A State-Based Programming Model for Wireless Sensor Networks

16 Chapter 2. Wireless Sensor Networks
it is first performed right after deployment, in the network initialization phase.
Then, changing parameters of a node and its environment may regularly prompt
reassignment of the node’s role.
2.2.11 Node Heterogeneity
While many sensor networks consist of identical nodes, for some applications it
can be advantageous to deploy multiple hardware configurations. For example,
nodes may be equipped with different sensors; some nodes may be equipped
with actuators, while others may be equipped with more computing power
and memory. Nodes with a powerful hardware configuration typically consume
more energy, but in turn can run more complex software and perform
special functions in the network, such as communication backbone, gateway to
the background infrastructure, or host for sophisticated computations. Low capability
nodes typically perform simpler tasks but have higher lifetimes at lower
cost. These setups can lead to clustered and multi-tiered architectures, where the
cluster-heads are equipped with more computing and memory resources.
2.3 Sensor Nodes
As we have seen in the previous sections, different applications have different
requirements regarding lifetime, size, cost, etc. While functional prototypes
have been realized for most the applications described above, many of the employed
sensor-nodes do not meet all their requirements. Concretely, most devices
are either too big, too expensive for the application at hand, or they fall
short of lifetime requirements. Therefore miniaturization, low-power and lowcost
designs are possibly the most pressing technical issues for sensor nodes.
Most sensor nodes have been built as research prototypes or proof-of-concept
implementations used to investigate certain aspects of future sensor nodes, networks,
algorithms, and applications. Indeed, even the designs which have been
commercialized are targeted at research institutions and very early adopters.
2.3.1 Device Classes
In today’s WSN deployments and research installations we see three classes of
sensor nodes of different degrees of maturity. The first class are commodity devices
that are being used as sensor nodes, though they are not actually built for
that purpose. Devices in that class are laptop computers, PDAs, mobile phones,
cameras, etc. Commodity devices are often used for rapid application prototyping,
where the actual device characteristics are less relevant. The second class
of sensor nodes are custom built from commercially-available electronics components.
These nodes are closer to the actual requirements of WSN and often
perform better than commodity devices. However, they still do not meet most
of the envisioned characteristics, such as “mote” size, years of lifetime, and a
price in the order of cents. Typically these nodes target at a broad applicability.
Some nodes designs are available commercially, such as the latest version
of the BTnode. Because of their availability and relatively low cost, these nodes