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SAE J2735-Draft-Rev29 [issued: 12-11-08]
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This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Updated Messages from Network
If a network user needs to update a message, it issues it again with the same message ID[RS28][CH29] but a
more recent start time. Note that a message ID is either an Advisory ID or Roadsign ID as shown
previously in Figure2. This allows for updated traveler information. A weather advisory issued in the
morning[RS30][CH31] may need to be updated later as conditions change.
Deleting Messages as Directed by Network User
There is also a mechanism for the Network User to delete, or recall, messages that have already made it to
the vehicle. It exploits the in-vehicle housekeeping algorithm to enable a network-directed deletion. An
updated message is sent where the Duration Time has already passed (or is set to zero). The updated
message will replace the old one in the vehicle’s data store. Subsequently, it will be purged by the local
housekeeping algorithm for being expired. A work zone warning issued early in the morning may need to
be deleted if the construction schedule is delayed.
Vehicle Power-Up Events
Need some more text here to deal with the user case of vehicle power down-purge and then a new power
up. Need to also discuss what happens when the power up occurs inside the footprint of these messages
being sent (rather then when outside) and recommend an algorithm to cope with that event.
It[RS32] is advised that current[RS33] messages be stored when the engine is stopped. Following engine start
old messages can then be purged.[CH34]
Presentation of Signs & Advisories in Vehicle
The specific presentation of road signs and traveler advisories is dependent upon vehicle manufacturer
HMI guidelines, display capabilities, etc. Some vehicles may only be capable of presenting a subset of the
message content. HMI design is out of scope for this document. However, three message attributes are
universal [RS35][CH36]– location, direction, and time. To ensure only pertinent information is presented to
the driver, all messages have a physical region, direction of travel and timeframe in which they are valid.
Valid Time
All messages have a valid time which begins at the Start Time and ends at this point in time plus the
Duration Time[RS37] for[CH38] that message. Advisories may exist for periods of time ranging from
minutes to hours to many days, and even years of duration in the case of planned construction. Physical
road signs exist twenty four hours a day and may be unchanged for years. Thus, during their valid time,
most road signs will be valid twenty-four hours a day. This does not imply that they are valid indefinitely.
When their expiration time is reached, they become invalid and are consequently purged from the OBU.
Exit service signs can contain a service provider with limited operating hours. The entire sign may be valid
twenty-four hours a day, but individual services can be presented to the driver during normal operating
hours.
Valid time is transmitted in the message with a start time element. It is expressed in a day of the year
(0..366) and the minute of the day (0..1440) format (as well as an optional year and other time elements)
which is part of the startTime. The duration (expressed in a number of minutes from the startTime), allows
a span of 45 days with a resolution of 1 minute, (as well as longer standardized durations lengths like one-
year). OBU devices can easily combine these elements to determine is a specific message is still valid.
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DCK: Extensive rework of time used here to save a few bytes and make the format tighter. Functional
requirements and abilities are the same but need committee review to confirm.