With the development of economy, traffic congestion, road blockage, traffic accidents and traffic pollution are increasingly plaguing cities around the world. The intelligent transportation system ITS (Intelligent Transportation Systems) that came into being has brought new hope to mankind by using advanced computer technology, electronic technology and communication technology to improve the efficiency of the existing transportation system. According to the definition of ITS AMERICA (Intelligent Transportation Society of America), the two basic components of ITS are the advanced traffic information system ATIS (Advanced Traveler Information Systems) and the advanced traffic management system ATMS (Advanced Traffic Management Systems). ATIS uses visual and auditory equipment to collect relevant traffic information, and then analyzes, transmits and provides information, thereby providing real-time assistance to travelers during the journey from the starting point to the end point, making the entire travel process comfortable, convenient and efficient; ATMS regards vehicles as part of the management system, using it to perceive and predict future traffic congestion, and provides the best strategy for traffic management.
The core method to ensure the operation of ITS (especially ATMS) is dynamic traffic assignment (DTA). The so-called dynamic traffic assignment is to reasonably distribute the real-time traffic flow on each section of the road network, provide travelers with departure time and mode options, provide vehicles with road guidance systems to guide vehicles to travel on the best routes, and provide mutual connection between the guidance system and the traffic control system.
In 2001, the University of Texas at Austin developed a real-time DTA system, DYNASMART-X. Based on its research results, this paper proposes a framework for a CORBA distributed real-time DTA system.
1 CORBA Technology
Since its establishment in 1989, the Object Management Group (OMG) has been committed to using object-oriented technology to make object-based software reusable, portable and interoperable in distributed heterogeneous environments. The Common Object Request Broker Architecture (CORBA) is an application software architecture and object technology specification proposed by OMG. Its core is a set of standard languages, interfaces and protocols to support interoperability between heterogeneous distributed applications and object reuse independent of platforms and programming languages.
CORBA technology is a major innovation that solves two well-known problems in system integration: (1) the difficulty of developing client/server applications; (2) the problem of quickly integrating new and old systems. It is considered to be the specification of the emerging distributed object management DOM (Distributed Object Management) technology. DOM technology provides a higher-level object-oriented interface on the basic distributed computing services. The highest-level specification is called Object Management Architecture (OMA), see Figure 1. Among them, the role of ORB is to transmit requests between other components; CORBA services provide some basic system services, such as naming, persistence and event notification; CORBA facilities include user interface, information management and other facilities; CORBA domains correspond to specific application domains, such as finance, manufacturing and telecommunications technology.
The key to integrating application objects is to use standard specifications defined by the Interface Definition Language (IDL). Once all applications and data have an interface compatible with IDL, communication will be independent of physical location, platform type, network protocol, and programming language. An information system created using CORBA arbitrates the control and information flow between these software objects.
The widely used CORBA2.0 ORB is middleware that establishes a client/server relationship between objects. Using an ORB, a client object can transparently call a method of a server object, which can be on the same machine or on a network. The ORB intercepts the call request and is responsible for finding an object, executing the request, passing parameters, calling methods and returning results. The client does not need to know the location of the object, the programming language, the operating system, or any other aspects that are not part of the object interface. It is important to note that the client/server role is only to coordinate the interaction between two objects.
2 Dynamic Traffic Assignment
The DTA system is a complex system that needs to manage historical, current, and predicted data of tens of thousands of road sections, controllers, and vehicles while ensuring real-time response to periodic and non-periodic events in the traffic system. The real-time operation of the DTA system requires the system to meet two conditions at the same time: (1) the system response avoids system failure; (2) the system response is timely. If it cannot respond in time, the system will not stop running. The computing environment and software tools are the two main factors to ensure the real-time response of a complex system.
2.1 Real-time operation mechanism
In order to meet the requirements of real-time operation, a mechanism is needed to enable the DTA system to receive measurement values in real time, start the corresponding algorithm units, and transmit the results to the corresponding external devices. Figure 2 shows this real-time operation mechanism. At the beginning of the current operation period Ti, the DTA system receives and evaluates the measurement values of the just-passed operation period Ti-1. Based on these measurement values, the entire system and the algorithm units therein respond and act in the current period. Each algorithm unit and the entire integrated system use the above mechanism in the internal logic and functional design, so that the algorithm steps and functions can be easily increased or decreased through the consistent definition of the operation period, which greatly improves flexibility.
