Introduction: Bird’s-Eye View of Wireless Sensor Networks - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 4

• Automobile traffic - sensors in cars and traffic infrastructure, for congestion

monitoring and prevention of road accidents
Home automation - temperature and humidity measurement, air conditioning

•

automatic control, alarm systems, etc.
Agriculture - humidity and temperature measurement, automatic control over •

water sprinklers etc., tracking of cattle movement [ 3 ]
Environmental issues - monitoring for hazardous gases, early earthquake and •

fire detection

1.3 Issues in Sensor Networks

In this section we give a short overview of some of the open issues in wireless sensor
networks, in order to better illustrate the kind of challenges sensor networks researchers
are facing.

1.3.1 Energy Conservation

By far the largest issue in sensor networks is battery conservation and careful use
of available energy. Some solutions in computer operation which are suitable for
desktop computers (with practically unlimited amount of energy at disposal) are
proving to be quite inefficient in sensor nodes. One such example is the busy-wait
loops and interrupt polling and masking.
Methods for energy conservation in sensor networks can be broadly divided into
two groups: passive and active methods.
Passive methods include using sophisticated energy sources to supplant the batteries
and placement of sensors into energy-efficient topologies. The alternate energy sources
include wind turbines (to allow sensor nodes deployed in the outdoors to run partly using
wind power), solar panels, micro-fuel cells, ultracapacitors (special capacitors which
hold electrical charge in the dielectric plus additional ionic charge in the double electrical
layer, thus increasing energy density by the order of magnitude). In wearable computing
use cases, the energy of the movement of the human body can be used as well [ 4 ].
Deployment of sensors into energy-efficient topologies benefits from a simple
principle: since energy required to bridge a distance d is proportional to the square
of that distance, K * d 2 , then if an intermediate node could be found on half of that
distance, d /2, the total sum of the energy required to bridge the half-distance twice
is ( K * d 2 ) /2, which is half of the original energy cost. In energy-efficient topologies,
the maximum distance between any two nodes that need to communicate is always
under some threshold which is a parameter of the topology (Fig. 1.4 ).
Active methods to conserve energy include using specialized operating systems
(one of which is described in Chapter 4, “Software Aspects of Wireless Sensor
Networks”), watchdog timers, which are able to reset the sensor node if the software