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SAE J2735-Draft-Rev15 [issued: 01-30-07]
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19 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
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.
5.1 Message: MSG_Ala Carte
Use: A message which is composed entirely of message elements determined by the sender for each message.
The message is fundamentally composed of two sections, a single byte tagged section and a two-byte tagged
section. Each may be used to create ala carte message contents as required. The single byte tagged section
allows transmission of elements (both complex and simple) which are defined in the standard to be sent (the tag
being used to inform the receiver what the next data element or data frame will be). The two-byte section expands
on this by allowing the transmission of data concepts whose definition and length are outside this standard but
which are known to both the sending and receiving entities. The elements valueCnt1 and valueCnt2 state how
many items follow in the message. This message allows extreme flexibility in that any defined data element may
be sent (in any order), followed by a section where additional data may be sent, whose definition is outside this
document.
ASN.1 Representation:
AlaCarte ::= SEQUENCE {
-- each send as required with short tags
valueCnt1 INTEGER (0..32),
items1 SEQUENCE (SIZE(0..32)) OF SEQUENCE {
tag StdTagList,
value ValueList
},
-- Part II, the ad hoc data section
valueCnt2 INTEGER (0..32),
items2 SEQUENCE (SIZE(0..32)) OF SEQUENCE {
tag TwoByteTagList,
data CHOICE {
payload Payload,
value ValueList
}
},
... -- # LOCAL_CONTENT