IntroductionPerformance evaluation of information-system development becomes more complex as the size and complexity of the system increases (c.f., [7,11,17]). Reliability is certainly one of the most important characteristics for communication networks. The measure of greatest interest is the distribution of the time to the first system failure. It is well known that the majority of problems can be treated with the help of serni-Markov processes (SMP). Since the failure-free operation time of the system corresponds to sojourn time problems, we can use the results obtained for SMP. If the exit from a given subset of the state space is a "rare" event, that is, it occurs with a small probability, it is natural to investigate the asymptotic behavior of the sojourn time in that subspace (see [5,6,8,10]). Realistic consideration of certain stochastic systems, however, often requires the introduction of a random environment, sometimes referred as to Markov-modulation, where system parameters are subjected to randomly occurring fluctuations. This situation may be attributed to certain changes in the physical environment such as weather, or sudden personal changes and work load alterations. In [4], Gaver proposee an efficient computational approach for the analysis of a generalized structure involving finite-state-space birth-and-death processes in a Markovian environment. Stern and Elwalid in [12] used Markov-modulated processes for analyzing some infinite-source information systems. This paper deals with a first-come, first-served (FCFS) queueing model to analyze the asymptotic behavior of a heterogeneous finite-source communication system with a single processor. The sources and the processor are assumed to operate in independent random environments. Each message is characterized by its own exponentially distributed source and processing time with parameter, depending on the state of the corresponding environment. Assuming that the arrival rates of the messages are many times greater than their service rates ("fast" arrival), it is shown that the time to the first system failure converges in distribution, under appropriate norming, to an exponentially distributed random variable. Some simple examples are considered to illustrate the effectiveness of the method proposed by comparing the approximate characteristics to the exact ones. This paper generalizes the results of Sztrik and Lukashuk [15] and Sztrik and Rigo [16], where the sources are homogeneous and the whole system is governed by a single random environment and two random environments, respectively. The technique used here is similar to the one applied in [9] and [13,14].
Preliminary ResultsIn this section, a brief survey is given of the most related theoretical results, mainly due to Anisimov (see ]1-3]), to be applied later on.9 Let (Xc(k), k _> 0) be a Markov chain with state space rn+l Uxq, x, nxj=o, i#j, q=O defined by the transition matrix (pc(i (q), j(':))), satisfying the following conditions: