 |
 |
 |
 |
|
 |
 |
SAE J2735-Draft-Rev29 [issued: 12-11-08]
-
265 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
DSRC system is first launched therefore, if a vehicle does not have the ability to send a certain element, it
should not send any reference to that element.
The specific encoding of data elements sent in snapshots follows the ASN and XML definitions provided
previously. The possible elements to be sent are enclosed in a simple CHOICE statement, followed by the
individual selected elements. When more than one element is required to be sent, i.e. a data frame (as in
the case of selecting a specific wheel and then providing data about it) the normal tagging rules are still
followed. The net effect of this over the air is typically a tag byte followed by a length byte, followed by
the data itself.
Periodic Snapshots
In order to obtain ubiquitous coverage nationwide, periodic snapshots are intended to distribute snapshots
between RSUs. To do this, the default method for the periodic snapshots is designed to space the snapshots
at regular intervals between RSUs.
The default method for generating periodic snapshots is to use time and the vehicle’s current speed to
linearly space the intervals between snapshots. Although the method could use distance, the arguments for
distance depend on uneven flow when incidents occur however, most flow occurs when there is no
incidents and thus using time as the default will provider more uniform distribution of snapshots. As
vehicle speed increases, the snapshot interval increases. This results in more widely spaced snapshots at
higher speeds and closer spaced snapshots at lower speeds. This approach is used because in general RSUs
will be further apart on higher speed roads.
The following assumptions were used to determine the default interval between snapshots:
For the rural case at 60 mph (26.8 m/s), the RSU spacing is 10 minutes, or 600 seconds. When
dividing this time by 30 snapshots it results in a snapshot interval of 20 seconds.
For the urban case at 20 mph (8.9 m/s), RSU spacing is 2 minutes and the trip between RSUs
would take 120 seconds or a snapshot interval of 4 seconds.
Thus the snapshot interval is:
4 seconds if speed is
= 20 mph and
20 seconds if speed is
= 60 mph
Between 20mph and 60 mph a linear spread of snapshot intervals would be used, this is achieved by using
the speed when a snapshot is taken to set a timer to count down to the next snapshot.
The exception to the above method is that periodic snapshots do not get collected after the vehicle is
stopped (see below under starts and stops).
An implementation of the message recording loop is shown in the figure below.