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SAE J2735-Draft-Rev29 [issued: 12-11-08]
-
268 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Start
snapshotctr
=
0
Number of Event
snapshots In Buffer
>
0
Number of Start/
Stop snapshots in
Buffer
>
0
Number of Periodic
snapshots in Buffer
>
0
Add oldest Event snapshot to
message
Clear Buffer cell
Add oldest Start
\
Stop snapshot to
message
Clear Buffer cell
Add oldest Periodic snapshot to
message
Clear Buffer cell
End
Y
Y
Y
snapshotctr
=
snapshotctr
+
1
snapshotctr
=
4
Transmit message
Snapshotctr =
0
Definition of variable
snapshotctr
=
counter of snapshots in each message
;
counts up to four for each
message
Note
:
Start
/
Stop and periodic snapshots may only be transmitted in a message with
the same PSN
–
a different PSN requires a new message
.
N
Y
N
N
N
Figure 13 Transmit Loop
Probe Segment Number (PSN)
The periodic snapshots are tagged with a short-lived Probe Segment Number (PSN). The PSN is regularly
changed to ensure privacy. This change occurs following either 120 seconds or 1 km whichever comes
last. To aid anonymity:
Snapshots within the same probe message shall not contain different PSNs.
The same PSN cannot be transmitted from the same vehicle to more than one RSU.
PSNs are limited in duration to 120 seconds or 1 km whichever comes last.
Separate messages can be transmitted to a single RSU with different PSNs.
When a new PSN is generated there is a random changeover gap of 50 to 250 m or 3 to 13 seconds
whichever comes first. Two random numbers should be used, one for distance one for time.
When the vehicle identifies a "Leave RSU" state, all remaining snapshots containing a PSN that
has already been sent to an RSU will be purged from the buffer. (A "Leave RSU" event/state is
when the RSU communications link is not available for 6 minutes or 4 km, which ever comes
first.)
Event snapshots do not contain a PSN.
The figure on the next page illustrates the reasons for changing a PSN.