Transponders and Standards for Dedicated Short-Range Communications

Transponders that communicate data between vehicles and the roadside are proliferating as the numbers of electronic toll collection (ETC) and electronic commercial vehicle credentials and inspection facilities in the United States and worldwide continue to grow. The data are transmitted over a wireless dedicated short-range communications (DSRC) link. Unfortunately, transponder manufacturers and user agencies are not agreed on one transmissions standard or protocol. Consequently, there is currently no national or worldwide DSRC standard to ensure interoperability among deployed ETC or commercial vehicle operations (CVO) systems. DSRC interoperability may be partitioned into three categories, as follows:

1. Contractual interoperability, which ensures that data and funds are reconciled between separate service providers 2. Procedural interoperability, which ensures that there is data connectivity between transponders and roadside systems programmed for different applications and charging mechanisms existing in different operating regions 3. Technical interoperability, which ensures physical link connectivity between transponders and roadside systems to support procurement of equipment from different manufacturers

351 352 Sensor Technologies and Data Requirements for ITS

This chapter describes the standards that are evolving to assist in achieving procedural and technical interoperability on electronic toll and traffic management (ETTM) systems.

7.1 Transponder Types

The transponder, also referred to as a tag or onboard equipment (OBE), is the radio frequency (RF) or infrared device in the vehicle that transmits a vehicle identity, account code, or other required data and messages to roadside reader (beacon)/antenna equipment (RSE), as depicted in Figure 7.1. Type I transponders are read-only tags, which store fixed information. Type II transponders are read/write tags, which contain an updateable area on which a roadside reader/antenna unit encodes information. The information may originate with the application services manager (i.e., the back office ITS application), such as transmission of a warning that credit in the account is approaching a critical value, or it may simply be an acknowledgment from the reader that the transaction is complete. Type III transponders incorporate a

Figure 7.1 DSRC operating environment. Source: IEEE Standard for Message Sets for Vehicle/ Roadside Communications, IEEE Standard 1455-1999, Institute of Electrical and Electronics Engi- neers, New York, NY, 1999. Copyright © 1999 IEEE. All rights reserved. Transponders and Standards for Dedicated Short-Range Communications 353 communications port, which allows data input by other electronic devices. In a CVO application, the data entry in a Type III tag might contain the trip or load number. In an advanced traffic management or advanced traveler information system application, the data might contain incident location information, which is transmitted to a traffic management center from the next available roadside equipment [1]. Tags are further categorized as passive or active, depending on whether or not they simply backscatter the received RF or infrared signals to the roadside reader or contain a transmitter. Passive RF transponders respond to information requests from a reader by reflecting (backscattering) and modulating the reader’s carrier signal in a manner that uniquely identifies the information on the tag. Passive tags are usually smaller, less expensive, and have a longer battery life than active tags, because they operate primarily from power supplied by the reader. Their dis- advantages are a shorter read range (up to approximately 78 meters) and a need for a high-powered roadside reader. The typical reader-to-passive transponder range is 5 to 30 meters [2]. Backscatter modulation requires a functionally matched set of transponders and readers. The transponder receives the carrier signal from the reader and reradiates a modulated form of the signal on a subcarrier frequency. The modulated subcarrier signal, which contains information previ- ously encoded onto the transponder, is multiplied with the reader carrier frequency before being retransmitted to the reader. The reader is designed to transmit a continuous wave (CW) carrier during the downlink phase of the communications sequence and to receive the modulated transponder signal during the uplink phase [2]. Active DSRC systems also require a functionally matched set of transponders and readers. Active RF transponders contain a transmitter, which generates a carrier signal that is modulated with information encoded into the transponder. The modulated waveform is sent to the roadside reader/ antenna combination in response to an interrogation. Since active tags operate solely from battery power, they reduce the power requirements of the reader and function over longer ranges (up to 100 meters). Typical reader-to- active transponder ranges are 5 to 100 meters [2]. Contactless infrared cards are read by a windscreen-mounted OBE, which conveys the information on the card, via a modulated infrared beam, to an infrared reader located over the roadway. Some types of RF transponders, known as smart cards, can also be inserted into OBEs for use on ETC systems. These OBEs can perform several functions for the driver, including emitting a warning signal when the balance is below a threshold value, 354 Sensor Technologies and Data Requirements for ITS changing the threshold value, reading the balance on the card, increasing the balance on the card, and visibly confirming that the system is operating cor- rectly. Some commercial tag applications, such as the International Trade Data System (ITDS), require a unique identifier, which changes with each border crossing. Types I and II RF tags or infrared technology do not easily satisfy these functions, since they do not incorporate a communications port for data entry [3, 4]. The back office equipment hosts applications and data required to facilitate the ITS services. For toll roads, these services include debiting of the user’s account, issuing warnings when the account drops below some threshold, and aggregating transactions for delivery to the toll road operator. For commercial vehicle inspection and weigh stations, the services include verifying credentials and sending clearance to the vehicle to pass through without stopping.

7.2 Open Systems Interconnection Communications Model

The open systems interconnection (OSI) model was developed by the Inter- national Standards Organization (ISO) in 1984 to standardize the transmission of data and messages in computer communications architectures and to provide a framework for developing protocol standards. A protocol consists of predefined, layered sets of rules, which govern the way two or more devices exchange information over a transmission medium. The protocol is usually written into software residing in the memory of a computer or transmission device and is executed when data are prepared for transmission. The protocol specifies the total number of bits in each transmission sequence by partitioning data and messages into a number of segments having a fixed or variable number of bits, depending on whether synchronous or asynchronous transmission is utilized. Synchronous communication transmits messages at a preestablished frequency. Successive bits, separated by a constant time interval, identify the beginning and ending of a data or message unit. Redundant information such as start and stop bits is not used. On the other hand, asynchronous communication does not use a specific frequency or timing for message transmission. The beginning and end of a transmission are characterized by bytes encapsulated with start and stop bits. However, once an asynchronous transmission is begun, each bit in the sequence is separated by a constant interval. The protocol may add headers to the front of segments to describe the information in the segment. At the receiving end, Transponders and Standards for Dedicated Short-Range Communications 355

Figure 7.2 OSI seven-layer communications model. the protocol software interprets the headers and predefined segment length to strip out the data and messages in the transmission sequence. OSI partitions the communications process into seven layers, as illustrated in Figure 7.2. In this instance, the user initiates the transaction by transmitting information that moves down the stack from Layer 7 to Layer 1, with each layer appending instructions to the information. After the information passes through the transmission medium, the receiving entity (e.g., the service manager or his agent) reverses the process. The OSI model is modular, since it defines the processes that occur at each layer. Each layer interfaces with the layer above and below it and, in theory, may use any protocol without affecting the operation of the neighboring layers. Interopera- bility is guaranteed when manufacturers design their products to conform to the protocols established by the layers. Each layer performs a subset of the functions required to communicate with another system. When the functions of each layer are properly defined, changes in one layer do not affect those of another. Thus, standards for each layer can be developed independently and simultaneously. 356 Sensor Technologies and Data Requirements for ITS

The top three layers, application (Layer 7), presentation (Layer 6), and session (Layer 5), control the processes required for the exchange of information in support of the application. They allow multiple applications to interact with each other through the capabilities provided by their respective operating systems. The bottom four layers, physical (Layer 1), data link (Layer 2), network (Layer 3), and transport (Layer 4), control the end-to-end processes that facilitate data and message transfer between transmitter and receiver. They contain the protocols that link the transmitter and receiver. Layers 1 through 6 support technical interoperability, while Layer 7 is needed for procedural interoperability. The functions of the seven layers in the OSI model are as follows [5]:

1. Physical layer: concerned with transmission of the unstructured bit stream over the physical medium. Defines characteristics such as the carrier frequency, modulation, data coding, and data bit rate; also specifies antenna and transponder parameters, such as location, polarization, and beamwidth. 2. Data link layer: defines a protocol to assemble data into a transmission sequence, which facilitates reliable transfer of information across the physical link. Some standards divide the data link layer in half because of the two general functions it performs. The first, referred to as logical link control (LLC), coordinates the physical transfer of data. The second, medium access control (MAC), manages access to the physical medium. One LLC sublayer can support several MAC sublayers, where each MAC sublayer supports a different physical layer. 3. Network layer: establishes, maintains, and terminates logical and physical connections. It translates logical addresses into physical addresses and routes and controls the flow of messages across the network interface. 4. Transport layer: ensures that data and messages are successfully transmitted and received across the physical medium. If data or messages are received with errors, the transport layer requests retransmission. 5. Session layer: controls the initiation and coordination of communications between the transmitting and receiving devices. It interfaces with any programs that are being executed in these devices to ensure efficient communication of data and messages. Transponders and Standards for Dedicated Short-Range Communications 357

6. Presentation layer: executes code conversion and data and message reformatting to ensure that information conforms to the protocol. 7. Application layer: provides the user interface to access the information transmitted over the network. In ETTM applications, it defines formats that support file and message transfer between vehicles and reader and vice versa.

