Dedicated Short Range Communications (DSRC)
Concept of Operations and
ISO Layer Implementation Summary

Submitted To:
DSRC Interface Definition Committee

Submitted by:

Peter B. Houser
Signal Processing Systems
13112 Evening Creek Drive South
San Diego, California 92128-4199
619.679.6466 voice
619.679.6076 fax


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

1 Background

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.

Figure 1 - Overall DSRC Communications System

2 Terminology

The following terms are used in this document:
MessageA series of bits which convey an information object.
Message PackageA 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.
FrameA series of bits which are transmitted as a group to implement the Data Link ISO Layer 2 definitions.
Message SetA group of logically related Messages which together support a specific ITS Application.

3 Assumed Transponder Capabilities

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:

  1. Agency Memory Region: Read-only Agency and Transponder ID information. This may be the only region required for Electronic Toll and Traffic Management. This document assumes that information in this area will allow a Reader to ascertain the Transponder's capabilities, including memory capacity. This region is assumed to be 32 bytes.
  2. Scratchpad Memory Region: Read and write area accessible only via Interface 4 (Reader). Typically used for real-time credentialing and weigh station bypass. This region is assumed to be 32 bytes.
  3. Inbound External Memory Region: Area is write enabled from Interface 5 (On Board Computer) and read enabled from Interface 4 (Reader). This area is currently used for Trip / Load Number transfers in Electronic Border Crossing systems.
  4. Outbound External Memory Region: Area write enabled from Interface 4 (Reader) and read enabled from Interface 5 (On Board Computer).

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.

Figure 2 - Transponder Memory Regions and Access

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:

  1. Layer 4 services will reference complete Transponder Memory Regions rather than addressed subregions. The High Speed Regions of the External Memories are considered complete Regions for these purposes.
  2. If multiple frames are required to complete the transfer of a complete Transponder Memory Image then the Layer 2 services will implement the necessary sequence of events.
  3. Some DSRC Message Packages are expected to require 150 bytes or more. It is desirable that the Transponder External Memory Regions be large enough to completely store such Message Packages. If this is not the case, a series of Transponder Memory Images may be required to exchange an entire Message Packages between the On Board Computer and the VRC Controller. This document assumes that the Transponder will not provide any additional capabilities to support such a protocol.

Additional Transponder capabilities related to Sleep Mode, LEDs, and audio alerts are assumed to be vendor specific.

4 Assumed Reader Capabilities

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.

5 Proposed OSI Model

The following sections summarize the proposed OSI model and also discuss specific implementation aspects which are central to the design

5.1 OSI Model Summary

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:

  1. The ITS Application registers with and then transmits Message Packages to the VRC Controller. The lower level protocol layers for this transfer are not specified and may be physically implemented using a variety of techniques, including dedicated serial channels and local area networks.
  2. Based upon the received Message Packages, the VRC Controller provides Layers 6 through 3 processing. This processing uses the contents of the received Layer 7 Message Packages to construct addressed Transponder Memory Images which can be transmitted to specific transponders via specific Readers. The Layer 4 processing will control decomposition of Message Packages into one or more Transponder Memory Images.
  3. The specified Reader exchanges addressed Transponder Memory Images with the specified Transponder. This document assumes that the Level 2 processing between the Transponder and the Reader will implement the frame movements required to transfer the Transponder Memory Image.
  4. The addressed Transponder Memory Image data is stored into or fetched from the Transponder Memory Region.
  5. The On Board Computer may exchange addressed Transponder Memory Images in a manner comparable to that used by the Transponder to respond to Reader transmitted data or data requests.

Figure 3 - OSI Protocol Layer Partitioning

5.2 OSI Layer Data Transformations

Figure 4 illustrates how data is formatted as it is transmitted through the DSRC System using the interface layers:

  1. The ITS Application generates Messages and groups them into Message Packages which will be transmitted to a specific Transponder.
  2. These Message Packages are sent to the VRC Controller, which formats the Packages into one or more Transponder Memory Images and then manages the sequential transfer of the Images to a specified Transponder.
  3. The Reader accepts Transponder Memory Images from the VRC Controller and implements the RF framing and control bit sequences which transfers the data into the Transponder's internal memory.
  4. The On Board Computer accepts the Transponder Memory Images, reforms them into a Message Package, and then extracts the individual Messages.

Figure 4 - DSRC Data Formatting and Control

5.3 Multiple Image Transmissions

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:

  1. The individual Transponder Memory Images could be sequentially loaded into the Transponder's External Memory Region and made available for transmit for a specific period of time (such as 500ms), without any provision for retransmit or acknowledgment. Image numbering would allow the receiving system to detect whether the entire Message Package was received successfully.
  2. Alternatively, the reverse data path could be used to transmit an Acknowledge message. However, this approach could limit backwards compatibility with existing Transponders.

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.

5.4 ITS Application Interoperability Using Shared Transponders

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:

  1. Prior to writing into any Transponder Memory Region the VRC Controller shall interrogate the region and determine if it contains previously stored messages which are assigned to ITS Applications other than the targeted ITS Application (which has registered with the VRC Controller). Such messages shall be stored within the VRC Controller and shall not be transferred to the ITS Application.
  2. If any non-targeted ITS Application messages are present in the External Memory Regions then Message Package transfers which would require a multiple Transponder Memory Image exchange sequence shall not be initiated. The VRC Controller shall notify the target ITS Application of the limited memory available in the Transponder Memory Region, excluding that which is already in use by the non-target ITS Application.
  3. If suitably sized Message Package is queued by the ITS Application for transfer to the Transponder then the VRC Controller shall create a Transponder Memory Image which restores the non-target ITS Application messages and appends the requested Messages within the remaining space in the Transponder Memory Region. The queued messages shall replace any messages corresponding to the target ITS Application which might have been previously present in the Transponder.

