Telecommunications infrastructure is important in enabling Intelligent Transportation Systems (ITS) to function, as it ties together and moves data between the major elements of an ITS, including roadside equipment, vehicles, the vehicle operator and central operations facilities (such as transportation management centers). Through integrating the individual elements of an ITS, telecommunications provides a critical technical function to the system, and can act as a mechanism for enhancing overall transportation efficiency. Telecommunications also comprises a significant share of the cost of an ITS, both in terms of implementation and operations and maintenance.
Arriving at the telecommunications solution that best suits agencies' needs is a high priority, but it can be a challenge. This is largely due to the rapid pace of change in telecommunications and the skills required to understand and assess different telecommunications alternatives. This report is designed to provide assistance on what processes work best and what factors should be considered when making telecommunications decisions. A number of the best techniques for exploring telecommunications alternatives are presented to help agencies determine the optimal alternative in support of their ITS program.
For this study, the telecommunications experiences of state Departments of Transportation (DOTs) and agencies from across the country were examined. In particular, examples of successful practices in ITS telecommunications were drawn from California, Georgia, Maryland, Michigan, Minnesota, Missouri, New York, Texas, Virginia, and Wisconsin.
One of the key decisions that an ITS implementing agency faces is the optimal level of distribution for the telecommunications system, ranging from a fully centralized system to a fully distributed one. A significant difference in centralized and decentralized systems pertains to the interfaces between the systems and technologies that are interconnected. More specifically, the interfaces for centralized and distributed systems will differ in:
The amount of information that is moved.
- The ability to transfer control capability from one center to another.
- The nature of the relationships between the systems that are interconnected (peer-to-peer in a distributed system and master/slave in a centralized system).
- Consider the strengths and weaknesses of a centrally controlled system. In a centrally controlled system, the network must support constant telecommunications between many endpoints and the central computer, and there is constant movement of both data and control. For example, the TransGuide Advanced Traffic Management System in San Antonio offers an example of a centralized system. It utilizes relatively simple field controllers for detector monitoring. Fiber is run from the control center to fiber hubs, which further divide services to multiple telecommunications cabinets. Data, control commands, and voice are transmitted as a single signal on each fiber. At the control center end, each of the fiber signals are demultiplexed, with data routed through further demultiplexers and intelligent analyzers. Primary computing takes place on a fault tolerant minicomputer, and telecommunications is constant, with a steady stream of status polls transmitted from the central computer to each field device.
The advantages offered by a centralized system include:
- Ease of management through use of centralized management tools.
- Ease of access for repair and modification since most processing and switching devices are centrally located.
- The opportunity to share the costs of resources such as large scale video displays or switches.
- Easier ability to take a corridor or regional approach, since data is centrally located for processing in this manner.
- Consider the strengths and weaknesses of a fully distributed system. In a fully distributed system, control is localized, and telecommunications with the central facility is less frequent. Distributed systems rely more extensively on processing resources near the field equipment, so the field processor is relatively complex and has a fair amount of local solid state memory. This type of environment requires much less capacity in the telecommunications network and usually can operate even if the field devices are disconnected from the control center. Minnesota is currently implementing a series of "virtual transportation centers" for small and medium urban areas with significant surrounding rural regions. Each center will consist of servers and workstations installed at the facilities of the individual agencies whose functions they support, networked together for sharing information. There will be no central facility or central support staff. In this environment information is shared liberally and there is the potential to integrate a range of other ITS applications, such as transit or traveler information. An example of linked remote systems would be the interconnection of the traffic signal computer(s) at the city DOT and the county department of public works, with pavement and road weather sensor information at the state DOT district office, and video signals from shared CCTV cameras on major routes and at common trouble zones.
Advantages of distributed systems include:
- Reduced costs of telecommunications and processing, since less information is transmitted and the systems rely on low cost field processing resources.
- Reduced vulnerability to a single point of failure such as a control center outage.
- Ability to be centrally managed under emergency or other relevant conditions.
- Reduced dedicated staffing, as system operation becomes one of a number of responsibilities of the operations staff at each agency's location.
The type and volume of the telecommunications workload will differ significantly based on whether a centralized or distributed system is utilized. Agencies must carefully consider which type of system best meets their needs, as their decision ultimately has a long-term impact on the agency, its network and its systems.
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