Some information transmission standards use the OSI model as a guide, but do not conform precisely to the functions of the layers discussed here. They combine some functions of a layer into another; others omit layers. However, the devices and services conforming to a specific protocol are able to interact with one another and access information transmitted according to that protocol. DSRC standards only address Layers 1, 2, and 7 of the OSI model because of the short range and short duration of the vehicle-to-road communication. An OSI model for DSRC is depicted in Figure 7.3, which shows the applicable U.S. and European standards [6, 7]. The applications depicted in the upper left portion of the figure are implemented through the back office equipment. The resource manager isolates the transponder from application-specific data requirements and manages the transponder resources. The alternative transmission paths, which directly connect the layers, accommodate standards other than those shown on the right of the figure.

7.3 DSRC Standards

Tables 7.1 and 7.2 list representative ETC deployments and the respective transponder standards and market penetration [1, 8– 10]. These locations were selected because of the relatively large numbers of transponders in use and to indicate the diversity in ETC equipment manufacturers. Figure 7.4 illustrates an example of ETC at the Sam Houston central toll plaza in Harris County, Texas. This tollway utilizes the passive backscatter EZ Tag, which allows nonstop passage through specially marked lanes. Additional lanes accommodate coins and tokens, while others issue change and receipts. The InterAgency Group (IAG) proprietary specification is dominant in the northeast United States. It is used in states as far south and west as West Virginia, as far north as New York, and as far east as Massachusetts [11]. Although the data and message protocols are not available for public distri- bution, the transponders and readers are similar to those designed by MARK 358 Sensor Technologies and Data Requirements for ITS ayers 1, ayers Simplified OSI architecture for dedicated short-range communications. Standards developed in the United States and Europe for L for Europe and States United in the developed Standards communications. short-range dedicated for architecture OSI Simplified 2, and 7 are indicated. Figure 7.3 Transponders and Standards for Dedicated Short-Range Communications 359

Table 7.1 North and South American ETC Deployments (Partial List)

Location Operator DSRC Manufacturer Standard Penetration

Garden State Pkwy, TRANSCOM MARK IV IAG (active) Deployed on NJ Turnpike, over 900 mi NY Thruway, (1,448 km) of PA Turnpike, roads, DE Port Authority, >1 million Mass Pike tags (E-ZPass) Highway 407, Ferrovial GM Delco Electronics, IAG 69 km, Toronto, Canada MARK IV ASTM Draft 6 180,000 tags Northern Virginia Dulles Toll Road MARK IV IAG Dallas, Texas North Texas Toll- Amtech Backscatter 86 lanes, (Toll Tag) way Authority 240,000 tags Houston, Texas Harris County Amtech ISO 10374, 130 lanes, (EZ Tag) Toll Road etc.* >500,000 tags Authority (backscatter) Oklahoma Oklahoma Turn- Amtech Backscatter† 980 km, pike Authority 440,000 tags SR 91 Express Lanes, New-Trac/ SIRIT (formerly Texas CA Title 21 100,000 tags Orange County, Cofiroute Instruments) (backscatter) California (FasTrak) Eastern, San Joaquin, Transportation SIRIT and Amtech CA Title 21 270,000 tags and Foothill Toll Corridor Agen- Roads, California cies, Orange County I-15, San Diego, San Diego SIRIT CA Title 21 10,700 tags California Assoc. of Gov- ernments Florida DOT, other state Amtech Intellitag CA Title 21 150,000 tags (SunPass) authorities Illinois Tollways and Illinois State Toll Mark IV IAG 350,000 tags Bridges (I Pass) Highway Author- ity Buenos Aires, Autopista del Amtech ISO 10374, 43 lanes, Argentina Oeste etc.* >70,000 tags 360 Sensor Technologies and Data Requirements for ITS

Table 7.1 (continued) North and South American ETC Deployments (Partial List)

Location Operator DSRC Manufacturer Standard Penetration

São Paulo, Brazil ABCR‡ Q-Free CEN standard 175 lanes, 500,000 tags São Paulo, Brazil Marechal Ron- Raytheon HTMS ASTM Draft 6 don Highway/ Catel Rio de Janeiro, Brazil Rio-Niterói Amtech ISO 10374, 8 lanes, Bridge etc.* 82,000 tags Osório-Porto Alegre CONCEPA EFKON CEN ITR 278/ 20,000 tags Highway, Brazil 9/#63 (infrared)§

* This passive tag is compatible with ISO 10374, Association of American Railroads S-918, American Trucking Association Standard for Automatic Equipment Identification, ANSI MH 5.1.9, and CEN EN 10374. † To be replaced with an active DSRC system. ‡ Associa Vão Brasileria de Concessionãnas de Rodovias. § The contactless card is compatible with ISO 14443 [4].

Table 7.2 European and Asian ETC Deployments (Partial List)

Location Operator DSRC Manufacturer Standard Penetration

Oslo, Trondheim, NPRA* Q-Free CEN standard 490,000 Rennfast, Hvaler, (Migrating to (>800,000 5.8 Norway 5.8 GHz stan- GHz tags dard) forecast) Sweden Öresund Bridge Combitech CEN standard 6 lanes, 100,000 tags Israel Cross Israel Raytheon HTMS ASTM Draft 6 86 km† Highway Malaysia Rangkaian Segar EFKON CEN ITR 200,000 tags§ Sdn Bhd 278/9/#63 (infrared)‡ Melbourne City Link, Transurban City Combitech CEN standard 600,000 tags Australia Link Ltd. anticipated Transponders and Standards for Dedicated Short-Range Communications 361

Table 7.2 (continued) European and Asian ETC Deployments (Partial List)

Location Operator DSRC Manufacturer Standard Penetration

Sydney, Australia Road Traffic Q-Free CEN standard 100,000 tags Authority of New South Wales Taiwan Highway Bureau EFKON CEN ITR 5,000 tags 278/9/#63 (infrared) ‡

* Norwegian Public Roads Administration. † The first 20 km is scheduled to begin toll operations in 2002. ‡ The contactless card is compatible with ISO 14443 [4]. § Additional tags issued for applications such as parking, gas stations, taxis, pay phones, public transit, retail stores, and theme parks.

IV for their ROADCHECK vehicle-to-roadside communications system [12]. These transponders are half-duplex devices, which use the same frequency (915 MHz nominal) and modulation method for uplink and downlink of information in the Layer 1 physical layer. The data or message capacity is 256 bits transmitted at 500 ±10% kbit/s. Data format is Manchester keyed carrier. The transponder utilizes a synchronous protocol for read/write of fixed, preprogrammed data and real-time variable data as a

Figure 7.4 Sam Houston central toll plaza depicting EZ Tag ETC lanes (courtesy of Harris County Toll Road Authority). 362 Sensor Technologies and Data Requirements for ITS vehicle passes under an antenna at highway speeds. Variable data include point of entry into a toll road for ETC applications and load status for CVO applications. With multiplexed antennas, each reader can monitor up to eight lanes of vehicles traveling at speeds up to 100 miles/hour (160 kilometers/hour) at data rates of 500 kbit/s. The antenna is typically mounted 16 feet (4.9 meters) above the road surface at a forward-looking angle, which optimizes the transfer of data to a windshield-mounted transponder. Ongoing standardization efforts are incorporating the ISO-OSI model. Standards developed by the American Society for Testing and Materials (ASTM), the Institute of Electrical and Electronics Engineers (IEEE), and the European Committee for Standardization (CEN) control the communications interface by defining the properties of the physical, data link, and application layers. Two standards prepared by ASTM define the physical and data link layers and one by IEEE specifies the application layer. CEN has developed four standards to span the physical, data link, and application layers. The following sections summarize the salient features of DSRC standards and proposals and provide additional information concerning their development.

