If the granularity of the
hardware clock is coarse, the error ?± contributed by the granularity should be
accounted for. As a result, the round-trip time measured with the hardware
clock is bounded by an error associated with the clock drift and granularity
as determined by
(1 ??’ ??A)(t4 ??’ t1) ?· H(t4) ??’ H(t1) < (1 + ??A)(t4 ??’ t1) + ?± (2)
The bound for the round-trip time fluctuates with respect to time since the
software and medium access fluctuate according to the load at the node and
in the channel. Although the transmission, propagation, and reception times
may be deterministic, they may contribute to the asymmetric delay that can
cause time o?®set between nodes A and B.
In the following section, di?®erent types of time synchronization protocols
are described. Each of them tries to minimize the e?®ect of the nondeterministic
and asymmetric delays. For sensor networks, it is best to minimize the
propagation delay variation. For example, the delays and jitters between two
nodes may be di?®erent in the forward and return paths. In addition, the jitters
may vary significantly due to frequent node failures, since the messages
are relayed hop-by-hop between the two nodes. The synchronization protocols
in the following section focus on synchronizing nodes hop-by-hop, so the
propagation time and variation do not play too much e?®ect on the error of
the synchronized clocks. Although the sensor nodes are densely deployed and
they can take advantage of the close distance, the medium and software access
times may contribute the most in the nondeterministic of the path delay
during a one-hop synchronization.
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