# Collecting Sensed Data with Opportunistic Networks: The Case of Contact Information Overhead
## 5 Evaluation
### Results and Discussion
#### 5.2.2 The Amount of Contact Information Maintained by PRoPHET and PoiFord  
The amount of contact information nodes exchanged per encounter and per day is shown in Figure 5. The results show that PoiFord is able to reduce contact information overhead when incorporated into an existing routing protocol. As shown in the figure, PRoPHET requires an average of 48 KB contact information to be exchanged (i.e., sent plus received) during each encounter before messages can be forwarded. PoiFord reduces the average and maximum contact information size maintained on a PRoPHET node by 77% and 54%, respectively. The figure also shows that, during an observation period of 1 day, the total amount of contact information exchanged by nodes running on PRoPHET reaches an average and maximum of 15.6 MB and 97 MB, respectively. PoiFord reduces these values by 78% and 57%, respectively.  
Figure 5: The size of contact information that nodes exchanged when running on PRoPHET and PoiFord.  
PoiFord is able to reduce contact information overhead due to the fact that it maintains encounter history of much fewer nodes--only about 9% of the number of nodes for which PRoPHET needs to keep history of (cf. Figure 6). In addition to this, it is also important to know how contact information size responds to increasing network size and node population. In order to investigate this, we analyze PRoPHET and PoiFord over three different movement scenarios of increasing network size in the next section.  
The (default) Helsinki simulation area in ONE simulator, which consists of four main districts, is modified to obtain three movement scenarios, namely small, medium, and large, correspond to their relative size. As shown in Figure 7, the Helsinki simulation area is used to represent the medium scenario, the small scenario is represented by one of the four main districts in the medium scenario, and the large scenario is artificially generated from the combination of four copies of the medium scenario. Nineteen, 76 and 304 mobile nodes move according to WDM in the small, medium, and large scenarios, respectively--the number of nodes is varied proportionally leaving the essential encounter characteristics unchanged. Next, we present the amount of contact information maintained by PRoPHET and PoiFord over each scenario.  
Figure 6: The average number of nodes for which encounter history is maintained.  
Figure 7: Movement scenarios (namely small, medium and large) used for evaluating the impact of increasing network size on the amount of contact information maintained by PRoPHET and PoiFord.  
Impact of Increasing Network Size on The Amount of Contact Information Maintained by PRoPHET and PoiFord  
The simulation results presented here augment the contact information overhead models presented in Sections 3.1 and 5.1.3 for PRoPHET and PoiFord, respectively. The following variables are used for computing contact information size with the models: (i) \(X_{D}\), a 64-bit double; and (ii) \(X_{S}\), a string of 80 bits (i.e., 5 characters). We consider 24 time slots for PoiFord, and take the number of POIs in the small, medium and large scenarios as 5, 12 and 35, respectively. Figure 8 shows the theoretical and simulation results for contact information size over increasing network size. The theoretical results obtained with the models agree with the simulation results.  
Owing to PRoPHET's transitive property, contact information overhead rises steadily with the number of nodes in the network. Due to location-based influence, the contact information overhead incurred by PoiFord remains quite stable over different node populations. PoiFord's stability in this regard is due to the fixed size of its location table. The slight rise over higher node populations results from increased node density in POIs, requiring encounter history to be maintained for more nodes with mutual POIs. This rise becomes less noticeable as the number of POIs increases, which is likely in city-wide scenarios.