7.3.1 Physical Layer Standards ASTM PS 111-98, Standard Specification for Dedicated Short-Range Commu- nication (DSRC): Physical Layer, provides a common framework for the design of onboard equipment and readers that use active and passive technologies in the 902 to 928 MHz band [2]. This standard is discussed further in Section 7.3.5. A transmission frequency of 5.8 GHz is specified by the CEN standard, Road Transport and Traffic Telematics (RTTT) Dedicated Short- Range Communication (DSRC): Physical Layer Using Microwave at 5.8 GHz, CEN/TC 278, ENV 12253 (1998). Support among industry and operating agencies in North America for continued transponder operation at 915 MHz or the 75-MHz spectral band between 5.850 and 5.925 GHz is still to be resolved. An industry group con- sisting of Amtech/Transcore, Sirit Technologies, Raytheon HTMS, and MARK IV Industries has proposed a set of 5.9-GHz specifications to support ETC and commercial vehicle regulation, as well as payments for goods and services provided by private businesses, highway safety improve- ments, and functions required by advanced vehicle control systems (see, for example, the functions listed in Table 7.3). Referred to as NA5.9, this proposal imposes complete interoperability among systems manufactured by Transponders and Standards for Dedicated Short-Range Communications 363

Table 7.3 Proposed Allocation of 5.850 to 5.925 GHz 75-MHz Spectral Band

Channel Function

1 In-vehicle signing and intersection information. Signing messages include traffic conditions, travel time, and construction ahead. Intersection information messages include railroad crossing warnings, intersection collision avoidance, vehicle signal priority status, and traffic network performance. 2 (same as In-vehicle signing and intersection information. Signing messages include traffic Channel 1) conditions, travel time, and construction ahead. Intersection information messages include railroad crossing warnings, intersection collision avoidance, vehicle signal priority status, and traffic network performance. 3 Commercial vehicle operations for publicly owned facilities. Applications include electronic clearance, international border clearance, toll collection, fleet and freight management, transit vehicle data transfer at bus stops and termi- nals, and parking payment and access control. 4 (same as Commercial vehicle operations for publicly owned facilities. Applications Channel 3) include electronic clearance, international border clearance, toll collection, fleet and freight management, transit vehicle data transfer at bus stops and termi- nals, and parking payment and access control. 5 Mobile location interrogation. Applications include electronic license plate reading, which downloads data via mobile, stationary, or hand-held readers, and downloading or uploading data in freight yards. 6 Automated highway system (AHS) to vehicle communications, which convey the status and position of the vehicle and send AHS operation instructions and roadway status to the vehicle. 7 Commercial vehicle operations for privately owned facilities providing services similar to those on Channels 3 and 4. Additional services include payment for services or products at drive-through businesses such as fast food and banking. 8 (same as Commercial vehicle operations for privately owned facilities providing services Channel 7) similar to those on Channels 3 and 4. Additional services include payment for services or products at drive-through businesses such as fast food and banking. Unallocated Afford some mobility in selecting frequencies to avoid interference to or from other users. Unallocated Accommodate several possibly competing DSRC technologies, which may require different channel bandwidths. 364 Sensor Technologies and Data Requirements for ITS any vendor that adopts the specification. Active synchronous transmission is mandated at a minimum data rate of 10 Mbit/s. The minimum downlink (RSE to OBE) range is 300 meters and minimum uplink (OBE to RSE) range is 90 meters. Europe faces similar issues with garnering acceptance of the CEN DSRC frequency of 5.8 GHz with a passive backscatter modulation tag. Japan has selected an active RF tag operating at 5.8 GHz. The Japanese pref- erence is for full-duplex operation (i.e., utilization of two frequency bands for simultaneous transmission of uplink and downlink information) at a transmission rate of 1 Mbit/s. The Japanese standard also allows a tag to operate at half duplex. In addition to ETC, Japan plans to use the 5.8 GHz tag to implement operational management systems for logistics; payments for buses, taxis, and ferry boats; parking services; drive-through shopping; and gasoline purchases. Accordingly, a multipurpose integrated circuit card, which is inserted into the onboard equipment, has been adapted as part of their DSRC standard [13]. Table 7.3 lists the recommendations of an Aeronautical Radio, Inc. (ARINC) study for partitioning the 75-MHz band centered around 5.8875 GHz into eight channels, each 6 MHz wide [14]. Each channel serves a group of applications using shared transceiver and antenna equipment. Although only 48 MHz is occupied by the eight channels, the remaining spectrum is required to ensure that DSRC operators have some mobility in selecting frequencies to avoid interference to or from other users. The additional spectrum is also needed to accommodate several possibly competing DSRC technologies, which may require different channel bandwidths. Proposals, such as the CEN TC 278 Road Traffic and Transport Telematics (RTTT) Dedicated Short-Range Communications (DSRC): Physical Layer Using Infrared at 850 nm, Internal Technical Report 278/9/#63, contain specifications for a physical layer based on infrared communications [15]. Wavelengths in the 800 to 900 nm near infrared are defined to transfer information from the onboard equipment to the roadside reader as described further in Section 7.3.6. The infrared proposal has been superceded by CEN/TC 278, ENV 12253, which specifies 5.8 GHz (nominal) as the transmitting frequency. An infrared DSRC system, developed in Europe by Entwicklung For- schung and Konstruktion GmbH (EFKON), is operating in Malaysia, Tai- wan, and Brazil [4]. The tag is activated when interrogated by a signal transmitted by an overhead reader. This contactless tag, estimated to cost drivers $3, is also marketed as an ATM card, public transit payment card, Transponders and Standards for Dedicated Short-Range Communications 365 and general charge card. During tests in Austria on the A9 highway over four years, a vehicle was equipped with six tags, which were read in real time while traveling at highway speeds. Even with icing, rain, snow, fog, spray, and lightning, EFKON guarantees 99.95 percent transmission reliability.

7.3.2 Data Link Layer Standards ASTM PS 105-99, Dedicated Short-Range Communication (DSRC) Data Link Layer, contains requirements for OSI Layer 2 [16]. It applies to active and backscatter transmission and allows for interoperability between systems that incorporate either of them. A discussion of this standard is found in Sec- tion 7.3.7. The CEN standard that applies to the data link layer is Road Transport and Traffic Telematics (RTTT) Dedicated Short-Range Communica- tions (DSRC)-DSRC Data Link Layer: Medium Access and Logical Link Con- trol, CEN/TC 278, ENV 12795 (1998).

7.3.3 Application Layer Standards IEEE 1455-1999, Message Sets for Vehicle/Roadside Communications, describes message set formats used in the application layer for transmitting information between transponders and readers in support of several ITS applications [6, 7]. This standard is discussed further in Section 7.3.8. The CEN standards that govern the application layer are Road Transport and Traf- fic Telematics (RTTT) Dedicated Short-Range Communication (DSRC): Appli- cation Layer, CEN/TC 278, ENV 12834 (1998), and Road Transport and Traffic Telematics (RTTT) Dedicated Short-Range Communication (DSRC) Automatic Fee Collection: Application Interface Definition for Dedicated Short- Range Communications, CEN/TC 278, ENV ISO 14906 (1998).

7.3.4 Multilayer Standards The California Department of Transportation (Caltrans) Compatibility Spec- ification for Automatic Vehicle Identification Equipment contains elements that concern the physical, data link, and application layers. It requires readers and transponders to comply with a two-way communication protocol for passive tags [17]. Known as Title 21, it was signed into state law in July 1992. The Caltrans specification also defines a set of data records to support toll operations. Additional information is found in Section 7.3.9. 366 Sensor Technologies and Data Requirements for ITS

In an attempt to coordinate the requirements of ETTM stakeholders, ITS America Standards and Protocols Committee’s ETTM User Group proposed DSRC performance requirements for transponders and readers (e.g., minimum capture rate, error rate, data security and encryption practices, and the coexistence of active and passive technologies via dual-use hardware); mounting, field of view, tag size, and connector interfaces; and other parameters associated with the physical, data link, and application layers of the OSI model [18]. A near-term operating frequency of 915 MHz was recommended for the physical layer, with future migration to 5.8 GHz to ensure compatibility with Europe and Japan. Data rates of 600 kbit/s were proposed, subject to further evaluation of their capability to support ETTM applications at required accuracy rates, transponder cost, and availability. The data protocol defined by the data link layer is synchronous transmission of binary data through a time division multiple access (TDMA) code, since this scheme supports both lane-specific and open-road communications.

Table 7.4 Physical, Data Link, and Application Layer Parameters Specified by ITS America ETTM User Group

Layer Specified Parameters

Physical Operating frequency, data rate, modulation type, polarization, power density, (OSI Layer 1) read/write field strength threshold, active tag transmit power, passive tag echo area.* Data link Data transmission format as defined in Figure 7.5, data transmission protocol, (OSI Layer 2) tag wake-up signal, polling message to define transaction type, data order, error checking code, data header format. Application Tag memory size, type, programmability; memory size and type for ETTM and (OSI Layer 7) CVO/EC† read/write data; data message format; reader-to-tag message formats: request tag ID format, request ETC data format, write ETC message format, request traffic management data format, write traffic management message format, request CVO/EC data format, write CVO/EC message format, serial interface control message, sign-off message; tag-to-reader message formats: send tag ID, send ETC data, send traffic management data, send CVO/EC data, send serial interface data, confirm write message, write ETC data, write traffic management data, write CVO/EC data; coding of read-only data: reader-to-tag downlink, tag-to-reader uplink, tag features, state/region/province/territory/dis- trict code, group/agency code, control features, vehicle type, HOV status, vehicle axles, vehicle class.

