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SAE J2735-Draft-Rev29 [issued: 12-11-08]
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231 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Prioritization of messages and message sets is provided so that systems can dynamically permit
transmission based upon the urgency and/or importance of messages.
Priority Related Terms
It is important for this discussion to note the meanings and differences between some terms used in various
standards:
Message Transmission Priority: As described within IEEE WG 1609 (1609.3 and 1609.4), a
three bit field represents Message Transmission Priority (sometimes called ‘User Priority’) which
determines how a given Medium Access Control (MAC) sub layer frame competes with other
MAC frames for access to the wireless medium. The priorities range from zero to seven (0-7)
where 7 is highest. Transmission priority 0 is higher than transmission priorities 2 and 1 due to
historical IEEE development evolution as a way to add a 'new' lowest priority. Note that the
default transmission priority is 0. Please note that J2735 priorities are not limited to the case
where messages are carried in 1609 packet.
Access Category: As defined in the IEEE WG 1609 standard, an access category is related to the
transmission priority and ranges from 0 to 3 where 3 is highest. Access Category is related to
transmission priority as follows:
Transmission Priorities 7 and 6 are Access Category 3.
Transmission Priorities 5 and 4 are Access Category 2.
Transmission Priorities 3 and 0 are Access Category 1.
Transmission Priorities 2 and 1 are Access Category 0.
The following table lists all Transmission Priorities from highest to lowest as well as their
corresponding Access Category:
Priority
Access
Category
7
Highest
6
AC3
5
4
AC2
3
0
AC1
2
1
Lowest
AC0
Message Priority (as considered in this annex): Philosophically, no high level stack layer
should have to know or actually know anything about lower layers. Given this, the applications,
rather than referring to the transmission priority or the access category from IEEE WG 1609.3, use
a common header byte for J2735 defined message priority. This byte is named the Message
Priority and is an integer with a range from 1 to 7 with 7 being the highest. To avoid any
confusion, a message priority of 0 is never used. Whether the protocol layers represented by IEEE