We study the impact of
heterogeneity of nodes, in terms of their energy, in wireless sensor
networks that are hierarchically clustered. In these networks some of
the nodes become cluster heads, aggregate the data of their cluster
members and transmit it to the sink. We assume that a percentage of the
population of sensor nodes is equipped with additional energy resources
 this is a source of heterogeneity which may result from the initial
setting or as the operation of the network evolves. We also assume that
the sensors are randomly (uniformly) distributed and are not mobile,
the coordinates of the sink and the dimensions of the sensor field are
known. We show that the behavior of such sensor networks becomes very
unstable once the first node dies, especially in the presence of node
heterogeneity. Classical clustering protocols assume that all the nodes
are equipped with the same amount of energy and as a result, they can
not take full advantage of the presence of node heterogeneity. We
propose SEP, a heterogeneousaware protocol to prolong the time
interval before the death of the first node (we refer to as stability
period), which is crucial for many applications where the feedback from
the sensor network must be reliable. SEP is based on weighted election
probabilities of each node to become cluster head according to the
remaining energy in each node. We show by simulation that SEP always
prolongs the stability period compared to (and that the average
throughput is greater than) the one obtained using current clustering
protocols. We conclude by studying the sensitivity of our SEP protocol
to heterogeneity parameters capturing energy imbalance in the network.
We found that SEP yields longer stability region for higher values of
extra energy brought by more powerful nodes.
