Preventive congestion control is more effective and widely used than reactive control. It is usually implemented in two ways: Connection Admission Control (CAC) and Usage Parameter Control (UPC). Leaky bucket (LB) is a typically used UPC mechanism. A traditional LB can either police a bursty source's Peak Cell Rate (PCR) or Average Cell Rate (ACR) but not both of them simultaneously, meanwhile, it introduces cell delays by buffering cells before they are transmitted into the network.  In this thesis, we propose a modified Leaky Bucket mechanism, in which a single LB can police both the PCR and ACR simultaneously. The first characteristic of the modified LB is that the token generator is designed to be dynamic which alternates between On and Off states, tokens will be generated during the token generator's On period at a speed equalling to the policed bursty source's PCR. The second characteristic of this modified LB is that no token pool or cell buffer is utilized. Therefore, cells generated by a bursty source can be transmitted into the network at its PCR only when the token generator is simultaneously in On state. Cells can either be transmitted or discarded, no cell delay occurs. As a result the source's PCR cannot be exceed the token generator's PCR