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.