Abstract: A method for distributing the available bit rates in a variable bit rate service under asynchronous transfer mode is provided by distributing the available bit rates in logarithmic-linear steps, so that the ratio between two successive available bit rates are substantially constant. To minimize computation required to calculate the next available allowable cell rate, a table look-up method can be used in conjunction with a state machine. In one embodiment, the table look-up operation includes accessing a rate increase table and a rate decrease table. A selection circuit is provided to choose the next allowable cell rate from the current cell rate, and the output values obtained from looking up the rate increase table and the rate decrease table.

Abstract: A segmentation and reassembly circuit under the ATM standard uses a transmit cell schedule table (TCST) to support real time transmission of ATM cells in multiple constant bit rate virtual channels. In one embodiment, null cells are intentionally scheduled in a TCST. Transmission of the scheduled null cells ("forced null cells") or non-time critical cells are skipped to compensate for delays in an ATM cell transmission schedule, e.g. delays due to a bus latency. During such latency, null cells are generated from a null cell generator and a negative credit counter is incremented for each ATM cell transmission time missed. When transmission of a forced null cell is skipped, the negative credit counter is decremented.