Resource Requirements for ABR Explicit Rate Flow Control: Deterministic and Probabilistic Analyses

ATM Available Bit Rate (ABR) service is intended to o er low cell loss for nonreal-time data sources that can respond to closed-loop ow control. ATM Forum Tra c Management Speci cation Version 4.0 de nes the various parameters used in the ABR ow control, as well as the source, destination, and switch behaviors. However, the switch designers and service providers are free to choose the method of congestion control to implement and the ABR Quality of Service (QoS) objective to o er. This paper addresses the interaction among the ow control algorithm, the switch resource requirements, and the resulting QoS characteristics. In this paper we propose and evaluate an Explicit Rate (ER) algorithm. The objective of this algorithm is to maintain the total bu er occupancy of all ABR connections to be close to a given threshold. By maintaining a non-zero queue, the ABR service can achieve a high utilization. The switch periodically determines its desirable ER value, based on the available capacity, the ABR bu er occupancy, and the number of active ABR sources. We develop analyses that relate ABR resources to QoS objectives for this algorithm. The rst approach is a deterministic, conservative analysis. It provides formulas for determining the ABR bu er and capacity requirements that can achieve zero bu er over ow. The second analysis determines an upper bound on the bu er over ow probability when the above requirements are not met. The result is most e ective when the number of active sources is a small fraction of the total ABR connections. Numerical examples show that by slightly relaxing the loss constraint, resource requirements can be signi cantly reduced.