* Echo area = change in backscatter area × backscatter antenna gain. † CVO/EC = commercial vehicle operations/electronic clearance. Transponders and Standards for Dedicated Short-Range Communications 367

Table 7.4 summarizes the physical, data link, and application layer parameters defined by the ITS America ETTM User Group. While the proposal incorporates features of existing systems, it renders most of them incompati- ble with the proposal. ASTM attempted to reach consensus on another multilayer proposal that exploited the TDMA protocol, namely the Standard for Dedicated, Short-Range, Two-Way Vehicle to Roadside Communications Equipment, Draft 6: February 23, 1996 [19]. Although this proposal was eventually superseded by separate ASTM and IEEE specifications for Layers 1, 2, and 7, it is discussed further in Section 7.3.10, since it is utilized in several ETC installa- tions. Bosch Telecom, Alcatel CGA Transport, and Combitech Traffic Sys- tems developed a Global Specification for Short-Range Communication based on CEN standards for the physical, data link, and application layers [20]. Its purpose is to ensure interoperability among DSRC passive transponders and readers manufactured by different companies through the standardization of Layers 1, 2, and 7 parameters and values. Additional information is contained in Section 7.3.11.

Figure 7.5 Reader-to-tag (downlink) and tag-to-reader (uplink) synchronous communications proposed by ITS America ETTM User Group. 368 Sensor Technologies and Data Requirements for ITS

In response to the U.S. FCC approval of the 5.850 to 5.925 GHz DSRC band, proposed standards from several companies and organizations are being reviewed by ASTM to define the physical and data link layers. Three standards were under consideration: the Association of Radio and Industry Businesses (ARIB) (Japan) T-xx, Motorola FreeSpace, and IEEE 802.11a/Orthogonal Frequency Division Multiplexing (OFDM). The ARIB option was subsequently eliminated and additional testing of the remaining two proposals was scheduled to assist in making a final decision.

Table 7.5 Key Operating Parameters in the Bluetooth Radio Transceiver Specification

Parameter Value

Carrier frequency band 2.4000 to 2.4835 GHz (defining 79 channels) in the United States, Europe, and most other countries* Transmitter output power Class 1: 1 to 100 mW; Class 2: 0.25 to 2.5 mW; Class 3: 1 mW Transmission range Class 1 and 2: 0.1 to 10 m; Class 3: up to 100 m Carrier modulation Gaussian frequency shift keying (GFSK) Communications type Spread spectrum with frequency hopping at 1,600 hops/s Network Point-to-point, point-to-multipoint, piconets,† scatternets‡ Message data Packet switched with a packet length of 0 to 2,745 bits, single or multipacket transactions Gross data bit rate 1 Mbit/s Maximum data transfer rate Asynchronous data channel: 723.2 kbit/s downlink, 57.6 kbit/s uplink, or 433.9 kbit/s symmetric Synchronous voice channel (up to 3): 64 kbit/s per channel (each direction) Bit error rate 0.1 percent at –70 dBm received signal power Error correction Forward error correction 1/3 rate Forward error correction 2/3 rate Auto retransmission query Parity and check scan Voice encoding/decoding Continuous variable slope delta (CVSD)

* Exceptions are Spain (2.445 to 2.475 GHz), France (2.4465 to 2.4835 GHz), and Japan (2.471 to 2.497 GHz). † Two or more devices sharing the same channel form a piconet. ‡ Multiple piconets with overlapping coverage areas form a scatternet. Transponders and Standards for Dedicated Short-Range Communications 369

Approximately 2,000 data and telecommunications companies around the world are utilizing the Bluetooth open technology standard to achieve voice and data transmission compatibility among wireless digital devices [21– 24]. The standard includes specifications for OSI Layers 1– 3. Table 7.5 shows the key operating parameters in the Bluetooth radio transceiver specification. Proponents of Bluetooth for DSRC claim better support for legacy technology and low-cost implementation, since the proposed chipsets for the tag and reader are already used in smart card applications, personal comput- ers, wireless telephones, pagers, and other digital devices that require synchronized data transfer. Other multilayer standards are also used to govern passive tag operation. They are ISO 10374-1991 Standard for Intermodal Freight Containers (see Section 7.3.12), Association of American Railroads (AAR) S-918 Stan- dard for Automatic Equipment Identification, American Trucking Associa- tion Standard for Automatic Equipment Identification, American National Standards Institute (ANSI) MH5.1.9-1990 Standard for Automatic Identifi- cation of Freight Containers, International Air Transport Association (IATA) Recommended Practice 1630 (Use of Radio Frequency Technology for the Automatic Equipment Identification of Unit Load Devices), and the CEN EN 10374 Standard for Automatic Container Identification for European Union, Norway, and Switzerland.

7.3.5 ASTM PS 111-98 Standard for the Physical Layer ASTM PS 111-98, Standard Specification for DSRC Physical Layer Using Microwave in the 902 to 928 MHz Band, allows for interoperability between active and passive tag technologies and for mixed time, frequency, and space division multiple access codes. The standard defines the physical layer for both wide-area (multilane, open road) and lane-based applications as an air interface, which uses half-duplex 902 to 928 MHz band RF for data transmission. ASTM PS 111-98 is compatible with a data link layer that uses a TDMA messaging protocol in which both the downlink and uplink are completely controlled by the roadside reader. The functions of other OSI model layers are included where necessary in the data link and application layers [2]. ASTM PS 111-98 specifies uplink and downlink frequencies and radiated power levels for active and passive DSRC systems. The band utilization is shown in Figure 7.6. Table 7.6 lists selected downlink and uplink parameters and values from ASTM PS 111-98. 370 Sensor Technologies and Data Requirements for ITS

Figure 7.6 902 to 928 MHz band utilization as specified in ASTM PS 111-98. Source: Standard Specification for Dedicated Short-Range Communication (DSRC) Physical Layer Using Microwave in the 902 to 928 MHz Band, Designation PS 111-98, 2000 Annual Book of ASTM Standards, Vol. 04.03 (West Conshohocken, PA: ASTM, 2000). Copyright © ASTM. Reprinted with permission. Transponders and Standards for Dedicated Short-Range Communications 371

Table 7.6 Selected Downlink and Uplink Parameter Values from ASTM PS 111-98 Standard

Parameter Downlink Value (RSE to OBE) Uplink Value (OBE to RSE)

Carrier frequency 902 to 904 MHz: unmodulated. Active: 915.000 MHz. 915.00 to 915.75 MHz: Class A Backscatter: OBE to reflect and modulated. modulate the RSE carrier with 912.75 to 918.75 MHz: Class B the modulated subcarriers. The modulated. OBE to generate the subcarrier frequency = 2.0 MHz. Carrier frequency tolerance Active (fixed): ±275 ppm.* Subcarrier: ±1,000 ppm. Backscatter (fixed): ±40 ppm. Active uplink carrier: ±819 ppm. Portable and handheld: ±275 ppm. OBE minimum operating Active: 915.00 to 918.75 MHz. Not applicable. frequency range Dual mode and backscatter: 912.75 to 918.75 modulated, 902 to 904 MHz and 909.75 to 921.75 MHz unmodulated. RSE receiver RF bandwidth Not applicable. Active: 3 MHz (nominal). Backscatter: 6 MHz (nominal). Equivalent isotropic radia- ≤ +40 dBm for Class A modulation. Active: Maximum of 3 ±3 dBm tion power (EIRP) ≤ +40 dBm or ≤ +44.77 dBm for for a range of 0 to +6 dBm (170 to Class B modulation depending on 350 mV/m at 1 m with a 0 dBi exact carrier frequency. horizontally polarized antenna). Backscatter (maximum single sideband EIRP): OBE antenna shall have a 45 to 100 cm2 delta RF cross-section. Antenna polarization Horizontal linear or left-hand circu- Horizontal linear. lar. Modulation Two-level AM. Active: Two-level AM. Backscatter: Multiplication of modulated subcarrier with carrier. Subcarrier: Modulation order phase shift keying, encoded data synchronized with subcarrier. Data coding Manchester. Active: Manchester. Backscatter: NRZI† Bit rate 500 kbit/s. 250 kbit/s (alternate: 125, 500, or 1,000 kbit/s). 372 Sensor Technologies and Data Requirements for ITS

Table 7.6 (continued) Selected Downlink and Uplink Parameter Values from ASTM PS 111-98 Standard

Parameter Downlink Value (RSE to OBE) Uplink Value (OBE to RSE)

