Before developing a solution to address these factors,
some basics of time synchronization for sensor networks need to be discussed.
These basics are to provide the fundamentals for designing a time synchronization
protocol.
If a better clock crystal is used, the drift rate ?? may be much smaller.
Usually, the hardware clock time H(t) at real-time t is within a linear envelope
of the real-time as illustrated in Figure 1. Since the clock drifts away from realtime,
the time di?®erence between two events measured with the same hardware
clock may have a maximum error of ??(b ??’ a) [5], where a and b are the time
of occurrence of first and second events, respectively. For modern computers,
the clock granularity may be negligible since the operating frequency is in the
MHz or GHz range, but it may contribute a significant portion to the error
223
Weilian Su
H(t)
2 3 4 5 6 7 8 9 10 11 12 13 14
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Hardware clock time
1
1+r
1??’r
t, Real??’time
Ideal time
1
Fig. 1. Drifting of hardware clock time.
budget if the clock of a sensor node is really coarse, running at KHz range
instead of MHz. In certain applications, a sensor node may have a volume of
cm3 [16], so a fast oscillator may not be possible or suitable.
Regardless of the clock granularity, the hardware clock time H(t) is usually
translated into a virtual clock time by adding an adjustment constant to
it. Normally, it is the virtual clock time that we read from a computer.
Pages:
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368