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4.9. THE TIME AND DISTANCE CRITERIA
Now an event can be created so that any time the traveling speed
exceeds
55mph , event 33 is generated:
>
SED33NV0;S00+
<
A configuration may also include an audible alert so that a speed
excess warning is generated before actually sending the violation re-
port. See the
Scenarios and Examples section for this.
4.9 The Time And Distance criteria
Up to 10 Time And Distance (T&D) criterion can be defined. Time
And Distance definitions are covered on the
TD
TAIP message.
A T&D signal attached to an event generates a T&D-like report
whose reporting-frequency changes according to the unit’s displace-
ment. With this, the unit increases the report frequency when the
unit is moving (accumulating traveled distance) and decreases it when
the unit is not moving. This is more efficient than having the unit
report by a
time-only criteria were almost all of the reports triggered
when the device is not moving are not relevant.
The unit can also be configured with a time-only criteria. Note:
How does this criteria work ?
First, the report has to be triggered by a distance threshold that tells
the unit to send a report whenever the accumulated distance exceeds
this value. This
distance-only scheme has two problems:
1. What if the unit is traveling too fast and/or the distance thresh-
old is low ?. It will report too frequent.
2. What if the unit moves too slow and/or the distance threshold
is large ?. Or worse, what if it does not move at all?. It will
report very few times or it may never report.
To solve this problem, the distance-only report is improved with a
Time and Distance report that adds two controlling parameters
for the Distance threshold . These parameters are the Minimum time
between reports and the Maximum time between reports .
The Maximum time between reports takes control of the report when
the unit has not exceeded the Distance threshold for a long time. So
this value ensures a minimum reporting frequency. One way to see
this parameter, is that this time will be the report period when the
device is stationary.
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