## 5 Experimental Section
### Overall Performance  
In Table 3, row 1 lists the average number of messages of meta-data exchange, row 2 lists the number of messages that upload data to \(MC\), row 3 lists the number of total messages per epoch, row 4 lists the _data coverage_, and row 5 lists the _efficiency_ of the algorithms. The results presented at each cell is the average of 6 rounds of simulations, given the default parameters such as number of nodes, the data gathering period, and etc. At the simulation all the nodes are mobile, and they take advantage of the opportunistic communication chances for the data gathering. So each node is randomly assigned to a home community at each simulation and has its own paths, where the paths are different from one simulation to another. For the overhead of message transmissions, all schemes except Epidemic incurs the cost of meta-data exchange and gathering. The number of messages for meta-data exchange is low, and it accounts for less than 15% of the overall message transmissions. Epidemic has no overhead of meta data exchange, yet it has the largest message transmission, as high as about 4.52E+5. This is because messages are exchanged among all the ordinary nodes; while at other schemes messages are selectively forwarded. At PROPHET, messages are forwarded to nodes who have larger probability to meet the \(MC\), and at other schemes messages are only forwarded to the proxy nodes, saving lots of in-network message transmissions. So the overall data transmissions of PDA, PROXY and ADG are relatively small, which is more than 40% less than that of Epidemic. The amount of data upload messages is directly related to the data coverage. ADG has the largest data coverage, so the number of messages that upload the data to \(MC\) is the largest. \(MC\) receives about 3.99E+4 messages of sensed data from other nodes. _Data coverage_ is defined as the amount of collected data divided by that of the total sensed data. From the table we could see that ADG has the largest data coverage, it gathers about 72.12% of the sensed data; while PROXY and PDA gathers 51.5% and 35.64% on average. The data coverage at ADG is about \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Id** & **Metric** & **Epidemic** & **PROPHET** & **PDA** & **PROXY** & **ADG** \\ \hline
1 & _Meta Exchange_ & 0 & 1.83E+4 & 2.89E+4 & 2.94E+4 & 2.62E+4 \\ \hline
2 & _Data Upload_ & 5.77E+3 & 4.80E+3 & 1.97E+4 & 2.85E+4 & 3.99E+4 \\ \hline
3 & _Total Mssage_ & 4.52E+5 & 3.39E+5 & 1.85E+5 & 2.16E+5 & 1.78E+5 \\ \hline
4 & _Data Coverage_ & 10.43\% & 8.45\% & 35.64\% & 51.50\% & 72.12\% \\ \hline
5 & _Efficiency_ & 4.88\% & 5.02\% & 28.36\% & 32.62\% & 43.21\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the overall performances. 6-8 times of those in Epidemic and PROPHET. \(MC\) at Epidemic and PROPHET could only collect data from whoever it encounters. Most of the data are not gathered because the data gathering period is relatively short, e.g., 1800 s. Instead, PDA, PROXY and ADG selectively compute the data gathering points; they guarantee a possible sojourn location that more data could be picked up by the \(MC\). For PDA, the data gathering location is a place where more nodes get together; while in PROXY and ADG the data gathering locations are a set of proxy nodes, which are visited by \(MC\) for data gathering. However, due to the movement of proxy nodes, at the PROXY scheme \(MC\) might lose the track of the proxy nodes. At this case, gathering data from proxy nodes might not be feasible. At the ADG scheme, the sojourn location and time for the proxy nodes are carefully calculated, so \(MC\) would encounter the proxy nodes and picks up the data with high probability, leading to higher data coverage. Metric _efficiency_ is defined as the number of \(valued\) messages divided by the number of total messages. Messages are defined as \(valued\) if they are related with successfully uploaded data.