3.3. Disseminating emergency information
In our experiment, we consider a scenario in which an emergency occurs in a place with high population density and an
alarm message must be broadcast to all the people located within a certain distance from the emergency location.476 A.S. Cacciapuoti et al. / Pervasive and Mobile Computing 9 (2013) 472–483
Table 1
Device classification summary.
Active A device allowed to re-broadcast an alarm
Passive A device never allowed to broadcast an alarm
Enabling An active device that starts the alarm broadcasting
Infected A device that has received an alarm
Target A device located inside the target area
For alarm dissemination, we have assumed that a fraction of the devices, namely the active ones, cooperate in spreading
the alarm by adopting a simple store-and-broadcast protocol: once an active node receives an alarm message (i.e., once it
becomes infected), it keeps broadcasting the message every τ s.4
The remaining devices, namely the passiveones, can simply
receive the alarm messages from the active infected nodes but they never re-broadcast such messages.
The classification of nodes into active and passive, summarized in Table 1, has a twofold purpose.
First, it is unreasonable to assume that all the mobile phones actively participate in spreading the alarm. In fact, it can
happen that only some special users (like police officers or firefighters) are authorized to forward alarms, and in this case the
active nodes represent the trustableusers. But it can also happen that some users could exhibit selfish behavior by disabling
the broadcasting functionality, i.e., by acting as passive nodes. Or even some devices could be passive as a consequence
of power or bandwidth management. As we do not have any information on the mobile phone users that could let us
differentiate them, in our experiment we randomly assign a certain percentage of users as active, and the remaining ones
as passive. We then repeat the experiments 50 times, each with a different selection of active users.
Moreover, by classifying nodes as active and passive, we can assess the alarm spreading as a function of the number of
active nodes, and therefore indirectly as a function of the system requirements, such as bandwidth and minimum number
of participating nodes.
In the following, we identify two circular areas centered on the emergency location and with different radii: the enabling
area and the target area.
The enabling area is the region where the emergency takes place and where people are close enough to be aware that
an emergency is occurring without the need to receive any message. As a consequence, alarm messages are originated by
the active devices that, at a certain time, are located in this area, namely the enabling devices. Clearly, all the active devices
contribute to diffusing the alarm by moving and by re-broadcasting a received alarm message, but only the enabling ones
are allowed to start the spreading.
Finally, the target area is the region where the alarm messages must be spread so that people can leave the area or
take a certain action to contain the emergency effects. Clearly, due to human mobility and message broadcasting, the alarm
messages can also spread outside the target area and, based on the case, this effect can be suitable or not. However, since
the target area is defined as the area where the emergency actually affects people, in this work we are mainly interested in
characterizing the alarm diffusion in such an area.