The
purpose of any time synchronization technique is to maintain a similar time
within a certain tolerance throughout the lifetime of the network or among a
specific set of nodes in the network. To achieve the purpose, the time synchronization
protocol has to take into account the energy as well as the low-end
nature of the sensor nodes in a multi-hop environment. Hence, this requirement
makes a challenging problem to solve.
In addition, the sensor nodes may be left unattended for a long period of
time, e.g., in a cave or on an ocean floor. When messages are exchanged using
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short distance multi-hop broadcast, the software and medium access time and
the variation of the access time may contribute the most in time fluctuations
and di?®erences in the path delays. Also, the time di?®erence between sensor
nodes may be significant over time due to the drifting e?®ect of the local clocks.
In this chapter, the background of time synchronization is provided, and
it is intended to enable new developments/enhancements of current timing
techniques. First, some sensor network nodes are described in Section 2. Second,
the factors a?®ecting the quality of the synchronized time are discussed
in Section 3. Afterwards, the design challenges are described in Section 4. In
addition, the fundamentals of time synchronization for sensor networks are
explained in Section 5. Lastly, the state-of-the-art of time synchronization
protocols for sensor networks is reviewed in Section 7.
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