JSE_V3_N2_RP4 C. Chandra Sekhar Reddy K.R. Prasad E. Mamatha A. Sathish Journal on Software Engineering 2230 – 7168 3 2 58 65 Unbounded queues, QBD process, Mean queue length, Markov processes, Exponential distribution The high expectation of performance and availability for communication systems has presented great challenges in the modeling and design of fault tolerant wireless systems. The proper modeling methodology to study the degradation of such systems is so-called performability modeling. In this paper, an overview of approaches for the construction and the solution of performability model spectral approximation method is given. In the existing exact solutions for Markovmodulated queues are computationally intensive and prone to numerical problems when the number of states of the Markovian environment becomes large. To address this problem, a simple technique, spectral approximation method is proposed. It uses the dominant eigenvalue of the characteristic matrix polynomial, together with the associated left eigenvector. This approximation is shown to be asymptotically exact in heavy arrival. July - September 2008 Copyright © 2008 i-manager publications. All rights reserved. i-manager Publications http://www.imanagerpublications.com/Article.aspx?ArticleId=250