A State-Based Programming Model for Wireless Sensor Networks

2.2. WSN Characteristics 15
2.2.8 Mobility and Network Dynamics
Sensor nodes may change their location after deployment. They may have automotive
capabilities, as described in the military application above or they may
be attached to mobile entities, like animals. Finally, nodes may be carried away
by air and water currents.
If nodes are mobile, the communication topology may change as nodes move
out of communication range of their former communication partners. However,
the network topology may change even if nodes are stationary, because nodes
fail or more nodes are deployed. Additionally, communication links may be
temporarily disrupted as mobile objects, such as animals or vehicles, or weather
phenomena obstruct RF propagation. Finally, some nodes may choose to turn
their radio off in order to save power. If the sensor network is sparsely connected,
individual node or link failures may also result in network partitions.
As a consequence of their integration into the environment sensor nodes may
be destroyed by environmental influences, for example, corroded by sea water
or crushed by a glacier. If batteries cannot be replaced, they may run out of
power. Increasing the node population (and hence the quality and quantity or
data readings) may be desirable at places that have been found to be interesting
after the initial deployment. After an initial deployment, a (possibly repeated)
re-deployment of sensor nodes may become necessary in order to replace failed
nodes or to increase the node population at a specific location. All these factors
may lead to a dynamic network, with changing topologies, intermittent partitions,
and variable quality of service.
2.2.9 Bursty Traffic
Wireless sensor networks may alternate between phases of low-datarate traffic
and phases of very bursty, high-datarate traffic. This may be the case, for example,
if the wireless sensor network is tasked to monitor the occurrence of certain
phenomena (as opposed to continuous sampling), and that phenomenon is detected
by a number of nodes simultaneously.
2.2.10 Dynamic Role Assignment
To optimize certain features of the sensor network, such as network performance,
lifetime, and sensing quality, the nodes of a sensor network may be
dynamically configured to perform specific functions in the network. This socalled
role assignment [42, 98] is based on static as well as dynamic parameters of
the nodes, such as hardware configuration, network neighbors, physical location
within the network (e.g., edge node or distance to the infrastructure gateway),
or remaining battery levels. For example, in dense networks, where all areas of
interest are covered by multiple nodes (both in terms of network and sensor coverage),
some of the nodes may be switched off temporarily to conserve energy.
These nodes can then later replace the nodes that have run out of battery power
in the meantime, thus increasing the overall lifetime of the sensor network [122].
As it is often difficult to anticipate the node parameters before deployment, role
assignment is typically performed in situ by the sensor network itself. Typically,