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SAE J2735-Draft-Rev18 [issued: 06-26-07]
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196 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
3.
Probe Data Message Sets
Research has indicated that the optimal DSRC packet size is around 400-500 bytes to account for the
effects of high-speed vehicles. Each Probe Data Message payload is limited to four (4) snapshots in order
to accommodate security and other message overhead in this packet size limitation. If a vehicle contains
more than 4 snapshots, it will send multiple messages. These multiple messages, each containing a
maximum of 4 snapshots, make up the Probe Data Message Set that is sent to an RSE. For Example if the
vehicle contains 30 probe data snapshots, it will send a Probe Data Message Set comprised of 8 individual
Probe Data Messages to the RSE. Figure 1 contains the layout of a Probe Data Message.
Probe
Header
Message
Header
Snapshot 1
Snapshot 3
Snapshot 2
Snapshot 4
Over the Air
Probe Message
Each snapshot contains the value of the respective sensor
at the time the snapshot was taken. Data Elements consist
of items such as:
-Lat
-Long
-Time
-Wipers
-Brake Applied
-Air Temp
...
The Probe Header consists of the following
items:
-ACID
-ACM
-Message Length
-Seconds since last Probe Message sent
Content provided
by lower layers
Fig 1: Probe Message Layout