  This paper considers the QBD approximation of a special class of
two-dimensional reflecting random walks (2D-RRWs). A typical example of the
2D-RRWs is a two-node Jackson network with cooperative servers. The main
contribution of this paper is to provide simple upper bounds for the relative
absolute difference between the time-averaged functionals of the original
2D-RRW and its QBD approximation.
