  We consider a two-node queue modeled as a two-dimensional random walk. In
particular, we consider the case that one or both queues have finite buffers.
We develop an approximation scheme based on the Markov reward approach to error
bounds in order to bound performance measures of such random walks in terms of
a perturbed random walk in which the transitions along the boundaries are
different from those in the original model and the invariant measure of the
perturbed random walk is of product-form. We then apply this approximation
scheme to a tandem queue and some variants of this model, for the case that
both buffers are finite. We also apply our approximation scheme to a
coupled-queue in which only one of the buffers has finite capacity.
