  Many studies in recent years have considered the use of mobile sinks (MS) for
data gathering in wireless sensor networks (WSN), so as to reduce the need for
data forwarding among the sensor nodes (SN) and thereby prolong the network
lifetime. Moreover, in practice, often the MS tour length has to be kept below
a threshold, usually due to timeliness constraints on the sensors data
(delay-critical applications). This paper presents a modified clustering and
data forwarding protocol combined with a MS solution for efficient data
gathering in wireless sensor networks (WSNs) with delay constraints. The
adopted cluster formation method is based in the 'residual energy' of the SNs
and it is appropriately modified in order to fit properly to the requirement of
length-constrained MS tour, which involves, among else, the need for
inter-cluster communication and increased data forwarding. In addition, a
suitable data gathering protocol is designed, based on an approximated TSP
route that satisfies the given length constraint, whereas the proper
application of reclustering phases guarantees the effective handling of the
'energy holes' caused around the CHs involved in the MS route. Extended
simulation experiments show the stable and energy-efficient behavior of the
proposed scheme (thus leading to increased network lifetime) as well as its
higher performance in comparison to other competent approaches from the
literature.
