## 2 Applications of Opportunistic Networking Protocols  
In this section, we first describe some of the notable application scenarios for OppNets. Then, we analyze them in terms of their node densities and size in general to extract relevant scenarios for evaluating forwarding protocols. OppNets applications have already been studied several times, e.g., recently in [3]. There, several applications have been proposed and discussed, which are also included here. The applications have been used from the papers in this survey, however the papers do not mention specific parameters for the used applications. Our main contribution is that we identify their realistic density properties. The following list of applications is not exhaustive. On the one hand, it cannot be exhaustive, because OppNets have not been leveraged broadly by real applications yet--who was able to predict applications such as Twitter or Facebook before cheap, affordable Internet connections emerged? On the other hand, we attempt to cover the most prominent and promising scenarios here. Along with the descriptions of the applications, we also discuss their estimated sizes, which are further summarized in Figure 1. We have considered the information in the publications to decide the density scale. All these publications state the applications their protocols are relevant for, but do not necessarily identify the application specific parameters. We take this a step further by trying to identify the realistic density properties that can serve these application areas. Our judgment on realism is based on our observations from the publications/expectations in the respective application area. **Group monitoring:** This is probably the simplest and most straight-forward application for OppNets. It refers to a group of students or tourists, who stroll around together through a city or museum. There are mainly two application goals: to identify if somebody is missing (not connected to the group any more) and to send information in real time to all members. This application is in fact a step between mobile ad-hoc networks and opportunistic networks. It was prototyped by [PERSON] et al. (2010). This application scenario can realistically scale up to 250 people at most and over a small geographical area. Even if the group moves, the relative area it covers remains small. Neighborhood/special interests: This scenario unites two different applications. First, neighbors would like to exchange information about local events, things to share, problems, discussions, etc. In this case, a well defined group of people, living close to each other (but not always connected), exchange data. In the second version of this scenario, another group of well defined people, such as the members of a sporting club, exchange data with each other. In comparison to the neighbors, the members of these groups are more spread, but still in relative proximity of each other. Since the application assumes that people know each other at least coarsely, the maximum number of people even in a large city can reach no more than 10,000 people over an area of a city such as Paris. We assume that, if larger cities are targeted, the application would rather split into several geographically isolated areas. Special professions/rangers: This application resembles the last one, but assumes that the people are brought together by their special profession or role, e.g., firefighters, policemen or rangers. The number of such people is not random and depends on the population or area. Thus, the node density tends to be more stable or inflexible. This can be clearly seen in Figure 1 (red area). Another typical property of this scenario is that it has rather sparse node densities. Figure 1: OppNets applications and their node densities. Six different node density areas have been identified and marked with dotted lines. Campus/office: In this scenario, people working together exchange information, e.g., in an office or campus environment.