Clock tolerance ±100 ppm. ±450 ppm. Bit error rate (BER) 10–6 in a nonfading channel (refer- 10–6 in a nonfading channel (ref- ence). erence). Signal-to-interference ≥15 dB at OBE. RSE must provide a BER ≥ 10–5 ratio with a signal-to-interference ratio of: Active Class A modulation: ≥15 dB for interference frequencies between 909.75 and 921.75 MHz. Active Class B modulation: ≥15 dB for interference frequencies between 912.75 and 918.75 MHz. ≥6 dB for interference frequencies ≤912.75 and ≥918.75 MHz. Backscatter: ≥6 dB. Slow wakeup process ≤50 ms with a required signal Minimum transmission time for for OBE strength ≥210 mV/m (–30 dBm OBE wakeup: 1.1 ms repeated at with 0 dBi antenna) horizontally least every 10 ms. polarized and with a downlink BER = 10–5. ≤50 ms with a maximum signal strength of 9,377 mV/m (+3 dBm with 0 dBi antenna) horizontally polarized and with a downlink BER = 10–5. Fast wakeup process ≤2.0 ms with a required signal Minimum transmission time for for OBE strength between 450 mV/m OBE wakeup: Shortest frame (–23.38 dBm with 0 dBi antenna) control message downlink trans- and 550 mV/m (–21.63 dBm with mission time including all pream- 0 dBi antenna) horizontally polar- ble and extended header bits. ized and with a downlink BER = 10–5. ≤2.0 ms with a maximum signal strength of 9,377 mV/m (+3 dBm with 0 dBi antenna) horizontally polarized and with a downlink BER = 10–5. Transponders and Standards for Dedicated Short-Range Communications 373

Table 7.6 (continued) Selected Downlink and Uplink Parameter Values from ASTM PS 111-98 Standard

Parameter Downlink Value (RSE to OBE) Uplink Value (OBE to RSE)

Time out Not applicable. 100 ms. Other defined parameters Transmitter spectrum mask. Transmitter spectrum mask. in ASTM PS 111-98 In-band and out-of-band interfer- Antenna beamwidth. ence. Antenna beam orientation. Reader operating range in terms of Antenna position tolerance. bit error rates and signal strength. Data modulation order. Preamble for data frames. Maximum off-carrier to minimum on-carrier ratio. Sideband suppression and isola- tion. In-band and out-of-band interference. Preamble for data frames.

* ppm = parts per million. † NRZI = nonreturn to zero with invert on ones encoding.

7.3.6 CEN TC 278 Internal Technical Report 278/9/#63 (Physical Layer Using Infrared at 850 nm) CEN TC 278 Road Traffic and Transport Telematics Dedicated Short-Range Communications: Physical Layer Using Infrared at 850 nm, Internal Technical Report (ITR) 278/9/#63, documents a consensus on a DSRC standard, but no ballot taken. The ITR describes an OSI physical layer operating in the near infrared that exchanges information between roadside equipment and onboard equipment [15]. The required parameters are defined in Table 7.7 and their corresponding downlink (RSE to OBE) and uplink (OBE to RSE) values are given in Table 7.8. In addition to these quantities, the technical report proposes energy loss budgets to ensure that adequate safety margins exist for reliable transmission of information between the RSE and OBE. The components that contribute to the path losses and typical loss values are shown in Table 7.9, while the loss path is illustrated in Figure 7.7. 374 Sensor Technologies and Data Requirements for ITS

Table 7.7 Parameter Definitions used in CEN TC 278 ITR 278/9/#63 (Infrared Physical Layer)

Parameter Definition

Transmission wave- Lower and upper optical wavelengths, which describe the optical band- length range width of an IR radiator. The lower and upper optical wavelengths are those where the peak intensity is reduced by 3 dB from its peak value. Temperature and component deviations are to be accounted for in this definition.

Coherence length (lc) Square of the transmitter wavelength divided by the minimal relevant λ2 ∆λ transmitter bandwidth (i.e., lc = / ). Visible light radiant Intensity of energy between 380 nm and 780 nm in which the RSE and intensity OBE must operate. Transmitter bandwidth Bandwidth where the peak radiant intensity is reduced by 3 dB in the RSE or OBE. Receiver sensitivity The range between the maximum and minimum optical wavelengths that bandwidth reduces the peak sensitivity of the RSE or OBE by 10 dB. Maximum radiated Transmitters must be classified into one of the maximum permissible peak intensity exposure classes—1, 2, 3A, 3B, or 4—according to IEC 825, depending on the potential danger to the cornea. Modulation Changing the shape of the transmitter waveform by coded data. Modula- tion methods include amplitude shift keying–on/off shift keying (ASK– OOK), phase shift keying (PSK), frequency shift keying (FSK), and amplitude modulation (AM). Duty cycle Transmitted pulse time divided by the bit time. Edge steepness of Edge steepness is defined by the rise time and fall time of the pulse. Rise transmitted pulse time is the time required to increase from 10% to 90% of the final value of the pulse amplitude. Fall time is the time required to decrease from 90% to 10% of the final pulse value. Data coding Manner in which information is captured in the baseband signal (i.e., the mapping of logical bits to the physical signals). Examples are biphase coding (such as Manchester, frequency modulation, and differential Manchester), nonreturn to zero (NRZ), and nonreturn to zero with invert on ones encoding (NRZI). Bit rate Number of bits per second, independent of the data coding. Clock tolerance Maximum deviation of the bit clock caused by any impact and expressed in parts per million (ppm) or in percent. Thus, a tolerance of 1% of 500 kbit/s permits the bit clock rate to lie in the range 500 ±5 kHz. Transponders and Standards for Dedicated Short-Range Communications 375

Table 7.7 (continued) Parameter Definitions used in CEN TC 278 ITR 278/9/#63 (Infrared Physical Layer)

Parameter Definition

Bit error rate Used as a reference value for Layer 1, BER is the averaged number of (BER) erroneous bits compared with all transmitted bits. The realized BER depends on the application. The effective BER within the communication zone may be different from the reference value due to time-varying and stochastic effects. Wakeup process A defined wakeup burst frequency, which switches a battery-powered for OBE OBE from sleep mode to active mode. Maximum start time Maximum time between the reception of a downlink wakeup signal by the OBE and the time at which the OBE switches to active mode and is ready for operation. Irradiance limits Maximum and minimum values of the optical wavelengths received by for the communica- the RSE or OBE. These values also govern the dynamic range of the RSE tions zone and OBE receiver sensitivity. Receiver sensitivity Minimum irradiance value at the front of the RSE or OBE. Irradiance is optical power density measured in units of W m–2. Receiver recovery time Maximum time delay to achieve 90% of RSE or OBE receiver sensitivity after a large signal saturates the receiver input. Optical interference by Maximum peak optical interference noise level caused by natural (e.g., natural or artificial sunlight illumination) or artificial sources in front of the receiver that sources increase the BER from 10–7 to 10–6.

Table 7.8 Downlink and Uplink Parameter Values Specified in CEN TC 278 ITR 278/9/#63 (Infrared Physical Layer)

Parameter Downlink Value (RSE to OBE) Uplink Value (OBE to RSE)

Transmission wavelength 800 nm to 900 nm (alternate: 900 800 nm to 900 nm (alternate: 900 range nm to 1,000 nm) nm to 1,000 nm)

Coherence length (lc) <1 mm <1 mm Visible light radiant <100 mW/sr <30 mW/sr intensity Transmitter bandwidth <100 nm <100 nm 376 Sensor Technologies and Data Requirements for ITS

Table 7.8 (continued) Downlink and Uplink Parameter Values Specified in CEN TC 278 ITR 278/9/#63 (Infrared Physical Layer)

Parameter Downlink Value (RSE to OBE) Uplink Value (OBE to RSE)

Receiver sensitivity 800 nm to 900 nm (alternate: 900 800 nm to 900 nm (alternate: 900 bandwidth nm to 1,000 nm) nm to 1,000 nm) Maximum radiated peak 100 W/sr 30 W/sr intensity Modulation ASK–OOK ASK–OOK Duty cycle 0.05 to 0.2 0.05 to 0.2 Edge steepness of transmitted pulse: Rise time <100 ns < µs Fall time <100 ns <1 µs Data coding NRZI – IR NRZI – IR Bit rate 500 kbit/s (alternate: 250 to 1,000 250 kbit/s (alternate: 125, 500, or kbit/s) 1,000 kbit/s) Clock tolerance 1% 2% Bit error rate (BER) 10–7 (reference) 10–7 (reference) Wakeup process for OBE: Burst frequency 85 kHz (alternate: 50 to 100 kHz) Not applicable Frequency on time 500 µs Duty cycle 0.1 to 0.5 Maximum start time 5 ms Not applicable Irradiance limits for the communications zone: Minimum irradiance 8 mW/m2 (alternate: 2 mW/m2) 2 mW/m2 (alternate: 0.2 mW/m2) Maximum irradiance 24 W/m2 24 W/m2 Receiver sensitivity 8 mW/m2 (alternate: 2 mW/m2) 2 mW/m2 (alternate: 0.2 mW/m2) Receiver recovery time <0.5 ms <0.5 ms Optical interference: From natural sources 100 W/m2 100 W/m2 From artificial sources 0.6 mW/m2 0.3 mW/m2 Transponders and Standards for Dedicated Short-Range Communications 377

