Generally, the sensor nodes communicate using RF, so broadcast is the fundamental communication primitive. The baseline protocols account for this property: on one hand it affects the trust assumptions, and on the other it is exploited to minimize the energy usage.
Figure 1: Communication organization within a sensor network at UC Berkeley's
Cory Hall. All messages are either destined for the base station or originate
at the base station. The routes are discovered so that the number of hops is
minimized and the reliability of each connection is maximized.
Figure 1 shows the organization of a typical SmartDust sensor network. The network forms around one or more base stations, which interface the sensor network to the outside network. The sensor nodes establish a routing forest, with a base station at the root of every tree. Periodic transmission of beacons allows nodes to create a routing topology. Each node can forward a message towards a base station, recognize packets addressed to it, and handle message broadcasts. The base station accesses individual nodes using source routing. We assume that the base station has capabilities similar to the network nodes, except that it has enough battery power to surpass the lifetime of all sensor nodes, sufficient memory to store cryptographic keys, and means for communicating with outside networks.
In the sensor applications developed so far, there has been limited local exchange and data processing. The communication patterns within our network fall into three categories:
Our security goal is to address primarily these communication patterns, though we do show how to adapt our baseline protocols to other communication patterns, i.e. node to node or node broadcast.