 |
 |
 |
 |
|
 |
 |
SAE J2735-Draft-Rev28 [issued: 11-10-08]
-
274 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Application and Use with DSRC
Network User
Network Users generate individual advisory or road sign messages. Network users need to assign unique
identifiers or “Advisory Numbers” to advisories. Road signs, however, are intrinsically identified by their
location, direction and MUTCD code. The individual messages are then propagated into the backhaul
network and eventually to the Road Side Unit (RSU). It is expected that this transmission will use the
defined XML message formats of this standard and TCIP/IP for such transfers.
Network User ? RSU
Individual advisories and road signs are combined together into packets, which must meet the 1200-byte
10
maximum 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.
10
DCK: Confirm this value, it was true for POC, but perhaps not true in IEEE 1609 right now. Do
we need a real link layer to support this from 1609?