SYSTEM ARCHITECTURE

We now describe the system architecture, functionality
of individual components and how they operate together.
We explain how they address the requirements set forth in
Section 2.
We developed a tiered architecture. The lowest level consists
of the sensor nodes that perform general purpose computing
and networking in addition to application-specific
sensing. The sensor nodes may be deployed in dense patches
that are widely separated. The sensor nodes transmit their
data through the sensor network to the sensor network gateway.
The gateway is responsible for transmitting sensor
data from the sensor patch through a local transit network
to the remote base station that provides WAN connectivity
and data logging. The base station connects to database
replicas across the internet. Finally, the data is displayed
to scientists through a user interface. Mobile devices, which
we refer to as the gizmo, may interact with any of the networks
– whether it is used in the field or across the world
connected to a database replica. The full architecture is
depicted in Figure 1.
The lowest level of the sensing application is provided by
autonomous sensor nodes. These small, battery-powered
devices are placed in areas of interest. Each sensor node
collects environmental data primarily about its immediate
surroundings. Because it is placed close to the phenomenon
of interest, the sensors can often be built using small and inexpensive
individual sensors. High spatial resolution can be
achieved through dense deployment of sensor nodes. Compared
with traditional approaches, which use a few high
quality sensors with sophisticated signal processing, this architecture
provides higher robustness against occlusions and
component failures.
The computational module is a programmable unit that
provides computation, storage, and bidirectional communication
with other nodes in the system. The computational
module interfaces with the analog and digital sensors on the
sensor module, performs basic signal processing (e.g., simple
translations based on calibration data or threshold filters),
and dispatches the data according to the application’s needs.
Compared with traditional data logging systems, networked
sensors offer two major advantages: they can be retasked in
the field and they can easily communicate with the rest of
the system. In-situ retasking allows the scientists to refocus
their observations based on the analysis of the initial results.
Suppose that initially we want to collect the absolute temperature
readings; however after the initial interpretation
of the data we might realize that significant temperature
changes exceeding a defined threshold are most interesting.
Individual sensor nodes communicate and coordinate with
one another. The sensors will typically form a multihop network
by forwarding each other’s messages, which vastly extends
connectivity options. If appropriate, the network can
perform in-network aggregation (e.g., reporting the average
temperature across a region). This flexible communication
structure allows us to produce a network that delivers the
required data while meeting the energy requirements. We
expand on energy efficient communication protocols in Section
6.