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SAE J2735-Draft-Rev29 [issued: 12-11-08]
-
269 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Initialization
(
OBE
turn
-
on
)
Current PSN
expires
(
time and
distance
)
All Snapshots
Transmitted
Lose RSE
Connectivity
New PMM
Request
Dump all
messages with
previously
transmitted PSNs
Under PMM
?
Under PMM
?
Generate GAP
Generate New
PSN
End
End
(
Do Nothing
)
Is Buffer Empty
End
(
Do Nothing
)
Y
Y
N
N
Y
N
Under Existing
GAP
Y
N
Wait for Gap to
elapse
Under PMM
?
N
Y
Figure 14 PSN Change Reasons
Buffer Overflow - Snapshot Deletion
The OBU should store a minimum of 30 Probe Data Snapshots to ensure data relevancy for areas of
sparsely deployed RSU. When the snapshot buffer is full the snapshots should be deleted in the in the
following order: first periodic, then start/stop and last events. The deletion of the periodic snapshots
should follow the following process:
The oldest periodic snapshot is deleted last. The first snapshot to be deleted is second oldest, then the
fourth, sixth etcetera. This is repeated until the snapshot in the position halfway between the oldest and the
newest period snapshot is met and then the process is repeated starting again at the snapshot in the second
position. This provides two features: the oldest periodic snapshot is kept to assist in the estimate of travel
time and the deletion of snapshots is preferentially applied to the older data that is less relevant. The
process is illustrated in the figure below. The figure does not illustrate the effect of the deletion process if
there are event snapshots; the effect of these is to reduce the point at which the deletion cycle is repeated.