying protocol of L-FSN, we observed that GHT protocol is the most energy-efficient protocol, and query resolving time is the lowest by the DQT protocol. Since we assume that gateways have enough storage capacity, we do not run the structured replication of GHT, which is a technique that distributes the data storage load, hierarchically. Without the structured replication, we categorize GHT as an _infrastructure-free_ querying protocol in which data and query packets are following a geographical path to the hash location, whereas DQT protocol is an _infrastructure-based_ protocol due to the construction of a quad-tree structure.  
As a second step of this experiment, we first ran L-FSN on the UB topology while running GHT protocol as the intra-island querying protocol of L-FSN for all islands, and, recorded the average consumed energy per node and the average query resolving time. Then, we repeated this  
Figure 6: The data delivery latency.  
experiment each time running DQT as an intra-island querying protocol on several islands depending on island sizes.  
Figure 7 demonstrates the percentages of additional energy consumption per node and the decrease on query resolving time at each DQT-GHT combination as intra-island querying protocols. In this figure, a data point label DQT n:m represents the DQT-GHT combination, where DQT protocol is running on islands of size n to m while GHT protocol is running on the rest of the islands. For example, data point DQT 4:9 represents that DQT protocol is running on islands of size 4, 6 and 9 while GHT protocol is running on the rest of islands in the UB topology. The size of islands and number of islands for each island size in the UB topology are shown in Table 2.  
We observed that if we ran DQT on all islands, we had 29.04% additional energy consumption per node with a 95.53% decrease on query resolving time compared to the case that GHT protocol was running on all islands, as shown with the DQT 1:9 point in Figure 7. While the rest of the islands are running GHT protocol, statistical variance of additional energy consumption per node as we increase the number of islands running DQT protocol is 0.91%, and statistical variance of decrease on query resolving time as we increase the number of islands running DQT protocol is 10.34%.  
Due to the tradeoff between energy-efficiency and query resolving time, we can select running DQT on some islands as an L-FSN intra-island querying protocol depending on island size. We observed that a high gain on query resolving time with a low additional energy consumption is achieved when we ran DQT protocol on large islands and GHT protocol on small islands, as shown with DQT 9:9, DQT 6:9 and DQT 4:9 points in Figure 7. If we run DQT on islands of size four and more, we had a low additional energy consumption \"1.46%\" with a high decrease on query resolving time \"37.40%\", as shown with DQT 4:9 point. Therefore, we suggest that, as intra-island querying protocol of L-FSN, _infrastructure-free_ protocols can be selected on small islands for conserving energy while _infrastructure-based_ protocols can be selected on large islands to gain benefit on query resolving time.