In the second tier, the collected traces from each static
tracker are stored at a local database - essentially records of
device traces with a corresponding tracker ID, a timestamp
and a RSSI value. These data are also forwarded to a
remote database and analysis server, typically accessible
over the Internet via a secure connection channel. In the
preprocessing stage, the trace data are filtered to remove
duplicate and invalid entries, as well as entries from devices
not participating. Furthermore, for each trace, a specific
location is assigned to the mobile device and hence to the
person carrying it, by considering the RSSI of the device
as measured from involved base stations in a short-time
interval around the trace timestamp. The remote DB adopts a
more advanced schema that allows taking into consideration
a time-schedule of events in different monitored locations,
the participants’ interests and personal attributes (e.g. age
range). During the analysis phase, possible interactions
among the population are inferred and correlated with their
self-reported attributes and scheduled events.