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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.
harmonious fashion. The message sets specified in this SAE standard therefore define the message content
delivered by the communication system at the application layer. This specification consequently defines the
message payload at the physical layer. However, the operations at the physical layer, for example, are specified
by IEEE P802.11p, and the actual content of over-the-air packets will be determined by layers below the
applications layer, as specified in the IEEE standards.
The following subsection provides an overview of the DSRC architecture and protocol stack. A subsequent annex
describes the concept of a Message Dispatcher function at the application layer, and the philosophy of the
message design used to develop the message sets specified in this Recommended Practice. These message
sets are presented in Section 5. The particular message design techniques described in this Recommended
Practice have allowed for the construction of a dictionary of reusable, relevant data frames and data elements that
are intended to expedite the development of future message sets. The standard data frames are presented in
Section 6 of this Recommended Practice, and the data elements are specified in Section 7.
4.2
DSRC Overview
The WAVE communications system is designed to enable vehicle-to-vehicle and vehicle-to/from-infrastructure
communications in order to provide a common platform to achieve the safety, mobility and commercial priorities
described in Section 4.1. Interoperability is a fundamental requirement of this common platform, and WAVE is
designed to provide the required interoperable wireless networking services for transportation. As well, the WAVE
system uniquely supports the high-availability, low-latency communications requirements of vehicle safety
applications, such as pre-crash collision mitigation, intersection collision avoidance and cooperative collision
avoidance.
The physical layer (PHY) of the WAVE system is defined in IEEE P802.11p. In general, the WAVE PHY provides
a control channel (CCH) and multiple service channels (SCH). The range of this system is generally considered to
be line-of-sight distances of less than 1000 meters. The PHY has been optimized to support usage by vehicles
traveling at highway speeds.
IEEE P1609.4 provides enhancements to the IEEE 802.11 medium access control (MAC) that support WAVE
safety, mobility and private applications in a multi-channel system by specifying mechanisms for prioritized
access, channel routing, channel coordination and data transmission.
The upper layers of the network stack, up to the application layer, are defined in IEEE P1609.3. There are two
pathways through the WAVE upper layers above the LLC layer: the Wave Short Message Protocol (WSMP) stack
and the UDP/IP stack. IEEE 1609.3 describes networking services for applications running over either of these
stacks, as well as describing the operation of the WSMP stack. Transmissions on the CCH are limited to WAVE
Short Messages (WSM). Either WSMP stack or UDP/IP stack may be used for communications on SCHs. The
WSMP stack is generally used for broadcast applications.
IEEE P1609.2 defines secure message formats, and specifies how these secure messages are processed within
the WAVE system. These security services are designed to protect messages from attacks such as
eavesdropping, spoofing, alteration and replay, while respecting end users’ rights to privacy. The messages
covered in IEEE P1609.2 security procedures include WAVE management messages and application messages,
but do not include vehicle-originating safety messages. Security services for vehicle-originating safety messages
have not yet been specified in any standard, but will be required before vehicle safety applications can be
deployed.
4.3
Philosophy of Message Design
The DSRC message sets which are the subject of this standard are transported over the protocol stack of the
WAVE Short Message (WSM), a finite resource which must be conserved in order to promote the best operations
for all vehicles. While other protocol stacks also exist over the DSRC media (and are in fact expected to
simultaneously carry a variety of other ITS related information including such things as ATIS information encoded in
XML forms), the WSM is characterized by short length packet message traffic, often broadcast to other vehicles in
an un-acknowledged delivery mode. Dialogs and transactions do take place, and such transaction can leave the
control channel in order to use a service channel as needed, but the general design goal is to maximize support for