Networked and Embedded Systems Laboratory

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Wireless ad-hoc sensor networks have emerged as an interesting
and important research area in the last few years. The
applications envisioned for such networks require collaborative
execution of a distributed task amongst a large set of sensor
nodes. This is realized by exchanging messages that are timestamped
using the local clocks on the nodes. Therefore, time
synchronization becomes an indispensable piece of
infrastructure in such systems. For years, protocols such as NTP
have kept the clocks of networked systems in perfect synchrony.
However, this new class of networks has a large density of
nodes and very limited energy resource at every node; this leads
to scalability requirements while limiting the resources that can
be used to achieve them. A new approach to time
synchronization is needed for sensor networks.
In this paper, we present Timing-sync Protocol for Sensor
Networks (TPSN) that aims at providing network-wide time
synchronization in a sensor network. The algorithm works in
two steps. In the first step, a hierarchical structure is established
in the network and then a pair wise synchronization is
performed along the edges of this structure to establish a global
timescale throughout the network. Eventually all nodes in the
network synchronize their clocks to a reference node. We
implement our algorithm on Berkeley motes and show that it can
synchronize a pair of neighboring motes to an average accuracy
of less than 20ms. We argue that TPSN roughly gives a 2x better
performance as compared to Reference Broadcast
Synchronization (RBS) and verify this by implementing RBS on
motes. We also show the performance of TPSN over small
multihop networks of motes and use simulations to verify its
accuracy over large-scale networks. We show that the
synchronization accuracy does not degrade significantly with the
increase in number of nodes being deployed, making TPSN completely scalable.

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