DSRC Interface Definition Committee
Peter B. Houser
Signal Processing Systems
13112 Evening Creek Drive South
San Diego, California 92128-4199
1 Background 1
2 Terminology 2
3 Assumed Transponder Capabilities 2
4 Assumed Reader Capabilities 45 Proposed OSI Model 4
5.1 OSI Model Summary 4
5.2 OSI Layer Data Transformations 5
5.3 Multiple Image Transmissions 6
5.4 ITS Application Interoperability Using Shared Transponders 7
5.5 Provisions for Privacy and Authentication 8
6 DSRC System Internal Interface Summaries 8
6.1 ITS Application with VRC Controller 8
6.2 VRC Controller with Reader 9
6.3 Reader with Transponder 10
6.4 Transponder with Transponder Memory 10
6.5 Transponder Memory with On Board Computer 10
7 Control Information 107.1 Message Control Fields 11
7.2 Transponder Memory Image Control Fields 11
7.3 Application Specific Transponder Memory Images 12
The IEEE SCC32 Working Group for DSRC Messaging is tasked with developing a common set of messages to accomplish vehicle to roadside communications. This is being done in cooperation with ASTM E17.51, which is tasked with developing Physical and Data Link specifications for DSRC.
In the context of Figure 1 below, it is understood that the goal of both committees is to enable communications between the On Vehicle Equipment and the Back Office Equipment in a manner which is interoperable across multiple vendors of individual DSRC components. The ASTM E17.51 has been chartered with specifying the OSI Layers 1 and 2 capabilities, which will be implemented as Interfaces 3 and 4 between the Reader and the Transponder. The IEEE Committee has been chartered with specifying the OSI Layer 7 capabilities, which will be implemented as Interface 1 between the ITS Application and the VRC Controller. This document proposes an approach to distributing OSI Protocol Layers 3, 4, 5, and 6 throughout the overall DSRC communications system. The required capabilities could then be formally controlled by specifying interfaces 2 and 5, while coordinating with the existing ASTM and IEEE efforts to ensure that layers 1, 2, and 7 provide the services and controls required to implement the middle OSI layers.
The following terms are used in this document:
|Message||A series of bits which convey an information object.|
|Message Package||A collection of Messages from a single Message Set which will be transmitted as a unit from the roadside to the vehicle or from the vehicle to the roadside.|
|Transponder Memory Image (TMI)||A series of bits which can be stored within a designated Transponder Memory Region.|
|Transponder Memory Region (TMR)||A physical area of memory within in Transponder suitable for storing a Transponder Memory Image.|
|Frame||A series of bits which are transmitted as a group to implement the Data Link ISO Layer 2 definitions.|
|Message Set||A group of logically related Messages which together support a specific ITS Application.|
Discussions of the overall DSRC system require some assumption about the capabilities of the individual components. This section lists the assumptions concerning the Transponder that were made to develop this document. In general the assumptions have been constrained to minimize Transponder expense and to provide compatibility with existing ASTM Draft 6 devices.
The Transponder is assumed to provide four regions of memory which may be referenced by commands received across Interface 4 or 5. These areas would be segmented as follows and are illustrated in Figure 2:
Both External Memory Regions are assumed to be at least 32 bytes in length. The first 32 bytes of the External Memory Regions are referred to as the High Speed Region and are assumed to be independently accessible via the Reader.
In some implementations the Inbound and Outbound External Memory Regions may be implemented as a single physical memory area, i.e., an Outbound Write operation will overwrite information which may have been placed in the Transponder Memory Region for Inbound Read by the Roadside Equipment.
This document assumes that efficient data transfers will require that each of these regions be at least 32 bytes in length. Many anticipated DSRC Message Packages will be longer than 32 bytes and it would be valuable if the Transponder provided either larger memory regions or handshaking signals so that larger Message Packages could be efficiently exchanged as a series of image transfers. However, to minimize Transponder complexity and to achieve backwards compatibility with existing Transponders this document assumes that such handshaking will not be provided. Inclusion of a Register in the future Transponder designs which provided Transfer Status to the On Board Computer would expedite the overall DSRC process.
This document assumes that the Transponder can respond to either broadcast commands or to commands which reference a specific Transponder ID.
The relationships between DSRC Message lengths, Transponder Memory Region sizes, and transfer frame lengths when exchanging data between the Reader and the Transponder are central to the interface specifications. Within this document the following assumptions are made:
Additional Transponder capabilities related to Sleep Mode, LEDs, and audio alerts are assumed to be vendor specific.
This document assumes that a Reader is capable of interrogating to determine what Transponders are in the read zone, requesting the contents of a specific Transponder's Memory Regions, and writing data into a specific Transponder's memory regions.
Additional Reader capabilities related to Sleep mode, LEDs, and audio alerts are assumed to be vendor specific.
