  We present the multi-hop extensions of the recently proposed energy-efficient
time synchronization scheme for wireless sensor networks, which is based on the
asynchronous source clock frequency recovery and reversed two-way message
exchanges. We consider two hierarchical extensions based on packet relaying and
time-translating gateways, respectively, and analyze their performance with
respect to the number of layers and the delay variations through simulations.
The simulation results demonstrate that the time synchronization performance of
the packet relaying, which has lower complexity, is close to that of
time-translating gateways.
