Wireless Sensor Networks : Technology, Protocols, and Applications

258 MIDDLEWARE FOR WIRELESS SENSOR NETWORKS
fusion point may be variable and dependent on such factors as system parameters,
network status, and performance requirements.
As middleware, DFuse comprises a data fusion API and a distributed algorithm
for energy-aware role assignment. It supports distributed data fusion with automatic
management of fusion point placement and migration to optimize a given cost function
[8.11]. Application programmers only need to implement the fusion functions
and provide a data flow graph. DFuse is suitable for applications that are hierarchical
fusion functions intended for deployment in a heterogeneous ad hoc sensor network
environment. It offers four sample cost functions for moving the fusion point:
(1) minimize transmission cost without node power considerations, (2) minimize
power variance, (3) minimize the ratio of transmission cost to power, and (4) minimize
transmission cost with node power considerations. DFuse provides a heuristic
role assignment algorithm that works as follows: First, run and deploy a naive
role assignment to the network nodes from the root node to the source node,
then allow every node to decide locally if it wants to transfer the role to any of
its neighbors [8.11].
8.4.10 DDS (Device Database System)
DDS [8.12] enables distributed query processing over a device network. It defines
three types of queries: historical, snapshot, and long-running. DDS is more suitable
for queries than are the traditional warehousing approaches. Each device is a
miniserver capable of supporting a set of functions and able to process portions
of the queries. This device capability results in improved aspects of query performance
such as throughput, response time, resource use, and time delay. Due
to its resource requirements, DDS is not effective in a resource-constraint WSN,
yet it can be useful for WSNs without resource limitations, such as when the batteries
can be recharged. DDS considers only the problem of queries.
8.4.11 SensorWare
SensorWare [8.13] provides a language and runtime environment to support WSN
programming. The language model is used to implement distributed algorithms
while hiding unnecessary details from the application programmer and to enable
sharing node resources among several applications [8.13]. A distributed algorithm
is a set of programs executed in a set of nodes. SensorWare calls these programs
mobile control scripts. The scripts are defined at the node level and can be recognized
by SensorWare at each node. SensorWare has event-driven behavior. It
resides on the top of operating system and uses functions and services of the
operating system.
SensorWare provides a compact runtime environment and script (180 kB). It
targets a specific type of distributed algorithm for a collaborative signal processing
task. It does not provide adaptation between applications and node resources or
among applications. SensorWare has a fixed addressing scheme.