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SAE J2735-Draft-Rev29 [issued: 12-11-08]
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239 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
changing data. Part I is mandatory in the Basic Safety Message, and so might be broadcast more frequently
than the optional Part II. The transmission frequency of the Basic Safety Message might be chosen so that
it provides an update rate that is consistent with the scan rates for on-board vehicle safety system sensors.
Part II of the MSG_BasicSafetyMessage is optional, and so might be included in only a subset of the
messages. The additional data provided in Part II is either required less frequently by vehicle safety
applications, or is less important, or both. Part II information, when present, might vary from message to
message. Part II can be included periodically or triggered by an event or a request. Locally defined content
can be sent in Part II as well, although this requires additional definition in the ASN and XML used. [JBK6]
MSG_BasicSafetyMessage Frame, Part III, which contains additional data frames or data elements with
open-ended tags. Part III is added on an ‘as required’ basis to allow the communication of data that is not
included in Part I or Part II.
2.
Applicable documents
A detailed description of the identification and selection of the high-priority vehicle safety applications, as
well as the background descriptions of the application scenarios, are included in the “Vehicle Safety
Communications Project Task 3 Final Report: Identify Intelligent Vehicle Safety Applications Enabled by
DSRC”, published by the National Highway Traffic Safety Administration in March 2005 and publicly
available from National Technical Information Service, Springfield, Virginia 22161 (or online at
3.
Application message sequences
The repetitive broadcast of vehicle safety messages is expected to increase the range of vehicle
environmental awareness beyond the range of any on-board sensors. Each vehicle will broadcast its
relevant information frequently via the MSG_BasicSafetyMessage and receive the equivalent messages
from all DSRC-equipped vehicles in the immediate vicinity. Messages from other vehicles can then be
analyzed by on-board processors to identify impending situations that would warrant warning the driver or
initiating other actions, for example, pre-tensioning of seat belts.
4.
Application use with DSRC
Basic Safety Messages will usually be transmitted using the Wave Short Message Protocol (WSM) stack on
a pre-agreed channel, to other devices (typically other mobile on-board units (OBUs)) which have
determined to receive this type of message. It will not be necessary for a sender to advertise a service, nor
for a receiver to undertake any confirm or join operation.
Receivers are expected to process all such messages. Upon receipt, a Basic Safety Message is examined for
message content and relevance at the application layer of the protocol stack.
Basic Safety Messages are expected to be broadcast at a rate sufficient to provide a level of data quality,
including data freshness, similar to that provided by on-board sensors used for vehicle safety systems.
However, to help prevent the possibility of vehicle broadcast messages congesting a channel, the frequency
of transmissions may need to be adjusted in dense traffic environments based on speed, number of vehicles
in close proximity or other parameters (e.g., a toll plaza).