1-24Configuring PVCs1.7 Traffic Policing (Usage Parameter Control)Traffic policing, also known as Usage Parameter Control (UPC), is a methodof assessing the cells entering the switch for conformance with pre-estab-lished traffic bandwidth contracts. Those cells that exceed the specified con-tract are “tagged” or “dropped,” depending on what is defined in thecontract. This ensures that the connections with reserved bandwidth are notexceeding their reservations. Cabletron Systems’ switches use a combinationof “leaky bucket,” or Generic Cell Rate Algorithm (GCRA) hardware in theswitch fabric and user-configurable parameters in AMI to perform thesepolicing functions.1.7.1 Setting the CLP BitFirst, it is important to understand the concept of tagging and dropping.Each ATM cell has a Cell Loss Priority (CLP) bit which indicates if the net-work can drop it under congested conditions. When the CLP bit is set to 0 (orCLP = 0), the cell will be assessed for compliance with traffic parameters. Ifthe traffic parameters dictate that non-compliant cells should be “tagged,” theCLP bit will be set to 1 (or CLP = 1), which means that upon experiencing con-gestion further in the network, these CLP = 1 cells will be dropped (becauseyou cannot tag them again).1.7.2 Leaky Bucket AlgorithmThe next important concept is the leaky bucket algorithm. Leaky buckets are amechanism by which cells entering the switch fabric are monitored for com-pliance with UPC traffic contracts that have been negotiated at connectionset-up time. Before the leaky buckets are discussed, it is important to under-stand the parameters that are being measured by the buckets. They are as fol-lows:• Peak Cell Rate (PCR) - the maximum number of cells per second.• Cell Delay Variation Tolerance (CDVT) - the tolerance for variation inthe inter-arrival time of these cells, or the amount of jitter that can beaccepted by the network.• Sustainable Cell Rate (SCR) - the average rate of cell transmission forthis connection, taking bursting into account.• Maximum Burst Size (MBS) - the maximum amount of cells that canbe transmitted at the PCR.