Figure 5 - Existing Message Preservation

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.

5.5 Provisions for Privacy and Authentication

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.

6 DSRC System Internal Interface Summaries

The following sections summarize the processing and data associated with each of the interfaces illustrated in Figure 3.

6.1 ITS Application with VRC Controller

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:

  1. The ITS Application will initially register with the VRC Controller and specify an application type. The VRC Controller will thereafter screen Message Packages arriving from the Reader and pass only those with a matching Application Identifier. This registration will also implicitly specify what Transponder Memory Regions are relevant. For example, an ETTM application may be concerned only with the Agency Data Region.
  2. When a Transponder enters the Reader's field, the relevant Transponder Memory Regions will be interrogated, the data will be formatted into Message Packages, and the Package will be passed to the ITS Application.
  3. The ITS Application may respond to such messages with a Message Package addressed to a specific Transponder. Note that the Transponder ID may be for the Transponder currently in the Reader Field or for a different Transponder ID. The lower protocol layers will queue the Message Package until the specified Transponder ID is encountered.

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.

6.2 VRC Controller with Reader

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:

  1. Layer 6 - Presentation: Layer 6 is defined to provide syntax negotiation and data representation transformations. In most cases the Layer 7 Messages will be directly transferred and no such processing will be required. There may be exceptions to this rule. For example, there has been some discussion that Trip / Load Numbers might be represented in highly compressed format to minimize the required Transponder Memory. In such a case, Layer 6 may transform the compressed format to match the Layer 7 data dictionary.
  2. Layer 5 - Session: Layer 5 is defined to provide dialogue and synchronization control. This primarily consist of commands which control Reader operations and which "sleep" specified Transponders.
  3. Layer 4 - Transport: Layer 4 is defined to provide end to end message transfer, including fragmentation and flow control. This will involve decomposing Message Packages into multiple Transponder Memory Images and managing their exchange with the Transponder and On Board Computer. It also incudes processing to ensure that information previously stored in the Transponder's Memory Regions by a different ITS Application is preserved. It is assumed that Transponder Memory Image fragmentation to match frame lengths will be accomplished within the Reader at Layer 2.
  4. Layer 3 - Network: Layer 3 is defined to provide network routing and addressing. This will consist of addressing the Reader and Transponder ID specified in the Layer 7 messages.

The following concept of operations and corresponding system capabilities are recommended:

  1. The VRC Controller will accept registration data from the ITS Application. It will use this information to configure the Reader to interrogate the Transponder Memory Regions which are applicable to the requested Application.
  2. When a Transponder enters the Read Zone the Reader will interrogate the desired Transponder Memory Regions and pass the Images to the VRC Controller. The VRC Controller will unpack the Images and pass them as Layer 7 Messages to the ITS Application. Image reassembly into a larger Message Packages will also be handled as described below.
  3. The ITS Application may then pass a Message Package to the VRC Controller. The VRC Controller will format this into one or more Transponder Memory Images and coordinate with the Reader to control transfer of each Image to the correct Transponder ID. If the Message Package is larger than the Transponder Memory Image it is presumably intended for the On Board Computer; the Transponder is being used as a "modem". In this case the VRC Controller will implement the protocol required to sequentially transfer the required Images. This protocol is likely to be relatively unsophisticated and may rely upon time-sequenced transfer of numbered Images.

6.3 Reader with Transponder

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.

6.4 Transponder with Transponder Memory

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.

6.5 Transponder Memory with On Board Computer

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.

7 Control Information

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:

  1. Message Identifier and Error Detection fields in each Message
  2. Image Identifier fields in each Transponder Memory Image

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.

7.1 Message Control Fields

Each Message shall include a 32 bit header, consisting of the following fields:

  1. ITS Application Identifier (8 bits). This field identifies a specific ITS Application, such as ETTM, Mainline Screening, or Border Crossing, and is used by the VRC Controller to route Message Packages to the connecting ITS Application(s). Each ITS Application Identifier will correspond to a unique Message Set.
  2. Message Identifier (8 bits). This field shall identify a specific Message within a Message Set. The combination of the ITS Application Identifier and the Message Identifier will always be sufficient to uniquely identify the format of subsequent bits in the message.
  3. Message Length (8 bits). This field shall specify the length of the message in bytes, not including the header bytes. This field is included in all messages to allow for encryption techniques which may change the length of the original, unencrypted message contents.
  4. Error Detection Code (8 bits). This field shall contain a value, such as a checksum or Cyclical Redundancy value, which may be used to verify the correct transmission of the subsequent message bits.

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.

7.2 Transponder Memory Image Control Fields

Each Transponder Memory Image shall include an 8 bit header, consisting of the following fields:

  1. Image Total (4 bits): This field shall specify the total number of Images required to transfer the overall Message Package. A value of zero ( 0 ) is undefined.
  2. Image Identifier (4 bits): This field shall identify a specific image within a group of Images required to transfer the overall Message Package. A value of zero ( 0 ) is undefined. The Image Identifier shall always be less than the Image Total, except as defined in Section 7.3 for non-standard Transponder Memory Images.

7.3 Application Specific Transponder Memory Images

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:

  1. Some Transponders provide only 8 bytes of External Memory, which is inadequate to contain the preferred Border Crossing messages. Unique methods of encoding Trip/Load Numbers may be required in these cases.
  2. The Transponder may be used as a modem to transfer application-specific files between the vehicle and the roadside. This would only be feasible when the vehicle is parked for an extended period of time, and might be used as part of a fleet management system.

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.

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