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SAE J2735-Draft-Rev29 [issued: 12-11-08]
-
253 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Network User ? RSU
Individual advisories and road signs are combined together into packets, which must meet the
maximum[CH23] size limitation of Wave Short Messages (WSM). Unique Packet IDs are assigned to these
combinations of specific messages. If any individual messages are altered or added to a packet, a new
packet is formed with a new Packet ID.
RSU?OBU Over-the-Air Traffic
The flow of traveler advisory and road sign packets is one-way from the RSU to the vehicle (OBU). All
traffic is transmitted via WSM. Very high priority packets can be transmitted over the Control Channel
(CCH). However, most packets will typically be transmitted over a Service Channel (SCH). A packet is
transmitted on the appropriate channel during the corresponding time slice. Depending on priority of the
packet, it may be repeated multiple times per time slot to ensure delivery.
Handling Repeated Packets
The Packet ID is used to determine if any new traveler information messages have been received by the
vehicle. If the data frames for a particular packet have already been stored locally, then subsequent receipts
of the packet can be ignored. The general flow of receiving a packet is shown below.
Handle Packet Receipt
Is Packet ID
already in
Data Store
?
Store Packet ID
In Data Store
Handle Packet Contents
No
Yes
New Packet is
Received
Wait for Receipt of
New Packet
Figure 5: Handle Packet Receipt