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The quest for maximizing the node lifetime has made multi-hop data routing a very
popular optimization scheme. Generally in order to achieve good signal to noise ratio,
the output power of a radio at the sender has to be proportional to dl where d is the
distance to the receiver and l . 2. Therefore, to save energy, sensors data is usually
relayed to the sink over multi-hop paths even if a sensor can directly reach the sink.
Since in most WSN applications, data is forwarded towards a single sink node, the
sensors close to this sink would get heavily involved in packet relaying and consume
their energy reserve rather quickly. At the time when a sensor node network configuration bridged unbridged close to the
sink runs out of energy, data paths are re-established and a node _gS2_h that is further
from the sink than S1 becomes the closet hop. Such a scenario increases the total
transmission energy and shortens the node_fs lifetime. Basically, S2 will consume more
energy to reach the sink than S1 and dies soon after. Such effect spreads outward and
may leave the sink unreachable to many sensors.
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