 |
 |
 |
 |
|
 |
 |
SAE J2735-Draft-Rev26 [issued: 09-18-08]
-
23 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
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 yet 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 widely 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 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 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
 |