Wiktor B. Daszczuk

Distributed algorithm for empty vehicles management in personal rapid transit (PRT) network

Summary In this paper, an original heuristic algorithm of empty vehicles management in personal rapid transit network is presented. The algorithm is used for the delivery of empty vehicles for waiting passengers, for balancing the distribution of empty vehicles within the network, and for providing an empty space for vehicles approaching a station. Each of these tasks involves a decision on the trip that has to be done by a selected empty vehicle from its actual location to some determined destination. The decisions are based on a multi-parameter function involving a set of factors and thresholds. An important feature of the algorithm is that it does not use any central database of passenger input (demand) and locations of free vehicles. Instead, it is based on the local exchange of data between stations: on their states and on the vehicles they expect. Therefore, it seems well-tailored for a distributed implementation. The algorithm is uniform, meaning that the same basic procedure is used for multiple tasks using a task-specific set of parameters. Copyright

Personal Rapid Transit (PRT) Computer Network Simulation and Analysis of Flow Capacity

Transportation problems of large urban conurbations inspire search for new transportation systems that meet high environmental standards, are relatively cheap and are user friendly. The latter element also includes the needs of disabled and elderly people. This article concerns a new transportation system - Personal Rapid Transit (PRT). In this article, the attention is focused on the analysis of the efficiency of the PRT transport network. The simulator of vehicle movement in PRT network as well as algorithms for traffic management and control are presented. The proposal of its physical implementation is also included.

TIMED CONCURRENT STATE MACHINES

Timed Concurrent State Machines are an application of Alur Timed Automata concept tocoincidence-based (rather than interleaving) CSM modeling technique. TCSM support theidea of testing automata, allowing to specify time properties easier than temporal formulas.Also, calculation of a global state space in real-time domain (Region Concurrent State Machines)is defined, allowing to store a verified system in ready-to-verification form, and tomultiply it by various testing automata.

Proposed benchmarks for PRT networks simulation

Personal Rapid Transit (PRT) is a promising form of urban transport. Its operation consists in the use of small unmanned vehicles which convey the passengers among stations within a dedicated network. Various aspects of the PRT network performance are frequently evaluated using the discrete-event simulation. The paper supports the need of establishing some reference models for the simulation of PRT networks, targeted mainly towards the needs of the research on network management algorithms. Three models of such PRT network models are proposed and discussed. The presented models can play the role of benchmarks which would be very useful for comparative evaluation of heuristic control algorithms, developed by different research groups.

Communication and Resource Deadlock Analysis Using IMDS Formalism and Model Checking

Modern static deadlock detection techniques deal with the global properties of the verified systems, using methods that explore the state space. Local features, like partial deadlocks or individual process terminations, are not easily expressed or checked by such methods. Also the distinction between communication deadlocks and resource deadlocks, common in dynamic waits-for methods, cannot be addressed or verified by static methods. An Integrated Model of Distributed Systems (IMDS) is proposed which specifies distributed systems as sets of servers’ states, sets of messages and sets of actions. The message passing/resource sharing dualism of distributed systems is provided by projections: on servers (server view) and on agents (agent view), yet the uniform specification of a verified system is preserved. A progress of computation is defined in terms of actions which change (local) states and generate messages. Distributed actions do not depend on global states and are independent from one another. Therefore, local features of subsystems can be easily described in IMDS. Communication and resource deadlocks can be handled separately and total and partial deadlocks and terminations can be distinguished from each other. Integration of IMDS with model checking is outlined and temporal formulas for deadlock and termination checking are discussed.

System modeling in the COSMA environment

The aim of this paper is to demonstrate how the COSMA environment can be used for system modeling. This environment is a set of tools based on Concurrent State Machines paradigm and is developed in the Institute of Computer Science at the Warsaw University of Technology. Our demonstration example is a distributed brake control system dedicated for a railway transport. The paper shortly introduces COSMA. Next it shows how the example model can be validated by our temporal logic analyzer.

Asynchronous Specification of Production Cell Benchmark in Integrated Model of Distributed Systems

There are many papers concerning well-known Karlsruhe Production Cell benchmark. They focus on specification of the controller—which leads to a synthesis of working controller—or verification of its operation. The controller is modeled using various methods: programming languages, algebras or automata. Verification is based on testing, bisimulation or temporal model checking. Most models are synchronous. Asynchronous specifications use one- or multi-element buffers to relax the dependence of component subcontrollers. We propose the application of fully asynchronous IMDS (Integrated Model of Distributed Systems) formalism. In our model the subcontrollers do not use any common variables or intermediate states. We apply distributed negotiations between subcontrollers using a simple protocol. The verification is based on CTL (Computation Tree Logic) model checking integrated with IMDS.

Data Model Design in Automatic Transit System (PRT) Simulation Software

Simulation has become a very important factor in the field of Automated Transit Network – Personal Rapid Transit (ATN-PRT) design. Multiple traffic conditions, as well as model structure and movement parameters lead to increase in the number of simulation experiments which must be performed to evaluate ATN control algorithms. This article aims to show some guidelines for design of such simulation systems, with particular emphasis on data model design in object oriented programming (OOP) for massive simulations. These guidelines are presented in the context of Feniks Personal Rapid Transit (PRT) simulator development, but are also valid for other graph-based simulation software.

Fairness in Temporal Verification of Distributed Systems

The verification of deadlock freeness and distributed termination in distributed systems by Dedan tool is described. In Dedan, the IMDS formalism for specification of distributed systems is used. A system is described in terms of servers’ states, agents’ messages, and actions. Universal temporal formulas for checking deadlock and termination features are elaborated. It makes possible to verify distributed systems without a knowledge of temporal logic by a user. For verification, external model checkers: Spin, NuSMV and Uppaal are used. The experience with these verifiers show problems with strong fairness (compassion), required for model checking of distributed systems. The problems outcome from busy form of waiting in some examples. The problem is solved by own temporal formulas evaluation algorithm, using breadth-first search and reverse reachability. This algorithm does not require to specify compassion requirements for individual events, as it supports strong fairness for all cases. Thus it is appropriate for verification of distributed systems.

Deadlock Detection in Distributed Systems Using the IMDS Formalism and Petri Nets

Integrated Model of Distributed Systems (IMDS) is a formalism which expresses duality of message passing and resource sharing and which highlights locality, autonomy of distributed elements as well as asynchrony of actions and communication. Combined with model checking, IMDS allows to verify numerous properties of modeled systems. It also provides insights into the behavior of model components (servers and agents) in the form of server view and agent view of the system. IMDS is used in the Dedan verification environment which can detect several types of deadlocks, including communication deadlocks (in the server view) and resource deadlocks (in the agent view). The paper also outlines a mapping of IMDS models into behaviorally equivalent Petri nets, opening the way for many analysis techniques developed for Petri nets to be used for analysis of IMDS models. In particular, structural (siphon-based) methods for deadlock analysis in Petri nets can be used for deadlock detection in IMDS models.