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A new, more realistic model for the available bit rate traffic class in ATM network congestion control with explicit rate feedback is introduced and analyzed. This model is based on recent results by M.M. Ekanayake ("Robust stability of discrete time nonlinear systems", Ph.D. dissertation, Univ. Miami, Coral Gables, FL, 1999) regarding discrete time models for time-variant delays. The discrete time model takes into account the effect of time-variant buffer occupancy levels of ATM switches, thus treating the case of time-variant delays between a single congested node and the connected sources. For highly dynamic situations, such a model is crucial for a valid analysis of the resulting feedback system. The new model also handles the effects of the mismatch between the resource management cell rates and the variable bit rate controller sampling rate as well as buffer and rate nonlinearities. A brief stability study shows that an equilibrium in the buffer occupancy is impossible to achieve in the presence of time-variant forward path delays. Stability conditions for the case of time-variant delays in the return path are presented. Finally, illustrative examples are provided.