2.2 Real-time DTA framework
The real-time DTA system consists of the following functional units: (1) consistency check; (2) consistency update; (3) OD estimation (O stands for Origin, D stands for Destination, and OD estimation is the estimation of the travel distribution matrix of the origin and destination points); (4) OD prediction; (5) state estimation; (6) state prediction; (7) traffic distribution; (8) user interface; (9) database: (10) management. These functional units interact with each other and with the ATMS database. Among them, (1) is responsible for checking the consistency between the real system and the DTA simulator and (3), mainly comparing the predicted state. The state variables and actual state variables are reported to (2) once they exceed the predetermined threshold; (2) based on the report of (1), the DTA simulator and (4) are updated; (3) based on the real-time measurements of the monitoring system and the historical OD data, the origin-destination travel matrix of the current road network is estimated; (4) based on the current OD estimation results, the current network status and the historical OD data, the OD prediction for the future period is generated; (5) the path decision of a given very short simulation interval (a few seconds) is combined with the adjustment generated by (2) to simulate the type of traffic flow; (6) the type of traffic flow for a longer period of time is simulated and provided for the future period
(7) Provide path decision according to different user requirements such as system optimization and user balance; (8) Provide user interface; (9) Minimize the waiting time of other unit requests and maximize throughput; (10) Provide control between all units to maintain system stability and prevent failures, while ensuring system synchronization. Obviously, the design of real-time DTA system should be based on a hierarchical structure. The highest layer is the management unit, and the other units are mapped to a different dedicated processor, as shown in Figure 3.
3 CORBA-Based DTA System
3.1 DTA system under the AMH framework
Multiprocessor/parallel computing is very important for real-time DTA systems. In real-time DTA systems, some functions are executed periodically; other functions are triggered by other functions non-periodically. Therefore, when designing, the most fundamental point is to grasp the execution cycle of each functional unit.
All loops can be integrated into an asynchronous multi-level AMH (Asynchronous Multi-Horizon) framework. In the AMH framework, each function is implemented on distributed processors at different levels. Each function runs in periodic mode, non-periodic mode or joint activity mode. In periodic activity mode, it is executed based on the execution cycle timing; in non-periodic activity mode, it is executed only when other functions issue an event call request; in joint activity mode, a function is executed regularly and other functions are allowed to trigger to start the operation of a new function. In other words, in the current execution cycle, when a call request is received, the operation of the new function will start from the starting point of the next execution cycle. This strategy is very important to ensure that the system responds to environmental changes in real time while maintaining the reliability and stability of the entire DTA system.
3.2 DTA system under ILU framework
It is best to use the Inter Language Unification (ILU) to implement a real-time DTA system in a CORBA environment, because ILU is shared and has better availability. ILU supports the creation of new objects, remote procedure calls, and asynchronous calls. Once an ILU object is created, it notifies the ILU server that it exists. Through this server, each object can obtain information about other objects. After that, each object can remotely access any other object, just like on the same machine.
A real-time DTA system can be composed of three main objects: operation objects, GUI, and ATMS database, as shown in Figure 4. Objects in CORBA need to be designated as servers or clients. A server is defined as an object that receives client requests and executes the request; a client is defined as an object that sends requests to a server. An object can also be designated as both a client and a server. It can both send and receive requests.
In the real-time DTA system, three objects are designated as clients and servers. Six sub-operation objects are designed under the operation object. Each sub-operation object works in one operation cycle. Two objects, consistency check and consistency update, are designed under the state estimation object because these two objects directly interact with the state estimation object. The GUI is responsible for inputting instructions and outputting results. The ATMS database includes real-time monitoring data, system output, historical data and the interactions among them.
Dynamic traffic allocation is the core means to ensure the operation of intelligent transportation systems. Designing a real-time dynamic traffic allocation system based on CORBA technology is an appropriate solution. International research in this area is still in the system testing stage, and domestic research has just started. This article makes a preliminary discussion on this framework, and hopes that more scholars will conduct in-depth research and develop a truly practical real-time dynamic traffic allocation system to promote the development of intelligent transportation systems.
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