1. Introduction
In typical wireless sensor network (WSN) applications,
sensor nodes are distributed in a certain region, and
empowered by small batteries irreplaceable. Under such
an energy constraint condition, sensor nodes can only
transmit a finite number of bits in their lifetime. Consequently,
energy consumption and data transmission are always
considered together in WSNs.
The main task of a sensor node is to sense the environment
and report what happens. Data collected by sensor
nodes are usually routed back to a sink node by a multiple-
hop infrastructure-less architecture. If a sensor node
can not reach the sink node in its transmission range, its
neighbor sensor nodes have to relay its data packets to
the sink node. Thus each sensor node has two roles, data
gathering and data relaying.