D. Sensor Network Topology (Data Linkage)
The topology is constrained by the requirements of the monitoring
and by the physical environment. Sensor nodes can be arranged
in either an ad hoc or a preplanned configuration [134].
In ad hoc arrangement, sensor nodes are randomly placed in the
monitored area. In preplanned arrangement, sensor nodes are
arranged in either a grid, optimal placement [100] (described
next), 2-D or 3-D placement [134].
Determining the optimum node placement is a complex
task and often requires a tradeoff. The network configuration
can be optimized against a number of different constraints.
A network may minimize relay nodes, may need to ensure a
minimum level of service (include a certain level of redundancy),
minimize energy usage to preserve battery life, or may
signal attenuation and loss, and it is not robust due to its
dependence on a single base station to manage the network.
A tree topology is a hierarchy of nodes, with the root node
serving client nodes that, in turn, serve other lower level nodes.
In a tree, the nodes can be grouped at each level. The network is
scalable. Messages pass from the sensor nodes through the tree
branches to the root. Sensor nodes can communicate with their
parent or with other nodes in a group within transmission range.
In contrast, in a mesh topology, any node in the network can
communicate with any other node in the network that is within
transmission range. This enables relaying of messages (multihop
communication) keeping the transmission power low,
saving energy compared with transmitting directly from sensor
nodes to base station in a single hop, and allowing nodes to
communicate indirectly with nodes outside their transmission
range. The network can be easily extended by adding new nodes
and also incorporates fault tolerance as failed nodes can be
bypassed using alternative routes.