# Collecting Sensed Data with Opportunistic Networks: The Case of Contact Information Overhead
## 1 Introduction
### Contact Information Overhead in OppNets  
The pervasiveness of portable handheld user devices (e.g., smartphones and tablets) makes them inseparable components of OppNets. These devices are able to serve as relay nodes and can cover areas of the city where other mobile nodes such as vehicles may be unable to reach. Unlike other mobile nodes, portable handheld user devices can readily participate in OppNets as they are already endowed with the necessary enablers such as short-range wireless communication interfaces (e.g., Bluetooth and Wi-Fi), computational capability and memory. They also present the primary user interface, as they are the main platform for human communication today. These devices can now be perceived as the users themselves: their attachment to humans allows them to follow and learn user movement patterns, gather social information about the user, as well as maintain user contextual information (e.g., contact information, occupation and preferences). They may even go as far as reflecting user emotion, as resource utilization on these devices could determine user satisfaction and willingness to participate in the network ([PERSON] et al., 2016; [PERSON] et al., 2017). Hence, accounting for them in OppNets is a necessity.  
Portable handheld user devices are resource constrained especially in terms of energy. In order to collect sensed data, they are required to interact with a wide range of nodes (e.g., sensors, smart vehicles, appliances and supporting infrastructure), and as per the message forwarding approach of most applicable OppNet protocols, contact information (i.e., encounter-based information) may need to be maintained, exchanged and updated each time nodes encounter each other. As human population--especially in urban areas--rises rapidly ([PERSON] et al., 2016), so is expected of the nodes that participate in OppNets. Considering bandwidth limitations and the multi-purpose nature of portable handheld devices, it is desirable that the toll OppNets take on available resources remains minimal, without the tendency to rise at a high rate with user and device population.  
Unfortunately, the current state of OppNet research shows that this is not the case. Emerging encounter-based protocols continue to adopt the same message forwarding approach: to compute nodes' ability to contribute to message delivery, they require information to be maintained for (at least) each encountered node, exchanged and updated during each encounter. Besides the large number of nodes expected in urban environments, there is also the tendency of high population density and dynamicity, which results in high encounter rates and frequent disconnections--imagine the number of encounters and the rate of disconnections a handheld device would experience in a shopping mall, train station, stadium or on the road during rush hours. Hence, the overhead associated with maintaining, exchanging and updating contact information tends to be significant. Unfortunately, the impact of this overhead remains unclear up to date, as it is often overlooked in OppNet protocol evaluations--for instance, [PERSON] et al. (2017) identify this overhead without studying its impact on available resources and network performance. Therefore, this paper investigates the impact of this overhead on: (i) data exchange success (i.e., the amount of data messages in node buffers that can be sent and received within the encounter duration), since encounters are often short-lived; and (ii) energy consumption on portable handheld devices.