4. Network operation
In this section we describe the WGSN normal, or routine, operation
and the sequence of actions that are performed if some
anomaly occurs (emergency situation). Since one of the potential
applications for the proposed network is the detection of a fire in
an apartment house, we have chosen a cluster topology for the
WGSN (Fig. 6) which provides secure and reliable operation. The
data routing in the network is performed by the ZigBee protocol.
The sensor nodes are deployed inside the apartments and are battery
operated devices. Conversely, the relay nodes are deployed on
a landing (one device per floor) in the electrical control unit of the
house and, therefore, have a stable power supply. However, these
nodes are also equipped with on-board batteries as a backup power
supply. The network coordinator is on the ground floor and collects
information from the relay nodes and the wireless actuators.
In the day-to-day routine, the sensor nodes (marked with the
letter ‘S’ in Fig. 6) periodically measure the methane concentration
in the environment, alternating with the ultra low-power sleep
mode: measurement mode–sleep mode–measurement mode. In
the measurement mode, all components of the sensor node are
functioning except for the ZigBee transceiver. Gas measurement
occurs as follows: in the measurement mode the voltage on the
sensor sensitive layer, which depends on the gas concentration,
is measured using the built-in ADC of the microcontroller. The
obtained value is compared with two preset thresholds specified
for the sensor node. For methane, the lower specified threshold is
set to 0.15–0.5% vol.; if the concentration rises above this threshold,
the WGSN will send an alert about the presence of methane in the
environment. The second limit is set to 0.5–1%, vol. In fact, the lower
limit of methane explosion is 5% vol. [9]. If the second threshold is
crossed, an appropriate alarm is transmitted over the data channel.
These specified limits could be changed by reprogramming.