The lower system tier includes the fixed location base
station trackers and the smartphones carried by the monitored
population, which are further divided in simple devices
which can be detected by the base station trackers, and
mobile trackers which are capable of detecting other mobile
devices and nearby base stations. The base station trackers
are placed at fixed locations throughout the monitored area
providing the coverage required. These trackers are interconnected
using a reliable and sufficiently high-throughput
technology (e.g., 100Mbps Ethernet, 802.11g). We currently
use the Bluetooth in our enabling devices - it is a ubiquitous
technology with which end-users feel comfortable, while IoT
nodes and smartphones also usually support it. The scan
range of a tracker is typically 10-20m, but the system does
not impose a specific constraint and can support trackers
with varying scan ranges. In most cases, the trackers are
placed in proximity of each other so that their scan ranges
overlap. In this way, we are able to infer presence of a
device at intermediate locations using the received signal
strength within a short-time window. Each base station
tracker maintains a local log of detected traces in addition
to forwarding them towards the local (on site) database.
The mobile trackers are utilised to complement the static
infrastructure and collect additional traces of mobile phones,
even when those are located outside the range of the static
base stations. They periodically attempt to transmit their
buffered trace data via a WiFi connection.