## 6 Scalability  
Scalability in OppNets is directly related to the network size, i.e., the number of nodes in the network. The network size considered in the studied protocols is represented in Figures 3 and 5. Most of the studies have been evaluated for a network size of fewer than 100 nodes. Very few evaluations have considered more than 200 nodes and the trend declines rapidly for 500 nodes, 1000 nodes and above. In one of the studies (Fresh [12]), the network size ranged 1000-64,000 nodes. In studies where  
Figure 4: Comparative studies of the surveyed OppNet protocols with the most compared OppNet protocols and their trends. mobility traces were used (see also the discussion in the next Section 7), the network size was limited by the number of nodes in the traces. We discuss realistic network densities and number of nodes in Section 2 above. Table 2 presents an overview of the here discussed protocols and the densities they have been tested for (Figure 1). Not all protocols provide enough details to evaluate the network density; some do not even provide the network size. Many of the papers refer to some of the application scenarios in Section 2, but the used parameters settings are rather small and thus not very realistic. However, it needs to be noted again that many simulators (refer to Section 4, Figure 3 and our previous work in [8]) do not cope well with large simulations. Nevertheless, simulations with approximately 500-1000 nodes can be considered realistic and are well supported by all simulators. There are also good mobility traces at this network scale (refer also to Section 7). _Take-home message: Large simulations with thousands of nodes are still hard to achieve nowadays. This is one of the most important challenges and goals for OppNet modeling. New studies should consider the targeted application scenarios (Figure 1) and target at least 500-1000 nodes. Some special application scenarios, e.g., group monitoring applications, might require a custom scale._  
Figure 5: Frequently used network size in the studies. Very large outliers are not shown for better visibility. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Protocol** & \begin{tabular}{l} **Extremely Sparse:** \\ **1’Device per km2** \\ \end{tabular} & \begin{tabular}{l} **Sparse: 1-25 Devices** \\ **per km2** \\ \end{tabular} & \begin{tabular}{l} **Populated: 26–500** \\ **Devices per km2** \\ \end{tabular} & \begin{tabular}{l} **Dense: 501–25,000** \\ **Devices per km2** \\ \end{tabular} & \
\begin{tabular}{l} **Very Dense:** \\ **25,001–1 Million** \\ **Devices per km2** \\ \end{tabular} &
\begin{tabular}{l} **Extremely Dense:** \\ **100 Million Devices** \\ **or More per km2** \\ \end{tabular} \\ \hline Epidemic & x & x & x & & \\ \hline Seek and Focus & x & & x & & \\ \hline Spray and Wait & x & x & x & x & \\ \hline Maxprop & x & x & & & \\ \hline Spray and Focus & x & x & & & \\ \hline Prioritized Epidemic & x & & & & \\ \hline CAR & x & x & & & \\ \hline HiBop & x & & & & \\ \hline GeoOpps & x & & & & \\ \hline Propicman & & & x & & \\ \hline Utility Based & x & x & x & & \\ \hline Rapid & x & & & & \\ \hline ORWAR & & x & & & \\ \hline EBR & & x & & & \\ \hline SimbeTs & & & x & & \\ \hline Prophet & x & x & x & & \\ \hline BubbleRap & & x & x & & \\ \hline dLife & & x & & & \\ \hline Sprint & & & x & \\ \hline Scorp & & x & & & \\ \hline HBPR & & x & & & \\ \hline CAER & & x & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Node densities for studied protocols (missing protocols do not provide enough information).