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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.
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 top level of complexity in the data structure standardization is called Messages
The above data concepts are all described in both Abstract Syntax Notation revision One (ASN.1, referred
to as ASN hereafter) and in an XML schema syntax. This process follows the typical style used for
message sets defined in ITS standards by SAE and the other SDOs engaged in ITS development. Complete
ASN modules and XSD schema sets of the standard are available for developers.
The ASN specified by this standard is then encoded for transport by the lower layers (the encoded stream
of bytes becomes the payload of that lower layer). The encoding style required to be used to conform with
this standard is the DER variant of BER. The Distinguished Encoding Rules are a specific sub-set of the
Basic Encoding Rules which were developed to allow one (and only one) encoding for any specific
message content. The DER style follows the normal byte-aligned Tag-Length-Value format of BER for
ASN, consult any textbook on ASN for further³ details.
5. Message Sets
This section defines the structure of the DSRC message sets. Each message set shall be further divided into
specific messages and elements as defined in this clause and those that follow. Typically, these messages
are made up of message content internal to this document (made up of entries that are either atomic or
complex) and message content external to this document (from other functional areas and companion
volumes).
Definitions for these messages are presented in the following subclauses. The ASN is presented in a
section called "ASN.1 Representation," formerly called "Format." In a similar manner, the equivalent
XML expression is presented in a section called "XML Representation" which follows the translation rule
set cited in Clause Two (SAE J2630).
Regarding equivalent entries to be placed into a data registry. The mapping between data elements and
analogous meta data entries have been explained in other ITS stds. In addition, some meta information is
constant in this entire standard and need not be repeated with each entry here. These include the sponsor
and steward of the entries [SAE], the registration status [registered once the standard is adopted] and the
revision date [the date of the standards adoption]. The class name is always ITS.
The productions of ASN.1 which follow shall be considered normative in nature. While the majority of the
normative content is reflected in the actual syntax of the ASN.1 some entries also have additional
statements in the ASN.1 comments which shall be considered to be normative as well. In addition, the
commentary provided with each entry may also provide additional normative restrictions on the proper use
of the entry which shall be followed. The XML productions follow directly from the ASN.1 specifications
and the same rules shall be applied.
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The DSRC committee has developed a (freely available) users guide to illustrate the proper use the
messages, and part of that guide provides additional data on the rules of encoding used in the message set.