Table 7.9 Downlink and Uplink Loss Budget Values Specified in CEN TC 278 ITR 278/9/#63 (Infrared Physical Layer)

Value

Component No Sunlight Direct Sunlight Comment

Minimum radiant RSE: 20 W/sr (downlink) Minimum radiated peak intensity at intensity OBE: 10 W/sr (uplink) which data are transmitted from RSE to OBE (downlink) or OBE to RSE (uplink) within the communication zone. Transmitter real- 1 dB Includes effects of component devi- ization margin ations, full operational temperature range, and misalignment. Aging loss 2 dB Maximum aging effect of IR transmitter device over 10-year lifetime. Path loss 20 dB* Equal to 10 log R 2, where R = RSE- to-OBE range. Additional atmo- 9 dB 1.6 dB Includes absorption and scattering spheric loss from rain, fog, dirt, and frost. Windscreen loss 7 dB Includes losses from metal-coated windows. Receiver realiza- 1 dB Includes effects of component devi- tion margin ations, full operational temperature range, and misalignment. OBE or RSE 0 dB 7.4 dB (varies with Based on 1,120 W/m2 sunlight irra- receiver sensitivity receiver technology) diance. loss from sunlight

* 20 dB corresponds to a communication distance of 10 m, where a 1-m distance is referenced to 0 dB.

7.3.7 ASTM PS 105-99 Standard for the Data Link Layer ASTM PS 105-99, Standard Provisional Specification for Dedicated Short- Range Communication (DSRC) Data Link Layer, defines the data link layer irrespective of the physical medium. (ASTM Standards PS 111-98 and PS 105-99 can be ordered by contacting ASTM at www.astm.org.) However, it assumes a bit rate equal to 500 kbit/s for both wide-area (multilane, open road) and lane-based applications [15]. The standard applies to active and 378 Sensor Technologies and Data Requirements for ITS

Figure 7.7 Downlink (RSE to OBE) and uplink (OBE to RSE) infrared loss budget paths. passive backscatter technologies and allows for mixed time, frequency, and space division multiple access data transmission. Synchronous and asynchronous medium access control (MAC) modes are allowed by the standard, which also addresses rules and conventions, data flow control procedures, acknowledgment procedures, error control procedures, services provided to data link users, and fragmentation. Both MAC modes support TDMA half-duplex communications combined with a slotted aloha protocol for activation. The preferred mode is specified by the application layer residing in the RSE and is transmitted to the OBE as part of the message control protocol. The synchronous mode utilizes a contiguous set of frames that is transmitted continuously with fixed polling, data communications, and activation phases. The asynchronous mode varies the Transponders and Standards for Dedicated Short-Range Communications 379

Figure 7.8 Generic frame structure defined by the ASTM PS 105-99 data link layer. transmission of the polling sequence, activation attempts, and data communications. All transmissions by the RSE or OBE consist of a preamble and a frame. The preamble is an 8-bit sequence used for bit synchronization, as specified in the physical layer. The frame, shown generically in Figure 7.8, is a data link layer entity that results from encapsulation of an application protocol data unit (APDU) or a portion of an APDU if it cannot be sent in a single transmission. An APDU is delivered from the application layer to the data link layer. APDUs are defined through a beacon service table (BST), which specifies the communications profile (e.g., bit rate, transmitted power, differentiation of sideband data in passive tag operation) for active and backscatter technologies utilizing fast or slow wakeup options. 7.3.7.1 Synchronous Transmission In synchronous transmission, a TDMA frame is divided into slots that transmit information in message control, transaction, or activation frames. An example of a TDMA frame is shown later in Figure 7.15, where the slotted aloha TDMA synchronous frame structure is defined [5]. The slots are combined to form a continuously repeated sequence of up to seven contiguous TDMA frames. The first frame in every TDMA frame sequence contains message control (also called frame control) information broadcast from the RSE that allocates the remaining frames in the sequence to data transfer transactions or activation. A data frame either uplinks or downlinks data. The data frame is allocated time for both data transfer and a corresponding acknowledge- ment. The activation frame consists of a series of activation windows that permit an OBE to request a private communications channel to the RSE. The message control frame in Figure 7.9 indicates the TDMA frame configuration through information contained in two fields: the frame structure identifier (FSI) and the slot allocation table (SAT). These are located between the logical link control (which supports “no response” and “response expected” services from the OBE data link layer) and the APDU field. The 380 Sensor Technologies and Data Requirements for ITS

Figure 7.9 Message control frame defined by ASTM PS 105-99 for synchronous data transmission.

FSI indicates the number of slots that are controlled by the SAT. The SAT consists of one, three, or six slot control identifier (SCI) fields, which allocate specific link addresses and one of four frame types to a slot. The four frame types supported by an SCI field are a message data slot, activation slot, empty slot, and idle signal slot. The empty slot is used, for example, to complete the specification of a six-slot SAT when activation of only five slots is required. An idle slot indicates transmission of “ones” during the time when a data message would normally be transmitted in a message data slot. Figure 7.10 provides an example of a synchronous transmission sequence that contains a full link negotiation followed by a read/write operation. In TDMA Frame 1, the OBE receives a BST from the RSE and decides to activate. The activation request is also transmitted in TDMA Frame 1. In TDMA Frame 2, the message control frame designates a downlink message data slot to obtain the link parameters. After the OBE transmits the link parameters in Frame 2, the RSE commands the OBE to support a read in TDMA Frame 3. In TDMA Frame 4, the message control frame designates an uplink message data slot to read the data. TDMA Frame 5 contains a message control frame that designates a downlink message data slot to write data to the OBE. An acknowledgment is also transmitted from the OBE to the RSE as part of Frame 5. 7.3.7.2 Asynchronous Transmission When the asynchronous MAC mode is activated, the frames that control link operation or convey data are not transmitted in synchronized slots allocated by a message control frame. Instead, they are allocated as needed by the RSE, which transmits a message control frame tailored for asynchronous operation, as shown in Figure 7.11. Here the FSI indicates that the link is operating asynchronously. Therefore, there is only one slot that is controlled by the SAT, which contains only one slot control identifier field. A message control frame allocates three activation slots in the asynchronous mode. Transponders and Standards for Dedicated Short-Range Communications 381

Figure 7.10 Sample read/write operation using the synchronous mode defined by ASTM PS 105-99.

Asynchronous data message frames are identical to synchronous data message frames with the addition of a link address field. The link address field defines the transmission between the RSE and OBE as either private (between the RSE and a single OBE) or broadcast (between the RSE and all OBEs within the RSE’s transmission range). 382 Sensor Technologies and Data Requirements for ITS

Figure 7.11 Message control frame defined by ASTM PS 105-99 for asynchronous transmission.

Figure 7.12 contains an example of an asynchronous transmission between an RSE and OBE that negotiates the link parameters for a read/ write operation. As in synchronous transmission, the OBE receives the BST from the RSE and attempts to activate. The activation frame is transmitted in activation windows that follow the message control frame. Once activa-

Figure 7.12 Sample read/write operation using the asynchronous mode defined by ASTM PS 105-99. Transponders and Standards for Dedicated Short-Range Communications 383 tion is established, the RSE commands the OBE to transmit link parameters and allocates an uplink window for the transmission. After the link parameters are transmitted, the RSE commands the OBE to support a read operation and allocates an uplink window for transmission of the read response. After the OBE transmits the data, the RSE writes data to the OBE and receives a reply acknowledging the success of the operation.

