3.2. Inferring contact events from human mobility
To evaluate the feasibility of a partially connected ad hoc network established by the mobile phones for disseminating
emergency information, we have supposed that no cellular infrastructure is working. As a consequence, the devices can communicate only via Wi–Fi ad hoc communications. The assumption of Wi–Fi enabled devices is justified by the latest market
research analyses, which estimate that 144 million Wi–Fi enabled mobile phones were shipped worldwide in 2009 [13] and
predict that the Wi–Fi enabled phone penetration will quadruple by 2015, reaching 66% of all mobile phone shipments [14].
To infer the contact events from the human mobility patterns derived in Section 3.1, we import the user traces into a
network simulator, Network Simulator 2 (ns-2) [15]. The adoption of ns-2 allows us to simulate the effects of all the involved
layers (physical, data link and networking) on the alarm dissemination process, which were not considered in previous work
using similar traces [11].
To take into account the Wi–Fi technology properties, we have assumed a transmission range shorter than 50 m, as shown
in Fig. 3, where the probability of correct packet reception between two neighboring devices is given as a function of the
distance. The adopted transmission range takes into account the effective indoor transmission ranges of the different Wi–Fi
standards, and further details about the adopted physical and data link layers, as well as the channel model, are described
in [16]. According to this, in a certain time slot two users experience a transmission opportunity with a probability of packet
delivery whose value depends on their mutual distance.
We note that the location measurements could be affected by a localization error greater than the adopted transmission
range. However, since the localization error of the measurements can be assumed independent and identically distributed
with zero mean, no biases are introduced in the statistics of inferred contact events, as shown in [11].