Hence, it is important for
the sensor nodes to remain awake for some time, in order to be ready to relay
the data from the other sensor nodes.
One way to curb the energy-expenditure due to idle-listening, is for the
nodes to use a power saving mode [26, 57]. In such schemes, nodes turn o?®
their radios periodically either independently [26] or in a co-ordinated fashion
[57]. This enables the node to save battery energy for the duration for which
its radio is turned o?®. In [57], the authors propose a scheme called S-MAC
in which nodes are organized into small groups called virtual clusters. All the
nodes in a virtual cluster have their sleep-wake schedule synchronized. Thus
multi-hop forwarding within the same virtual cluster, can take place without
requiring to wait for the next-hop node to wake up. The disadvantage of
using such a scheme is that the co-ordination of the sleep-wake cycles requires
some communication between the nodes, which in turn amounts to additional
protocol and energy overheads.
The power saving mode presented in [26] does not require the nodes to
co-ordinate their sleep-wake cycles. Nodes are assumed to execute their sleepwake
cycles independent of each other. The per-hop latency of communication
then depends on the frequency of these sleep-wake cycles, and the spatial
density of the sensor nodes. The authors provide tight bounds on the average
latency of end-to-end packet delivery.
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