The following sections summarize the proposed OSI model and also discuss specific implementation aspects which are central to the design
Figure 2 below illustrates the proposed model for distributing OSI services throughout the DSRC Communications system to achieve end-to-end messaging. The overall message flow is as follows:
Figure 4 illustrates how data is formatted as it is transmitted through the DSRC System using the interface layers:
The protocol for using sequential Transponder Memory Images to accomplish the transmission of a larger Message Package is a significant open issue. If control signals were available at the lower OSI Layers then acknowledgment signals could be defined to control the sequential transfers. This protocol could be handled either within the Reader or the VRC Controller. Lacking such signals, two possibilities seem available:
In either case, the situation is complicated by the slow serial interfaces currently provided to connect the Reader with the VRC Controller. Because of this limitation, use of the VRC Controller to provide sequential Image transfer protocols can significantly slow the overall transmission process.
It is essential that the overall transfer processes not interfere with existing data which may have been previously stored into a Transponder's Memory Regions by a different ITS application. This interoperability shall be implemented by the VRC Controller as follows, and is illustrated in Figure 5 below:
These processes shall be applied to the Scratchpad Memory Region. They shall also be applied to the External Memory Regions if thse regions are implemented as a single physical memory region within the Transponder.
The proposed OSI Layers and processing do not provide
privacy (encryption) or authentication (electronic signature).
This approach has been taken because it is assumed that the ITS
Application managers will be unwilling to release the required
encryption keys to the VRC Controller or Reader software developers.
This document assumes that all privacy and authentication will
be accomplished at Layer 7 by (a) providing for message whose
contents are encrypted, and (b) defining Electronic Signature
fields or messages.
The following sections summarize the processing and data associated with each of the interfaces illustrated in Figure 3.
This interface will provide the Application Layer communications between the ITS Application and the VRC Controller. The lower layers of communications are not specified in the document and system specific techniques may be selected. Ethernet using TCP/IP and RS-232 utilizing minimal intermediate layers are examples of suitable lower interfacing layers.
This interface is gernally aligned with the CVISN DSRC Interface Requirements and to the initial charter of the SCC32 Working Group. However, some modifications to the CVISN concept of operations are proposed.
The following concept of operations and corresponding system capabilities are recommended:
This concept differs somewhat from that in the CVISN DSRC Interface Requirements. That specification is essentially a polled protocol whereby the ITS Application requested specific information from the Transponder. The proposed concept is essentially asynchronous: the ITS Application receives whatever inbound data for which it has registered from all passing Transponders and queues outbound data transfers which will be passed to the Transponders as they enter the Reader Zone. This modified approach requires less sophistication in the Transponder and is more easily mapped to the existing ASTM Draft 6 capable equipment.
This interface provides for the exchange of Transponder Memory Images which are addressed to specific Transponder IDs. In general, the VRC controller provides routing, Application filtering, Message Package decomposition into Transponder Memory Images, and multiple Image transfer protocols with the On Board Computer. To accomplish this the VRC Controller will provide the following services at Layers 3 through 6:
The following concept of operations and corresponding system capabilities are recommended:
This interface is being specified by the ASTM E17.51. However, that committee is currently chartered only for Layers 1 and 2. It is recommended that the committee also consider Layer 3, Network Routing and Addressing. This will support transmitting Transponder Memory Images to specific Transponder IDs.
This "interface" is included only to emphasize that Transponder Memory limitations fundamentally constrain the overall DSRC interface. The specific methods for implementing this interface are at the discretion of the vendor.
This interface is analogous to that between the VRC Controller and the Reader. It provides for transfer of entire Transponder Memory Images between the Transponder and the On Board Computer with no other protocols. Layer 4 processing within the On Board Computer is responsible for the protocol with Layer 4 within the VRC Controller to provide for transfer of large Message Packages using multiple Transponder Memory Images.
In future Transponder designs, it would be valuable to include a Status Register that can be interrogated by the On Board Computer to determine whether a specific Inbound Transponder Memory Region has been transmitted and whether any transfer is currently in progress. Such a capability would increase the speed and robustness of the end to end DSRC transfers.
In general, the OSI Layers are implemented by attaching control information, i.e., headers, to the data payloads which will be transmitted. The Layering approach recommended within this document will require such control information in two areas:
The following sections recommend an approach to these fields and to achieving backwards compatibility with existing transponders which have External Transponder Memory Regions smaller than the 32 bytes recommended in Section 2.
Each Message shall include a 32 bit header, consisting of the following fields:
These fields shall not be encrypted. The balance of the message bits may be encrypted by the ITS Application to ensure privacy. The choice of encryption algorithms is solely the responsibility of the ITS Application.
Each Transponder Memory Image shall include an 8 bit header, consisting of the following fields:
It may be necessary in certain systems to transfer Transponder Memory Images which do not correspond to all of the DSRC layer specifications. For example:
To accommodate these situations, specific values
for the Transponder Memory Image Control Fields shall be used
to flag Transponder Memory Images which cannot be parsed by standard
Message processing. In particular, Transponder Memory Image Control
Fields in which either the Image Total is 0, the Image Identifier
is 0, or the Image Identifier is greater than the Image Total,
shall designate a Transponder Memory Image which is outside the
usual Message Formats. The IEEE SCC32 committee shall control
the assignment of such Control Field values. All VRC Controllers
shall be capable of evaluating such Transponder Memory Images
without error, though they need not be capable of parsing the
subsequent bits in the Image.
Back to Home Page E-mail to Sue Vogel, Staff