Ad hoc Networks
The Defense Advanced Research Projects Agency (DARPA) was among the original patrons of the mote idea. One of the initial mote ideas implemented for DARPA allows motes to sense battlefield conditions.
For example, imagine that a commander wants to be able to detect truck movement in a remote area. An airplane flies over the area and scatters thousands of motes, each one equipped with a magnetometer, a vibration sensor and a GPS receiver. The battery-operated motes are dropped at a density of one every 100 feet (30 meters) or so. Each mote wakes up, senses its position and then sends out a radio signal to find its neighbors.
All of the motes in the area create a giant, amorphous network that can collect data. Data funnels through the network and arrives at a collection node, which has a powerful radio able to transmit a signal many miles. When an enemy truck drives through the area, the motes that detect it transmit their location and their sensor readings. Neighboring motes pick up the transmissions and forward them to their neighbors and so on, until the signals arrive at the collection node and are transmitted to the commander. The commander can now display the data on a screen and see, in real time, the path that the truck is following through the field of motes. Then a remotely-piloted vehicle can fly over the truck, make sure it belongs to the enemy and drop a bomb to destroy it.
This might seem like an awful lot of trouble to go to, until you realize the system that these motes replace. In the past, the tool a commander used to prevent truck or troop movement through a remote area has been land mines. Soldiers would lace the area with thousands of anti-truck or anti-personnel mines. Anyone moving through the area -- friend or foe -- is blown up. Another problem, of course, is that long after the conflict is over the mines are still active and deadly -- laying in wait to claim the limbs and even lives of any passerby. According to this UNICEF report, over the last 30 years, landmines have killed or maimed more than 1 million people -- many of whom are children. With motes, what is left behind after a war are tiny, completely harmless sensors. Since motes consume so little power, the batteries would last a year or two. Then, the motes would simply go silent presenting no physical threat to civilians nearby.
This concept of ad hoc networks -- formed by hundreds or thousands of motes that communicate with each other and pass data along from one to another -- is extremely powerful. Here are several examples of the concept at work:
- Imagine a suburban neighborhood or an apartment complex with motes that monitor the water and power meters (as described in the previous section). Since all of the meters (and motes) in a typical neighborhood are within 100 feet (30 meters) of each other, the attached motes could form an ad hoc network amongst themselves. At one end of the neighborhood is a super-mote with a network connection or a cell-phone link. In this imagined neighborhood, someone doesn't have to drive a truck through the neighborhood each month to read the individual water or power meters -- the motes pass the data along from one to another, and the super-mote transmits it. Measurement can occur hourly or daily if desired.
- A farmer, vineyard owner, or ecologist could equip motes with sensors that detect temperature, humidity, etc., making each mote a mini weather station. Scattered throughout a field, vineyard or forest, these motes would allow the tracking of micro-climates.
- A building manager could attach motes to every electrical wire throughout an office building. These motes would have induction sensors to detect power consumption on that individual wire and let the building manager see power consumption down to the individual outlet. If power consumption in the building seems high, the building manager can track it to an individual tenant. Although this would be possible to do with wires, with motes it would be far less expensive.
- A biologist could equip an endangered animal with a collar containing a mote that senses position, temperature, etc. As the animal moves around, the mote collects and stores data from the sensors. In the animal's environment, the biologists could place zones or strips with data collection motes. When the animal wanders into one of these zones, the mote in the collar would dump its data to the ad hoc network in the zone, which would then transmit it to the biologist.
- Motes placed every 100 feet on a highway and equipped with sensors to detect traffic flow could help police recognize where an accident has stopped traffic. Because no wires are needed, the cost of installation would be relatively low.