The first part, which was a
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single-hop network, consisted of nodes which sampled the sensory data every
five minutes, and communicated the data with the gateway using direct
(single-hop) communication. There were no routing issues in this network because
of the direct single-hop communication. The second part, which was a
multi-hop network, consisted of nodes which sampled the data every twenty
minutes. These nodes employed adaptive multi-hop routing to route the data
packets to the gateway. A sleep and wake duty cycle of 2.2% was fixed to
maximize the node lifetime. The gateway sent routing beacons periodically to
initiate the network discovery process.
Observations
Following are some of the important observations made from both the experiments.
1. Overhearing and Node Lifetime: Severe weather conditions (Hurricane
Isabel) forced the base station to shut down for a month a?®ecting
the lifetime and correct operation of the overall network. Also the observed
sensor node lifetime was considerably smaller than expected. It
was concluded that the shortened lifetime of the multi-hop nodes was a
result of the high overhearing. Long preambles (designed to work with
the sleep and wake cycles of sensor nodes) coupled with the availability of
many neighboring nodes for multi-hop routing resulted in a high degree
of overhearing. The power consumption observations indicated that the
overhearing consumed nearly eight times more power than the power consumed
in the packet transmission.
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