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SAE J2735-Draft-Rev18 [issued: 06-26-07]
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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,
}
},
... -- # LOCAL_CONTENT