OSDI '02 Abstract
Fine-Grained Network Time Synchronization Using Reference Broadcasts
Jeremy Elson, Lewis Girod, and Deborah Estrin, University of California, Los Angeles
Recent advances in miniaturization and low-cost, low-power
design have led to active research in large-scale
networks of small, wireless, low-power sensors and actuators.
Time synchronization is critical in sensor networks
for diverse purposes including sensor data fusion,
coordinated actuation, and power-efficient duty cycling.
Though the clock accuracy and precision requirements
are often stricter than in traditional distributed systems,
strict energy constraints limit the resources available to
meet these goals.
We present Reference-Broadcast Synchronization, a
scheme in which nodes send reference beacons to their
neighbors using physical-layer broadcasts. A reference
broadcast does not contain an explicit timestamp; instead,
receivers use its arrival time as a point of reference
for comparing their clocks. In this paper, we use measurements
from two wireless implementations to show
that removing the sender's nondeterminism from the
critical path in this way produces high-precision clock
agreement (1.85 + 1.28Ásec, using off-the-shelf 802.11
wireless Ethernet), while using minimal energy. We also
describe a novel algorithm that uses this same broadcast
property to federate clocks across broadcast domains
with a slow decay in precision (3/68 + 2.57Ásec
after 4 hops). RBS can be used without external references,
forming a precise relative timescale, or can maintain
microsecond-level synchronization to an external
timescale such as UTC. We show a significant improvement
over the Network Time Protocol (NTP) under similar
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