7 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 elec- tronic commercial vehicle credentials and inspection facilities in the United States and worldwide continue to grow. The data are transmitted over a wire- less dedicated short-range communications (DSRC) link. Unfortunately, transponder manufacturers and user agencies are not agreed on one transmis- sions 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 fol- lows: 1. Contractual interoperability, which ensures that data and funds are reconciled between separate service providers 2. Procedural interoperability, which ensures that there is data connec- tivity 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 tran- sponders are read-only tags, which store fixed information. Type II transpon- ders 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 appli- cation), 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 infor- mation system application, the data might contain incident location informa- tion, 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 tran- sponder range is 5 to 30 meters [2]. Backscatter modulation requires a functionally matched set of tran- sponders and readers. The transponder receives the carrier signal from the reader and reradiates a modulated form of the signal on a subcarrier fre- quency. 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 tran- sponders 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 oper- ate 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 transpon- ders, 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 transmis- sion 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 trans- mission 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 asynchro- nous transmission is utilized. Synchronous communication transmits mes- sages 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 sep- arated 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 illus- trated 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 infor- mation 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 pro- tocol 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 func- tions of each layer are properly defined, changes in one layer do not affect those of another. Thus, standards for each layer can be developed indepen- dently and simultaneously. 356 Sensor Technologies and Data Requirements for ITS The top three layers, application (Layer 7), presentation