EFFECTIVENESS OF THE LEAKY BUCKET POLICING MECHANISM IN ATM NETWORKS

This paper presents an analysis of the performance of the "Leaky Bucket," a device proposed [1] for controlling the source traffic parameters of an ATM network (policing function). The most critical situation, i.e., an on/off bursty source, is assumed. Moreover, the delay jitter introduced by a possible multiplexing of sources inside the customer premises network is not considered. The Leaky Bucket can be analyzed as a G/D/1/N queue with finite waiting room N and a suitable arrival process. But the complexity of the arrival pattern brings only to numerical solutions for the system performance. An alternative approach is presented in the paper: the "fluid flow" approximation, by which the bit flow is considered as a continuous variable. By comparison with the exact model, it is shown that the accuracy of the fluid flow approach can be considered sufficient for practical purposes. Following the fluid flow approximation, an explicit formula for the cell loss probability, in case of exponential distributions for both the burst duration and the inactivity period of the source, is derived. On the basis of that formula, some numerical evaluations with the aim of assessing the effectiveness of the Leaky Bucket mechanism are presented. Two approaches are investigated. The first approach considers the selectivity characteristics of the Leaky Bucket, i.e., the efficacy of the device in detecting a critical source behavior. According to the second approach, the Leaky Bucket is regarded as a device filtering the traffic offered by the source and allowing to load the network with a controlled flow. The conclusions are that, while the Leaky Bucket can easily control the peak rate, difficulties arise in controlling the mean bit rate (because of the long time required) and the burst duration (because of the poor selectivity).