of data quality applied to conventional
scientific instrumentation.
some myths
First, let us dispel a few common myths
about sensor network field deployments.
Myth #1: Nodes are deployed randomly. A common assumption in sensor network papers is that nodes will be
randomly distributed over some spatial
area (see Figure 2). An often-used idiom
is that of dropping sensor nodes from
an airplane. (Presumably, this implies
that the packaging has been designed
to survive the impact and there is a
mechanism to orient the radio antennas vertically once they hit the ground.)
Such a haphazard approach to sensor siting would be unheard of in many
scientific campaigns. In volcano seismology, sensor locations are typically
chosen carefully to ensure good spatial
coverage and the ability to reconstruct
the seismic field. The resulting topolo-gies are fairly irregular and do not exhibit the spatial uniformity often assumed
in papers. Moreover, positions for each
node must be carefully recorded using
GPS, to facilitate later data analysis. In
our case, installing each sensor node
took nearly an hour (involving digging
holes for the seismometer and antenna
mast), not to mention the four-hour
hike through the jungle just to reach the
deployment site.
Myth #2: Sensor nodes are cheap
and tiny. The original vision of sensor
networks drew upon the idea of “smart
dust” that could be literally blown onto
a surface. While such technology is still
an active area of research, sensor networks have evolved around off-the-shelf
“mote” platforms that are substantially
larger, more power hungry, and expensive than their hypothetically aerosol
counterparts (“smart rocks” is a more
apt metaphor). The notion that sensor
nodes are disposable has led to much
research that assumes it is possible to
deploy many more sensor nodes than
are strictly necessary to meet scientific
requirements, leveraging redundancy
to extend network battery lifetime and
tolerate failures.
It should be emphasized that the
cost of the attached sensor can outstrip
the mote itself. A typical mote costs approximately $100, sometimes with on-board sensors for temperature, light,
Working with domain
scientists has taught
us some valuable
lessons about sensor
network design.
and humidity. The inexpensive sensors
used on many mote platforms many not
be appropriate for scientific use, confounded by low resolution and the need
for calibration. While the microphones
used in our volcano sensor network cost
pennies, seismometers cost upward of
thousands of dollars. In our deployments, we use a combination of relatively inexpensive ($75 or so) geophones
with limited sensitivity, and more expensive ($1,000) seismometers. The
instruments used by many volcano deployments are in the tens of thousands
of dollars, so much that many research
groups borrow (rather than buy) them.
Myth #3: The network is dense.
Related to the previous myths is the idea
that node locations will be spatially homogeneous and dense, with each node
having on the order of 10 or more neighbors in radio range. Routing protocols,
localization schemes, and failover techniques often leverage such high density
through the power of many choices.
This assumption depends on how
closely aligned the spatial resolution of
the desired network matches the radio
range, which can be hundreds of me-
ters with a suitably designed antenna
configuration. In volcanology, the prop-
agation speed of seismic waves (on the
order of kilometers per second) dictates
sensor placements hundreds of meters
apart or more, which is at the practical
limit of the radio range. As a result, our
networks have typically featured nodes
with at most two or three radio neigh-
bors, with limited opportunities for
redundancy in the routing paths. Like-
wise, the code-propagation protocol we
used worked well in a lab setting when
all of the nodes were physically close
to each other; when spread across the
volcano, the protocol fell over, prob-
ably due to the much higher degree of
packet loss.
Lessons Learned
Working with domain scientists has
taught us some valuable lessons about
sensor network design. Our original intentions were to leverage the collaboration as a means of furthering our own
computer science research agenda, assuming that whatever we did would be
satisfactory to the geophysicists. In actuality, their data requirements ended
up driving our research in several new
directions, none of which we anticipated when we started the project.
Lesson #1: It’s all about the data.
This may seem obvious, but it’s interesting how often the actual data produced
by a sensor network is overlooked when
designing a clever new protocol or programming abstraction. To first approximation, scientists simply want all of the
data produced by all of the sensors, all of
the time.
The approach taken by such scientists is to go to the field, install in-
GPS receiver
for time sync
Base station
at observatory
FreeWave
radio modem
Long-distance
radio link (4km)
figure 1. sensor network design for monitoring active volcanoes.