This paper presents an approximation method for numerically solving general
Markov-modulated fluid models which are widely used in modelling communica
tions and computer systems. We show how the superposition of a group of het
erogeneous sources (normally modeled by a multidimensional Markov process)
can be approximated by a one-dimensional Markov process, which is then used
as the modulating process of the buffer content process. The method effect
ively reduces the computation that is usually required to find exact (or as
ymptotic) solutions of fluid models. While this method is general, we focus
our discussion on the models with only ON/OFF traffic sources. Numerous nu
merical results are provided to show the accuracy of the approximation.