Over the past decade, as information- and computer-based systems have become larger and more complex, the importance of and reliance on systems architectures have grown substantially. A systems architecture is the framework that guides and moderates the evolution of a complex system's many interrelated elements. It provides order and rules so that hardware, software, data, and communications can work together.

"Systems architectures have emerged as the centerpieces of system development programs," notes the General Accounting Office.

The centerpiece of intelligent transportation system (ITS) development is the National ITS Architecture -- a systems architecture developed by a combined Lockheed Martin and Rockwell International team (representing the public sector, private sector, and academia) for the U.S. Department of Transportation (DOT). The architecture was completed in June 1996, after nearly three years of effort, including more than 50 stakeholder meetings, briefings, and workshops.

The architecture's development has been an important first step toward achieving the DOT's goal of encouraging states and localities to build an intelligent transportation infrastructure in metropolitan and rural areas, as well as interstate corridors. Without a systems architecture, ITS deployment would be analogous to states, cities, and counties each building small parts of a limited-access freeway with little regard for its connections to segments in other jurisdictions. The National ITS Architecture shows how individual ITS services can be linked together to create intermodal and interoperable systems that better serve travelers and system managers in all areas. The architecture achieves three specific objectives:

• Provides a theoretical and practical framework in which 30 ITS services can operate together to provide various levels of service to travelers and system operators.
• Illustrates how ITS services can be incorporated within an existing infrastructure and can accommodate future technological advances.
• Supports "open" standards that enable multiple public- and private-sector organizations to independently deploy ITS components that work together, regardless of who designed or manufactured the particular hardware or software.

The architecture is quite extensive and contains more than 5,200 pages of documentation in 18 volumes. It is also the world's first systems architecture for ITS development and deployment.

A Framework for Building ITS Infrastructure

Any complex system that consists of many technical components and subelements carries the inherent risk of technological chaos -- incompatibility, premature obsolescence, costly inefficiency. A system architecture provides a unifying framework to ensure that technologies can work together smoothly and effectively. The system architecture for a home entertainment system, for example, allows a television, videocassette recorder, audiotape deck, compact disc player, radio, headphones, speakers, and remote control, as well as future add-ons, to function as a unified system, even when individual components are designed and produced by different manufacturers.

In much the same way, the National ITS Architecture defines major ITS components and describes how system elements can interact compatibly. It provides a comprehensive technical and institutional framework that allows individual ITS services and technologies to work together and share information. More specifically, the architecture defines the following aspects of ITS infrastructure:

• Functions necessary to perform a given ITS service.
• Subsystems that enable ITS services to function (e.g., roadway or transit management center).
• Interfaces and information flows between subsystems.
• Wired or wireless communication modes available for transmitting ITS information.
• Requirements for developing standards to support national ITS interoperability.

The ITS architecture consists of a logical architecture and a physical architecture. The logical architecture defines eight major processes and associated information flows: (1) managing traffic, (2) managing commercial vehicles, (3) providing vehicle monitoring and control, (4) managing transit, (5) managing emergency services, (6) providing driver and traveler services, (7) providing electronic payment services, and (8) planning system deployment and implementation.

The physical architecture allocates the processes of the logical architecture to 19 major physical subsystems that are organized into four basic physical classes: transportation management centers, roadside equipment, vehicles, and travelers.

The physical architecture addresses three major layers of infrastructure: transportation, communications, and institutional. The transportation layer deals with the transportation infrastructure and operations of ITS. The communications layer identifies how existing and developing commercial or dedicated communications systems and open protocols can support ITS services. And the institutional layer outlines possible roles and relationships among public and private institutions -- particularly how the federal government, state and local governments, vehicle manufacturers, and information and communications service providers could work together to implement ITS services.

The Architecture's Benefits: Tinker Toys vs. Lincoln Logs

The framework provided by the architecture is inclusive, flexible, and expandable. These characteristics promote production economies by allowing ITS products and services to be assembled in a variety of ways to address a range of needs and service levels and to evolve in step with new technologies and requirements. In a sense, the architecture and the associated interface standards allow the elements of ITS infrastructure to be assembled with the flexibility and dexterity of Tinker Toys, whose standardized sprockets and uniform connectors can be easily linked together and rearranged in multiple combinations. In contrast, ITS deployment without an open architecture is analogous to building a cabin out of Lincoln Logs; the slightest change to the cabin's structure, such as putting in a new window, requires major reconstruction or, worse yet, starting over from scratch.

The architecture also allows ITS products from competing vendors to work together and ITS infrastructure implemented by one jurisdiction to work with systems in neighboring jurisdictions. As a result, "vendor lock-in," functional isolation, and obsolescence --all of which can be experienced with closed proprietary solutions -- are less likely with the adherence to a national framework.

