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SAE J2735-Draft-Rev15 [issued: 01-30-07]
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This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
short broadcast style messages. To that end, a dense encoding of information is used in defining the message
sets of this Standard. Several of the design aspects of this encoding are discussed below.
This dense encoding uses a three-way approach:
1)
The smallest divisions of information content to be standardized are called Data Elements
2)
Data Frames are the next, more complex data structures to be standardized in this dense encoding
3)
The next level of complexity in the data structure standardization is called Messages
First and foremost, data elements and data frames are typically used in a known order and with a known element
lengths and often appear without any form of element tagging info encoding because the specific data element in
question (and hence its definitions) can be determined by inspection. This style of encoding provided a dense set
of packed bytes where the decoding of any byte can be determined by the definitions. This style of encoding is
used for many common messages (both complete messages and messages which begin with known elements).
In these messages, other elements many be appended using a simple byte-long tag encoding to separate the
elements which follow. The tag bytes relate to specific elements of known length, and therefore a length value is
not needed. The range of tags in the two-byte case follows the ITS practices for enumerations wherein the (upper)
first 127 values of a byte are used for national codes (in this case specific data elements and data frames) and the
remaining 127 entries can be used for local codes as needed. Messages allow appending 2-byte tagged style to
provide an open –ended means to convey elements that were unknown when the standard was adopted and which
are defined and known only between local implementations. Further, a range of the 2-byte tags can be used to
send variable length data elements (such as a string). In this usage the 2-byte tag is followed by a single byte
representing the length of the data to follow. This allows sending octets or strings up to 255 bytes in length, and it
allows receivers that do not comprehend the tag to calculate the word length and skip over it to the next tag in the
sequence.