Protocols for Secure Communication in Wireless Sensor Networks

2.2. Sensor Node Characteristics 17
the clients themselves. Cooperation between the clients is required in order
to assure fair use of each other’s resources. Examples of such networks are
overlay peer-to-peer networks, wireless ad hoc networks, and amateur radio
networks (ham radio). Sensor networks based on this paradigm are simpler to
set up than those of the previous type. Often, it is possible to simply deploy the
nodes randomly within an area. However, the algorithms and protocols used in
such networks are usually more complicated since the nodes have to collaborate
in providing network services and there is no global view of the network’s
state.
In practice, most networks will probably be a mixture of these two extreme
incarnations. Both designs may exist in parallel in the same network: inaccessible
areas may be covered by randomly deployed, self-organizing nodes,
while populated areas may be covered with the use of base stations. Or, a network
may be basically self-organizing with few base stations spread randomly
throughout the network area, providing access points for external clients, for
example, but without full coverage. A network may also exist temporarily in
environments where no base station is available, for example a group of mobile
sensors during transportation.
Base stations can provide certain security services, such as authenticated
broadcast (cf. the µTESLA protocol [142]), for example for distributing code
updates, or acting as trusted third parties for establishing secure links between
nodes as each node maintains a trust relationship to a base station. Generally,
they are not as restricted as sensor nodes and thus the extensive use of public
key cryptography is possible.
For the purpose of our considerations regarding security, we do not rely
on base stations or any supporting network infrastructure for communication
within the sensor network. The deployment of an infrastructure is costly and
not always possible, therefore we want to avoid relying on it for security purposes.
In fact, base stations do not only offer new opportunities for security
services, but also introduce risks similar to those in heterogeneous networks
(see the previous subsection).
The lack of an infrastructure means that all security-relevant decisions have
to be made autonomously by the network, for example whether a query is authorized
to access certain information. Such access control decisions should be
made by collaborations of sensor nodes [17].