As a result, time
synchronization with these sensor nodes presents a di?®erent set of problems.
Some of the factors influencing the quality of the synchronized time in large
220
Chapter 9 Time-Synchronization Challenges and Techniques
Table 1. Factors influencing the quality of the synchronized time.
Factor Description
(1) Temperature -Since sensor nodes are deployed in various places,
the temperature variation throughout the day may
cause the clock to speed up or slow down. For a
typical PC, the clock drifts a few parts per million
(ppm) during the day [15]. For low end sensor
nodes, the drifting may be even worse.
(2) Phase noise -Some of the causes of phase noise are due to access
fluctuation at the hardware interface, response
variation of the operating system to interrupts,
and jitter in the network delay. The jitter in
the network delay may be due to medium access
and queueing delays.
(3) Frequency noise -The frequency noise is due to the unstability of the
clock crystal [2]. A low-end crystal may experience
large frequency fluctuation, because the frequency
spectrum of the crystal has large sidebands on
adjacent frequencies. The drift rate ?? values
for quartz oscillators are between 10??’4
and 10??’6 [5].
(4) Asymmetric delay -Since sensor nodes communicate with each other
through the wireless medium, the delay of the
path from one node to another may be di?®erent
than the return path.
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