# Building Topic-Driven Virtual IoTs in a Multiple IoTs Scenario
## 6 Experiments
### Our Approaches' Capability of Improving the Efficiency of Information Dissemination  
This experiment was devoted to measure the efficiency of both supervised and unsupervised approaches. The rationale underlying this experiment is that if some information must be transferred from a source object \(o_{s}\) to a target one \(o_{t}\), the number of objects to be contacted for this task should be minimized. At the same time, if an object is involved in an information dissemination task, it would be desiderable that the information it is transmitting is also useful for it (which, in our case, means that it is in line with the interests of its profile). In order to perform this experiment, we randomly selected some pairs of (source, target) nodes from our MIoT. Let \((n_{s},n_{t})\) be one of these pairs. We verified if there existed at least one virtual IoT comprising both \(n_{s}\) and \(n_{t}\) (This is always true for the unsupervised approach, whereas it could not happen for the supervised one.). In the negative case, we discarded that pair. Let \(\mathcal{V}\) be a virtual IoT comprising both \(n_{s}\) and \(n_{t}\). After this, we computed the number \(num_{st}^{\mathcal{V}}\) (resp., \(\widehat{num_{st}^{\mathcal{V}}}\)) of MIoT nodes involved in the dissemination of information in presence (resp., absence) of the virtual IoT \(\mathcal{V}\). Specifically, we computed \(num_{st}^{\mathcal{V}}\) by performing the information dissemination task only through its nodes; instead, we obtained \(\widehat{num_{st}^{\mathcal{V}}}\) by performing the same task on the whole MIoT. Finally, we computed: \(f_{st}=\frac{num_{st}^{\mathcal{V}}}{num_{st}^{\mathcal{V}}}\). Clearly, the lower \(f_{st}\), the higher the contribution of the virtual IoTs in reducing the number of nodes necessary for the information dissemination task and, consequently, the higher the contribution that our virtual IoT detection approach can provide to information dissemination. Figure 3: Computation time (in seconds) against the size of MIoTs (unsupervised approach)  
We computed the average values of \(f_{st}\) by operating on the ten MIoTs introduced in Section 6.2 and by performing the same tasks described therein for both the supervised and the unsupervised approaches. The obtained results are reported in Tables 10 and 11. From the analysis of these tables we can observe that both the supervised and the unsupervised approaches really contribute to decrease the number of the nodes of a MIoT involved in the information dissemination, and, therefore, to increase the efficiency of this task. As for the supervised approach, we observe that the decrease of the number of involved nodes is always high. It becomes very high as the MIoT size and the number of keywords composing the query increase. As for the unsupervised approach, we observe that it leads to a decrease of the number of the MIoT nodes involved in the dissemination task. However, this decrease is minimum for small MIoTs, whereas it becomes significant for large ones (i.e., for MIoTs with a number of nodes higher than 1256). We performed a second experiment in this direction. Specifically, given a pair \((n_{s},n_{t})\) of a MIoT such that information must be disseminated from \(n_{s}\) to \(n_{t}\) and there exists at least one virtual IoT \(\mathcal{V}\) comprising both \(n_{s}\) and \(n_{t}\), we computed the fraction \(g_{st}^{\mathcal{V}}\) (resp., \(\widehat{g_{st}^{\mathcal{V}}}\)) of the nodes of the MIoT involved in the diffusion of information from \(n_{s}\) to \(n_{t}\) and having at least one content of the disseminated information registered in their profile (which implies that, in principle, they could benefit from the information they are required to disseminate). As in the previous experiment, we computed \(g_{st}^{\mathcal{V}}\) by assuming the existence of \(\mathcal{V}\) and, hence, by performing the information dissemination task through it; by contrast, we computed \(\widehat{g_{st}^{\mathcal{V}}}\) by carrying out the information dissemination task through the whole MIoT.