Introduction to Wireless Sensor Networks - Security for Wireless Sensor Networks Using Identity-Based Cryptography - page 13

interference and cause congestion in the channel and the network. Therefore, power
control can be categorized as a cross-layer design problem, as it affects each and every
layer of the stack.
1.7.2 Power Conservation Mechanism
Power conservation can be divided into active and passive schemes (Figure 1.7). Passive
schemes simply turn off the transceiver when there is no sensing, processing, or com-
munication activity; active schemes work toward improving node operations.
Power conservation above the data link layer can be categorized into structured and
nonstructured mechanisms. In structured mechanisms, all nodes are organized into a
structure, such as a group of clusters. Each cluster contains a head node, and the power
conservation mechanism is performed on each head node. It may perform a synchroni-
zation of sleep/awake schedules on a group of nodes in a cluster to conserve bandwidth,
and assign proxy nodes in situations of head-node failure. SPAN is an example of such
a mechanism.
On the contrary, a nonstructured based mechanism allows each sensor node to
schedule its sleep/awake cycles independently. A node may consider neighboring
nodes' sleep/awake cycles in tuning its own cycle. Beacon/hello messages are used
to discover its nearest neighbors. Some examples include TDMA scheduling, energy
efficiency using sleep-mode TDMA schedule, S-MAC, and self-stabilizing determin-
istic TDMA.
Figure 1.7. Power Conservation Mechanism