7.3.8 IEEE 1455-1999 Standard for the Application Layer IEEE 1455-1999 addresses the application layer by defining commands and message sets for a DSRC device [6, 7]. The standard is designed to maximize compatibility with open-road and lane-based tolling and other standards such as those developed by CEN for Layer 7, ASTM standards, and Title 21 migration toward IEEE standard messaging. The IEEE 1455 Layer 7 standard provides the services and protocols needed to transfer resource-specific commands and responses across an air interface from the transponder to the reader/antenna. (IEEE Standard 1455-1999 can be ordered by contacting the IEEE at 345 East 47th Street, New York, NY 10017-2394.) Cooperation between IEEE 1455 Layer 7 and the ASTM Layer 2 standards is required to enable high-efficiency data and message transfers. Compliance with CEN Layer 7 is achieved by a 1455-compliant transponder through incorporation of a subset of CEN-specified services. As some CEN features were deemed unnecessary during the development of 1455, compliance was maintained by assigning fixed values to the appropriate command fields. Transponder cost was minimized by assigning functionality to the roadside reader rather than the tag. Backward compatibility is supported by preserving the basic architecture of existing products, allowing existing fixed data fields to be placed in protected memory pages, and allowing messages to be defined to mimic existing data structures. The IEEE standard assigns privacy responsibility to the ITS applications that reside above Layer 7, as illustrated in the DSRC architecture of Figure 7.3. Security-related features incorporated into the standard are as follows:

• Data protection using access controls on transponders and page identifiers • Data privacy using access controls or encryption of messages • Data authentication using access controls, encryption of message bodies, or inclusion of digital signatures in messages 384 Sensor Technologies and Data Requirements for ITS

Figure 7.13 IEEE 1455-1999 resource manager interfaces. Source: IEEE Standard for Message Sets for Vehicle/Roadside Communications, IEEE Standard 1455-1999, Institute of Electrical and Electron- ics Engineers, New York, NY, 1999. Copyright © 1999 IEEE. All rights reserved.

The data marketplace is supported by a set of application-independent transponder commands. Manufacturer-specific test commands may be implemented within the standard. The transponder commands and application-specific messages are implemented through read-only, fixed-format memory areas; sharing of public memory by the different applications; shared access to credentials; and incorporation of all current billing processes with provisions for adding new approaches.

Table 7.10 Message classes supported by IEEE 1455-1999

Message Class Number of Messages in Class

Electronic toll and traffic management 4 Commercial vehicle management 9 Common utility 3 Private (unreserved) Messages are uncontrolled Private (reserved) None as yet defined Transponders and Standards for Dedicated Short-Range Communications 385

Figure 7.14 IEEE 1455-1999 message protocol.

A resource manager, shown in Figure 7.13, is defined to perform the following tasks:

• Isolate the transponder from application-specific data requirements • Manage the transponder resources (e.g., RF interface; controller; read-only memory; read/write memory; displays; and digital interface to the controller, extended read/write memory, and any onboard devices) to ensure compatibility among multiple back office applications

Application messages provide the structure to store information into the transponder’s memory for retrieval by a reader or an onboard device. The messages are designed to satisfy nonredundant user requirements. Five classes of messages are currently defined, as shown in Table 7.10. The message protocol is displayed in Figure 7.14.

7.3.9 California Title 21 DSRC Standard Title 21 defines a half-duplex passive backscatter protocol for two-way transmission that supports open-road and lane-based communications. The functions of the reader are to activate the transponder, poll it for specific information, and provide an acknowledge message to the transponder after a valid response to the polling message is received [17]. Transponders are encoded with unique identification data and other application-specific data. When passing through a reader zone, the transponder transmits the coded data upon receipt of a valid reader polling command. The transponders can read and write information. Table 7.11 gives specifications for the Title 21 compatible readers and transponders. Unlike the ASTM and CEN standards, California Title 21 specifies a 300 kbit/s data transmission rate. The communications protocols for a reader polling message and a reader acknowledge message are shown in Tables 7.12 and 7.13, respectively. The protocol for a transponder data message for toll collection is shown in Table 7.14. 386 Sensor Technologies and Data Requirements for ITS

Table 7.11 Reader and Passive Transponder Specifications from California Title 21 Standard

Parameter Characteristic

Reader: Trigger signal 33 µs of unmodulated RF Carrier frequency 915 ± 13 MHz Carrier modulation Unipolar amplitude shift keying, Manchester encoded Data bit rate 300 kbit/s Number of data bits Application specific Field strength at transponder antenna 500 mV/m minimum* Transponder: Technology type Modulated backscatter Antenna polarization Horizontal Field of view Operational within 90-deg conical angle Location Front of vehicle Carrier frequency Same as in reader send mode Carrier modulation Subcarrier amplitude modulation Subcarrier modulation Frequency shift keying Subcarrier frequencies 600 kHz ± 10% and 1,200 kHz ± 10% Data bit rate 300 kbit/s Number of data bits Application specific Receiver field strength threshold 500 mV/m ± 50 mV/m minimum*

* mV/m specifications are root mean square (RMS) voltage.

Table 7.12 Reader Polling Message Protocol from California Title 21 Standard

Field Definition Number of Bits Hexadecimal Value

Header code: Selsyn 8 AA Flag 4 C Transaction record type code 16 8,000 Agency code 16 — Error detection code 16 — Total = 60 Transponders and Standards for Dedicated Short-Range Communications 387

Table 7.13 Reader Acknowledge Message Protocol from California Title 21 Standard

Field Definition Number of Bits Hexadecimal Value

Header code: Selsyn 8 AA Flag 4 C Transaction record type code 16 C000 Transponder identification 32 — number Reader identification number 32 — Transaction status code 16 — Error detection code 16 — Total = 124

7.3.10 ASTM DSRC Draft 6 Proposal for Physical, Data Link, and Application Layers Before the development of ASTM PS 111-98 for the DSRC physical layer and PS 105-99 for the data link layer, ASTM proposed a “ Draft 6” DSRC standard for an active tag that attempted to define the physical, data link, and application layers [19]. ASTM does not allow the contents of a draft standard to be used as guidance for equipment design. Therefore, the information presented in this section is merely to indicate the salient features of

Table 7.14 Transponder Data Message Protocol from California Title 21 Standard

Field Definition Number of Bits Hexadecimal Value

Header code: Selsyn 8 AA Flag 4 C Transaction record type code 16 1 Transponder identification 32 — number Error detection code 16 — Total = 76 388 Sensor Technologies and Data Requirements for ITS

Table 7.15 Active Transponder Operational Characteristics from ASTM DSRC Draft 6 Proposal

Parameter Characteristic

Carrier frequency Country and application specific Carrier modulation Unipolar amplitude shift keying, Manchester encoded Data bit rate 500 kbit/s Message data 512 data bits per TDMA packet using single or multipacket transactions Technology type Two-way active RF Transponder antenna location Application specific Polarization Transponder transmission: Horizontal Reader transmission and reception: Application specific Protocol TDMA/Adaptive slotted aloha access this proposal as they appear in several toll road applications. Unlike the ITS America ETTM User Group proposal, which also defined the physical, data link, and application layers, ASTM Draft 6 does not contain a specific RF carrier frequency or bandwidth for the physical layer. Rather, it states that the carrier frequency shall contain sufficient bandwidth to maintain system signal reliability. The frequency and bandwidth assignments are left to the telecommunications agencies in each country. ASTM Draft 6 also differs from the ITS America proposal in that ASTM Draft 6 provides specificity for the data link layer. Table 7.15 lists several of the active transponder operational characteristics found in the Draft 6 proposal. The slotted aloha TDMA data link protocol proposed for an active tag by Draft 6 is based on a cyclic structure called a frame, illustrated in Figure 7.15. The frame consists of a message control phase, a transaction phase, and an activation phase. The message control phase specifies the frame structure, synchronization, message slot assignments, transaction type, and data link control. The transaction phase indicates the transaction type and contains the slots in which the data and messages are stored. Transactions consist of internal or external data and messages that are transmitted or received and addressed or broadcast. In this synchronized transmission protocol, the reader transmits a frame control message (FCM) at the beginning of each frame to define the frame structure, enable activation, and establish synchronization with the transponders. Transponders that successfully decode the Transponders and Standards for Dedicated Short-Range Communications 389 Source: Standard for Dedicated, Short-Range, Two-Way Vehicle to Roadside Communica- Roadside to Vehicle Two-Way Short-Range, Dedicated, for Standard Source: , Draft 6: FebruaryASTM,1996). 1996 Conshohocken, 23, Reprinted (West with permission. PA: Slotted aloha TDMA synchronous frame structure. Figure 7.15 Equipment tions 390 Sensor Technologies and Data Requirements for ITS

FCM randomly choose an activation slot and send a transponder identification message to the reader. The protocol specified by the data link layer permits multiple transponders to simultaneously request permission to perform a transaction. These open-road communications are supported by a wide-area data transmission protocol containing four message slots in the transaction phase and 16 activation slots in the activation phase. Lane-based communications support one transponder at a time through a protocol containing one message slot and four activation slots. Draft 6 supports active tag interoperability among itself and tags that use the HELP Specification for Automatic Vehicle Identification Equipment and Title 21. It also supports passive tag interoperability among itself and tags compatible with ISO 10374-1991, ANSI MH5.1.9-1990, or equivalent protocols. The HELP protocol is similar to Title 21, except for the transaction phase, which specifies different time intervals for executing the events in the phase.

