Data from within a cluster is forwarded to the clusterhead
according to a Time Division Multiple Access (TDMA) schedule maintained
by the clusterhead. The clusterhead will aggregate all of the data and forward
it to the sink after compression. The clusterheads are chosen according to
equation 1 or 2, where P is the number of nodes desired per cluster, r is the
round number, rs is the successive amount of rounds in which the node has
not been a clusterhead, En max is the maximum energy available to a node,
and En current is the current energy available to a node.
T(n) =
P
1 ??’ P(r mod 1
P )
, (1)
T(n) =
P
1 ??’ P(r mod 1
P ) ?·En current
En max
+ ?µrs
1
P ?¶?µ1 ??’
En current
En max ?¶??. (2)
Equation 1 is a stochastic threshold, which decreases as the amount of
successively-failed selection rounds pass [19]. Equation 1, starting at P, gradually
increases the threshold every round until it reaches 1. The threshold will
reach 1 every 1/P rounds. In the last round, before the threshold wraps back
to P, every node that has not been chosen as a cluster head in the last 1/P
rounds will now be a clusterhead.
Equation 2 is a deterministic threshold, which decreases according to Equation
1 but also takes into account the energy available at the node [4]. Equation
1 tries to stochastically distribute which nodes are clusterheads every round,
e?®ectively distributing the workload. The problem is that the workload might
369
Fernand S.
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