  Usage of mobile sink(s) for data gathering in wireless sensor networks(WSNs)
improves the performance of WSNs in many respects such as power consumption,
lifetime, etc. In some applications, the mobile sink $MS$ travels along a
predefined path to collect data from the nearby sensors, which are referred as
sub-sinks. Due to the slow speed of the $MS$, the data delivery latency is
high. However, optimizing the {\em data gathering schedule}, i.e. optimizing
the transmission schedule of the sub-sinks to the $MS$ and the movement speed
of the $MS$ can reduce data gathering latency. We formulate two novel
optimization problems for data gathering in minimum time. The first problem
determines an optimal data gathering schedule of the $MS$ by controlling the
data transmission schedule and the speed of the $MS$, where the data
availabilities of the sub-sinks are given. The second problem generalizes the
first, where the data availabilities of the sub-sinks are unknown. Plane sweep
algorithms are proposed for finding optimal data gathering schedule and data
availabilities of the sub-sinks. The performances of the proposed algorithms
are evaluated through simulations. The simulation results reveal that the
optimal distribution of data among the sub-sinks together with optimal data
gathering schedule improves the data gathering time.