7.3.11 Global Specification for Short-Range Communication To ensure the interoperability of passive transponders and readers manufactured by different vendors using CEN standards, Bosch, Alcatel, and Com- bitech defined the physical, data link, and application layer parameter values for DSRC applications [20]. The resulting Global Specification for Short- Range Communication (GSS) standardizes the following:

• Procedures that support interoperability as vehicles travel through DSRC systems manufactured by different companies • Default values for physical layer parameters • Timing parameter and data flow control • Uplink and downlink window management • Application data management • Initialization procedures that establish interoperable communication • Measures that allow flexibility for operator specifications

Table 7.16 displays several of the major parameters and values found in GSS. Transponders and Standards for Dedicated Short-Range Communications 391

Table 7.16 Passive Transponder Operational Characteristics from GSS

Parameter Characteristic

Carrier frequency 5.7975, 5.8025, 5.8075, 5.8125 GHz Carrier modulation Downlink: Two-level amplitude modulation Uplink: Multiplication of modulated subcarrier with carrier Subcarrier modulation Biphase shift keying; uplink data NRZI encoded Data bit rate Downlink: 500 kbit/s ±100 ppm Uplink: 250 kbit/s ±0.1 percent Frame size Downlink window: 128 octets maximum Private uplink window: 128 octets maximum Public uplink window: 9 octets maximum Number of bits Dependent on bit stuffing Technology type Two-way passive RF Transponder antenna location Lateral center of vehicle at a nominal height of 1.5 m: Automobiles—behind rear view mirror Heavy commercial vehicles—middle of lower rim of windshield Polarization Transponder: Left hand circular Reader: Left hand circular

7.3.12 ISO 10374-1991 Standard for Freight Containers—Automatic Identification The ISO 10374 standard describes a system that uses a passive tag to auto- matically identify freight containers and electronically transfer the identity of the container and permanent related information to third parties [25]. The tags respond to an interrogating signal within the 850 MHz to 950 MHz band or within the 2,400 MHz to 2,500 MHz band. The standard gives minimum and maximum RMS signal strengths returned by the transponder in response to a request from the reader. The signal strengths are specified as a function of signal frequency for a ten-meter range and one-watt effective isotropic radiated power emanating from the reader. Equipment that conforms to the ISO 10374 standard is required to read a tag located on the container within 1 to 13 meters of the container, 392 Sensor Technologies and Data Requirements for ITS depending on the passing speed. The distance needed to discriminate between two tags is also a function of the passing speed and varies from ten meters at a passing speed of 130 kilometers/hour to 1.2 meters at 30 kilometers/hour. The discrimination distance for stationary containers is 1.5 meters. The tag can be programmed while attached to the container, with the exception of permanent data that cannot be changed while the tag is affixed to the container. The minimum information provided by the tag consists of a code that identifies the tag type in terms of content (i.e., less than basic information, basic information, more than basic information, spare), codes to identify the equipment and owner, serial number, check digit, length of container, height of container, width of container, container type code, maximum gross mass in hundreds of kilograms, and tare mass in hundreds of kilograms. The reader provides a reader identification number, date, time, and freight container movement status.

7.4 Summary

Multiple active and passive DSRC systems are currently supporting ETTM applications. The need to increase the accuracy of the transponder reads and to achieve interoperability with other systems is causing some toll authorities to switch from passive to active technologies [26]. In a passive DSRC system, transponders modulate the received carrier frequency in a manner that conveys the information on the tag and then backscatters the modulated signal to the roadside reader at a subcarrier frequency. In active DSRC systems, transponders generate their own carrier and transmit a modulated signal that contains the transponder information when they are interrogated by the reader. Standards relating to the operation of transponders and roadside readers for ETTM and other CVO applications are continuing to evolve. The many organizations participating in the ETTM standards-setting process have a common goal: develop specifications to ensure interoperability among products from different manufacturers and roadside readers that may lie in different jurisdictions and support a variety of applications. Although most developing standards are backward compatible with older standards, some are not. For example, carrier frequencies, technologies, modulation, and data transmission rates may differ among ETTM standards, as shown in Table 7.17. The OSI multilayer architecture appears to be the model of choice for developing the newer standards. This approach assists in isolating the functions of the modules or processes in the communications architecture and Transponders and Standards for Dedicated Short-Range Communications 393 10% ± 10% and ± Message Coding Active: Manchester. Passive: NRZI.Sup- time, mixed ports and frequency, space divisionmulti- ple access codes. TDMA/ Manchester. slotted adaptive access. aloha Downlink: Manches- ter. Subcarrier Uplink: of frequencies 600 kHz 1,200 kHz modulated with FSK. Synchronous protocol Message Data Message Packet Size Application specific 512 data bits per TDMA packet,single or multipacket transactions Application specific 256 bits/packet Data Bit Rate Bit Data 500 kbit/s downlink, 250 kbit/s uplink kbit/s 500 300 kbit/s downlink, 300 kbit/s uplink kbit/s 500 Table 7.17 Table Carrier Modulation AM. Active: Two-level Passive: PSK, encoded data synchronized with subcarrier. Unipolar ASK. Downlink: Unipolar ASK. Uplink: Subcarrier amplitude modulation RF carrier. the of keyed Manchester carrier Summary of Physical Layer ETTM Standards for DSRC ETTM of Physical Layer Summary 13 MHz 13 ± Carrier Frequency Carrier or Band 904 to 902 Downlink: MHz unmodulated. MHz 915.75 to 915.00 modulated.Class A MHz 918.75 to 912.75 modulated. Class B Uplink: 915.000 MHz. applica- and Country tion specific 915 nominal MHz 915 Technology Type Technology activeRF Two-way backscat-or passive RF ter activeRF Two-way passive Two-way backscatter RF activeRF Two-way Specification 111-98 PS ASTM DSRC ASTM 6 proposal Draft 21 Caltrans Title IAG 394 Sensor Technologies and Data Requirements for ITS

† 0.1%2.0 or MHz 0.1%. Message Coding switched Packet NRZI. Subcarrier frequencies of 1.5 MHz ± ± NRZI CSMA and TDMA under consideration under 512 bits Message Data Message Packet Size Packet length of 0 to or bits, single 2,745 transac- multipacket tions Number of bits depen- bit stuffing on dent Application specific ≥ 100 ppm ppm 100 ± 10 Mbit/s 10 0.1% uplink 0.1% Data Bit Rate Bit Data 1,000 kbit/s (data lower throughput kbit/s 1,000 than because of protocol overhead) kbit/s 500 500 kbit/s downlink, 250 kbit/s uplink ≥ downlink, 250 kbit/s ± (continued)

OOK – Table 7.17 Table Carrier Modulation GFSK Downlink: Two-level AM. Uplink: Subcarrier- shift keying; biphase multiplication Carrier of modulated subcar- carrier. with rier ASK FSK under and OFDM consideration * Summary of Physical Layer ETTM Standards for DSRC ETTM of Physical Layer Summary Carrier Frequency Carrier or Band GHz 2.4835 to 2.4000 (United States, Europe, countries) most and GHz 5.8125 to 5.7975 nm 900 to 800 in GHz 5.925 to 5.850 UnitedStates; 5.825 to in Canada 5.925 Technology Type Technology activeRF Two-way spectrum spread hop- frequency with chan- at 1,600 ping nels/s passive Two-way backscatter RF passive Two-way infrared activeRF Two-way Carrier sense multiple access. Specification Bluetooth on CEN (based GSS for standards Layers 1, 2, and 7) CEN TC 278 ITR 278/9/#63 (infrared) Stan- GHz 5.9 ASTM dard for Physical Layers Link Data and (in development) * are Exceptions Spain, France, and Japan. † Transponders and Standards for Dedicated Short-Range Communications 395 allows standards to be created independently for any of the layers. Some standards, such as those of ASTM, IEEE, and CEN, support interoperability among an assortment of ETTM and CVO communication protocols (e.g., California Title 21, HELP, and ISO10374) and active and passive tags.

Exercises

Answers to the exercises aare provided in Appendix L of the accompanying CD-ROM.

1. Define the different levels of interoperability. Which levels are addressed by DSRC standards? Which layers in the OSI communications model relate to each interoperability level? 2. What are the design and operational differences between passive and active tags? 3. What carrier frequencies are currently utilized in the United States, Europe, and Japan for ETC with DSRC? Which of these frequencies appears to be the choice for future applications worldwide? 4. Describe the functions of the OSI layers that are utilized in DSRC applications. 5. What standards have been proposed in the United States and Europe to guide the development and interoperability of these layers? 6. Is it possible for a motorist who possesses a DSRC tag that operates in the northeastern United States to currently drive across the United States and use the same tag to pay tolls on all highways he or she will encounter? Explain. 7. Differentiate between synchronous and asynchronous transmission protocols.

References

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