Networked and Embedded Systems Laboratory

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Abstract

Time synchronization is an extremely important service in
wireless sensor networks. The notion of time on a node is
maintained using crystal oscillators. These oscillators are
designed to oscillate at a particular frequency called the
nominal frequency, but typically there is always some deviation.
This deviation is caused due to a wide variety of reasons
which include manufacturing variations, environmental
factors such as temperature, pressure, humidity. Even aging
of the crystal is a long-term factor influencing the oscillation
of a crystal. Various time synchronization protocols
have been developed over the years which perform message
exchanges at different points in time to keep the clocks in
sync. These points are called the resynchronization points,
and are essential because in spite of periodic synchronization,
the clocks continue to drift apart with time. One way
to avoid frequent resynchronization is to locally compensate
for the drifts once a synchronization is performed. Ideally,
these drifts are due to a combination of various factors and
we should compensate for each of them. In this report, we
focus on the drift caused due to temperature(which is also
the most dominant factor) and ways to compensate for it.
There are various techniques which have been developed to
perform compensation for temperature induced drifts. The
techniques rely on creation of a look-up table which relates
the different temperature ranges with the corresponding frequency
drifts. Since, this table is highly specific to a crystal,
we need to perform the measurements for the look-up table
during the manufacturing phase of every crystal. Thus, most
of these techniques turn out to be costly.
In this report we try to use the communication network between
nodes, to populate the entries of the look-up table
online. We also exploit the known relationship between frequency
drift and temperature (which is a cubic polynomial)
to estimate the parameters of the polynomial and use it for
local compensation.

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