# Collecting Sensed Data with Opportunistic Networks: The Case of Contact Information Overhead
## 6 Conclusions and Future Work  
Due to the distributed nature of OppNets, most routing protocols guarantee acceptable throughput by following an approach that requires contact information to be maintained for every encountered node and exchanged via summary vectors during each encounter. However, this exchange of contact information is often neglected in OppNet simulations. For instance, PRoPHET routing protocol in the latest release of ONE simulator (v1.6.0) does not implement summary vector exchange when nodes encounter each other. Nodes rather obtain the necessary encounter-based knowledge directly from the simulator's internal memory, which is globally and readily available. Unfortunately, this  
Figure 12: Number of failed transmissions (aka. aborted messages in ONE simulator) and number of dropped messages for PRoPHET and PoiFord. unrealistic abstraction has prevented researchers from gaining insight into the impact of contact information exchange on routing performance. Since opportunistic IoT scenarios envision a wide range of participating nodes (e.g., portable handheld devices, sensors, smart vehicles, appliances and other infrastructure), high node population and encounter frequency may result in large amounts of contact information and frequent exchanges, respectively. Our contribution in this regard is twofold:  
1. First, we modelled contact information overhead and investigated its impact on routing performance. We observed that the size of contact information exchanged when nodes encounter each other is related to routing performance in terms of data exchange success as well as the energy utilized in the process. We verified these observations via experiments in ONE simulator, which indicated that exchanging more contact information reduces the number of data messages that can be forwarded per encounter. The results also indicated that exchanging more contact information consumes more energy, especially in high node population and encounter frequency. 2. Second, we proposed PoiFord, a message forwarding algorithm that can be incorporated into existing encounter-based routing protocols to reduce contact information overhead without compromising throughput. The operation of PoiFord is based on the spatial and temporal regularity embedded in human movement and its design is fully distributed and does not require global knowledge or central management. PoiFord reduces contact information overhead by maintaining encounter history for only nodes that have at least one mutual POL. Suitable forwarders are selected based on a forwarding utility derived from the combination of location-based information and available encounter-based information. Incorporating PoiFord into PRoPHET improved routing performance in terms of throughput, message transmission overhead, and energy consumption. Lessons learned from this study include:
* Contact information may account for a significant portion of summary vectors, especially in scenarios with high node population;
* The energy utilized in exchanging contact information increases with node population and encounter frequency;
* Neglecting the exchange of summary vectors in simulations may lead to exaggerated results and unfair performance comparison--a protocol that performed better may perform worse when summary vector exchange is implemented;
* Maintaining history of every encounter may degrade routing performance and is not required to achieve an acceptable overall performance; and
* A suitable forwarding utility can be derived without using the transitive property--since the transitive property requires a history of every encounter to be maintained. We expect our results and findings to motivate further research contributions. Since this work incorporated PoiFord into PRoPHET, future work could investigate the contact information overhead reduction, as well as overall network performance improvement for different encounter-based routing protocols.