REAL-TIME COMMUNICATION 'VIRTUAL TIME CSMA Vs TIMED TOKEN BUS

Abstract

In implementing a nc1.woi•k, IL is neceavy 1'1r•sL to determine if the network will function properly in the specific enviroment and then to predict the performance of the network and compare it to the system requirements. ..Simulation modeling is presently the most effective means of performing a detailed analysis of a network. The objective of this work is to study the performance of a new Carrier Sense Multiple Access (CSMA) algorithm, called Virtual Time CSMA, as proposed by Zhao and Ramamritham [1], for the real time communication systems. Where each node maintains two clocks: a real time clock and a virtual time clock. The virtual clock runs at a higher rate than the real clock. A node transmits a waiting message when the time on the virtual clock is equal to some parameter of the message. Message arrival time, message length, message laxity, and - message deadline can be used as this parameter to implement FCFS, Mimimum Length Firtst, Minimum Laxity First, and Minimum Deadline First transmission policies, respectively. In this dissertation ,simulation program has been developed to study the performance of these four variations of proposed protocols, and of the prioritized Token Bus access mechanism, and to assess their real time capabilities. II First, models for these protocols based on the petri nets and it's modified versions have been constructed. A package which offers a menu driven user friendly environment for the user is then discussed. Network parameters such as the number of nodes, the virtual clock rate, the normalized propagation delay, and the offered load have been varied to demonstrate their effect on the network performance metrics. Finally, extensive simulation results for the above algorithms are presented, including comparison between the above protocols.