32 Fundamentals of WSANs design
A quick diagnosis of conditions will be realized. In case a relevant parameter on a
machine is approaching a critical threshold, Ada will be alerted on her device by an alert
message. For example, sensors could detect that the fruit press machine overheats and
that the level of lubrication oil is abnormally low. An alert is sent to Ada with the infor-
mation about the thresholds and the location of the machine.
2.2 Event detection and spatial and time random
process estimation
With the aim of creating a very general taxonomy of WSN applications, they could be
classifi ed into the following two categories: event detection (ED) and spatial and time
random process estimation (PE).
In the fi rst type of application, sensors must detect an event, for example a fi re in a for-
est, a quake, etc. Therefore, no periodic monitoring by the sink(s) is required. Rather, the
nodes must be able to detect an event that might happen at any time in any location within
the monitored space.
In the second case, the WSN aims at periodically estimating a given physical phenomenon
(e.g., the atmospheric pressure in a wide area or the ground temperature variations in a
small volcanic site) which can be modelled as a bi-dimensional random process (generally
non-stationary). If we consider a typical application related to data gathering from an area
and forwarding data to a fi nal sink(s), when sensors receive a service request they take a
sample from the environment and transmit it by following an appropriate communication
protocol to the fi nal sink(s), which is in charge of collecting all information detected by
nodes and estimating the process realization.
The two types of applications are characterized by separate user requirements and
impose specifi c characteristics at the WSN.
In the ED applications the signal processing within devices is very simple, owing to
the fact that each device has to compare measurement results to a given threshold and
send the binary information to the fi nal sink(s). The density of nodes must ensure that
the event is detected with given probability; therefore the network must have a suffi -
cient coverage. The network coverage is related to the sensing range of nodes and the
event type. Moreover, distributed localization algorithms could be used to identify the
exact position in which the event occurs. The report can be received by the sink(s) with
given probability; connectivity issues, related to transmission range of nodes, must be
kept under control and the communication protocols must be designed so that the alarm
arrives at the sink(s) with a high probability and in a short time.
Applications of WSANs 33
In case the sink(s) periodically queries the nodes, the frequency of queries must be
chosen so that the event is detected with a given probability and must also ensure that
the reports reach the fi nal sink(s) in time.
Therefore, the main issues which characterize this type of application are:
coverage
distributed localization
connectivity
communication protocols (they must ensure low losses of packets and low delays).
Typical requirements for ED applications are therefore:
minimum probability of coverage
maximum localization error
minimum probability of connectivity
maximum packet loss probability
maximum packet delivery delay
and so on.
As far as PE applications are concerned, the density of nodes must ensure that the pro-
cess is accurately estimated; therefore signal processing is one of the main issues. The
samples can be received by the sink(s) with given probability; connectivity of the net-
work, related to transmission range of nodes, must be kept under control and the com-
munication protocols must be designed so that the process estimation error is maintained
under a given threshold. Here the sampling frequency must be chosen so that the process
evolution is tracked.
Therefore the main issues which characterize this type of application are:
signal processing
connectivity
time synchronization
communication protocols (they must ensure low process estimation error).
Typical requirements for PE applications are therefore:
maximum estimation error
maximum localization error
minimum probability of connectivity
maximum packet loss
and so on.

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