Communications Are Key

A worker tests the communications equipment that transmits volume, average speeds, and video images of the traffic on Interstate 10 to the traffic operations center in Phoenix. Communications networks are among the fundamental structural elements that make up ITS infrastructure.

Communications networks are among the fundamental structural elements that make up an intelligent transportation infrastructure, because they serve as the paths ITS services use to access and share information. These networks link disparate ITS applications to each other and to centralized management centers, allowing for the key functions of data gathering, synthesis, delivery, and broadcast to occur in real time. Because of their importance to ITS, the architecture advocates five types of communications links:

• Wide-area broadcast, such as that provided to an automobile's FM radio receiver.
• Wide-area two-way wireless, which allows more advanced, interactive services over, for example, a cellular phone link.
• Dedicated short-range communications (DSRC), such as wireless vehicle "tags" for toll collection.
• Vehicle-to-vehicle communications, which will someday endow vehicles with collision-warning and avoidance capabilities and will play a critical role in automated highway systems.
• Wireline communications, which include regular "phone line" devices, such as phone, fax, modem, video, and high-speed data networks.

The National ITS Architecture notes that ITS communications needs can be met through a combination of owned facilities, leased services, or both, including emerging digital communications developments associated with the national information infrastructure. For the most part, as noted by a member of the architecture development team, "The choice of which particular communications technology is used becomes a local design decision."

Establishing a Common Vocabulary With Standards

Although the National ITS Architecture defines how ITS components and subsystems can be linked together, the development of standards is essential in achieving national compatibility and interoperability.

Standards and protocols establish a common vocabulary so that ITS elements can understand each other. Without them, systems conforming to the national architecture will have similar conceptual designs and, perhaps, functionality, but they may be incapable of seamless information exchange. For example, without standards for traffic controller interfaces, local authorities might have to choose proprietary solutions that are not easily upgraded or purchase expensive software customized for a particular equipment configuration. With appropriate standards, compatibility, interoperability, and expandability are much easier and more affordable.

The development of ITS standards is not a simple exercise. Four types of standards are required for many of the interfaces between ITS services and subsystems:

• Data standards provide uniform formats for data sharing across ITS applications.
• Message sets define message content and transmittal procedures among various ITS elements and subsystems.
• Communications standards, many of which already exist as industry standards, can be modified to serve the communications needs of ITS applications more effectively.
• Performance standards include guidelines that could be the basis for safety standards or "human factors" guidelines that ensure that people use ITS services effectively.

Consensus has already been secured on many ITS standards, such as the National Transportation Communications ITS Protocol (NTCIP), which facilitates wireline communication between traffic management centers and field equipment. In 1996, DOT executed cooperative agreements with five standards development organizations to accelerate the nonproprietary development of standards for other key elements of ITS infrastructure.

The Architecture's Future

Long-term stewardship of the architecture is an important goal for DOT. As a result, the department has established the ITS Architecture Task Force to guide; promote; and facilitate effective, efficient, and rapid adoption of the national architecture. Key goals include:

• Increasing awareness of the architecture among highway and transit agencies.
• Mainstreaming the architecture into transportation planning and acquisition processes.
• Making sure the architecture remains current and relevant to transportation decision-makers.
• Developing preliminary guidance on what constitutes local system conformance with the architecture.

Preliminary guidance has already been produced on how to use the architecture to integrate the components of a metropolitan ITS infrastructure. A draft guide to architecture implementation for transit applications has been completed. Four similar guides covering other applications will be completed in 1997.

A critical next step is to validate the efficacy of the architecture through field tests. The model deployments of metropolitan ITS infrastructure and commercial vehicle information systems and networks (CVISN), which were launched in 1996, will help DOT to refine or expand the architecture as a comprehensive guiding framework for building ITS infrastructure.

In addition, as proposed in the National Economic Crossroads Transportation Efficiency Act (NEXTEA), ITS projects in the future must adhere to the national architecture in order to receive federal funding. DOT is now preparing guidelines to help states and localities develop regional frameworks for ITS deployment that conform to the architecture. Even without federal guidance, some areas already see the architecture's value. For example, rather than starting from scratch, the Minnesota DOT used the national architecture as a basis from which to develop Minnesota's own architecture for ITS deployment.

Global Appeal

The National ITS Architecture is being examined and considered carefully by ITS advocates in the United States, Europe, Asia, and other parts of the world. DOT is particularly committed to achieving internationally consistent ITS architecture principles and standards, especially in North America, where implementation of compatible ITS services between the United States and its neighbors to the north and south can facilitate trade.

In the United States, the architecture is the framework that makes possible a national infrastructure of integrated, intermodal, and interoperable ITS. As such, its development is a cornerstone achievement of the national ITS program.

Lee Simmons is the systems engineering team leader in charge of the National ITS Architecture program for DOT. Previously, he worked as a civilian for 22 years on the development, production, and deployment of electronic systems to increase the Navy's aviation capabilities.