  In this paper we study energy-efficient time synchronization schemes with
focus on asymmetric wireless sensor networks, where a head node, which is
connected to both wired & wireless networks, is equipped with a powerful
processor and supplied power from outlet, and sensor nodes, which are connected
only through wireless channels, are limited in processing and battery-powered.
It is this asymmetry that we focus our study on; unlike existing schemes saving
the power of all sensor nodes in the network (including the head node), we
concentrate on battery-powered sensor nodes in minimizing energy consumption
for synchronization. Specifically, we discuss a time synchronization scheme
based on source clock frequency recovery, where we minimize the number of
message transmissions from sensor nodes to the head node, and its extension to
network-wide, multi-hop synchronization through gateway nodes.
