Image Processing Reference
In-Depth Information
H
(
t
)
Ideal time
1+ρ
1
1−ρ
1
2
3456
7
8
9
10 11 12 13 14
t
, Real-time
FIGURE .
Drifting of hardware clock time.
to be discussed. hese basics are to provide the fundamentals for designing a time synchronization
protocol.
If a better clock crystal is used, the drift rate ρ may be much smaller. Usually, the hardware clock
time
H
at real-time
t
is within an linear envelope of the real-time as illustrated in Figure ..
Since the clock drifts away from real-time, the time difference between two events measured with
the same hardware clock may have a maximum error of ρ
(
t
)
[], where
a
and
b
are the time of
occurrence of first and second events, respectively. For modern computers, the clock granularity may
be negligible, but it may contribute a significant portion to the error 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 cm
[], so a fast oscillator may not be possible or suitable.
Regardless of the clock granularity, the hardware clock time
H
(
b
−
a
)
is usually translated into a vir-
tual clock time by adding an adjustment constant to it. Normally, it is the virtual clock time that
we read from a computer. Hence, a time synchronization protocol may adjust the virtual clock time
and/or discipline the hardware clock to compensate for the time difference between the clocks of the
nodes. Either approach has to deal with the factors influencing time synchronization as described in
Section ..
When an application issues a request to obtain the time, the time is returned after a certain delay.
This software access delay may fluctuate according to the loading of the system. This type of fluc-
tuation is nondeterministic and may lessen if real-time operation system and hardware architecture
are used. For low-end sensor nodes, the software access time may be in the order of few hundred
microseconds. For example, a Mica mote is running at MHz [] having clock granularity of .
µ
s
. If the node is % loaded and it takes cycles to obtain the time, the software access time is
around µ
s
.
In addition to the software access time, the medium access time also contributes to the nonde-
terministic delay that a message experiences. If carrier-sense multiple access is used, the back off
window size as well as the traffic load affect the medium access time [,,]. Once the sensor node
obtains the channel, the transmission and propagation times are pretty deterministic, and they can
be estimated by the packet size, transmission rate, and speed of light.
In summary, the delays experienced when sending a message at real-time
t
and receiving an
acknowledgment (ACK) at real-time
t
are shown in Figure .. The message from node A incurs
(
t
)
