Abstract
Considerable research activity has been directed towards estimating the Cell Loss Probability (CLP) at the buffer of an ATM switch and understanding the ways in which it can occur. Much of this activity has been conducted by modeling the cells which enter the buffer as a continuous fluid. This model can capture the variability of a source at a burst level, but it ignores the fact that the workload actually arrives in discrete cells. Nonetheless the fluid model can give accurate estimates of the cell loss probability when the buffer size is not very small. If the switch hasa very small amount of buffer per source then cell level effects cannot be ignored. We consider both Constant Bit Rate (CBR) and periodic on/off Variable Bit Rate (VBR) sources, and apply large asymptotic techniques to a cell level model of an ATM output link. Our analysis simultaneously captures both the cell scale and burst scale effects, enabling us to study the boundary between regions in the parameter space where cell level effects are or are not signicant. In addition to accurately computing the CLP, we are able to give an insightful qualitative description of how cell loss occurs in very small buffers.}