Editing Asynchronous Transfer Mode (section)

=== Traffic policing ===
To maintain network performance, networks may apply [[Traffic policing (communications)|traffic policing]] to virtual circuits to limit them to their traffic contracts at the entry points to the network, i.e. the [[user–network interface]]s (UNIs) and [[network-to-network interface]]s (NNIs) using [[UPC and NPC|usage/network parameter control]] (UPC and NPC).<ref name="UPC_NPC" >ITU-T, ''Traffic control and congestion control in B ISDN'', Recommendation I.371, International Telecommunication Union, 2004, page 17</ref> The reference model given by the ITU-T and ATM Forum for UPC and NPC is the [[generic cell rate algorithm]] (GCRA),<ref name="ITU-T-GCRA" >ITU-T, ''Traffic control and congestion control in B ISDN'', Recommendation I.371, International Telecommunication Union, 2004, Annex A, page 87.</ref><ref name="ATMF-GCRA" >ATM Forum, The User Network Interface (UNI), v. 3.1, {{ISBN|0-13-393828-X}}, Prentice Hall PTR, 1995.</ref> which is a version of the [[Leaky bucket#As a Meter|leaky bucket algorithm]]. CBR traffic will normally be policed to a PCR and CDVT alone, whereas VBR traffic will normally be policed using a dual leaky bucket controller to a PCR and CDVT and an SCR and maximum burst size (MBS). The MBS will normally be the [[Network packet|packet]] ([[Segmentation and Reassembly|SAR]]-[[Service data unit|SDU]]) size for the VBR VC in cells.

If the traffic on a virtual circuit exceeds its traffic contract, as determined by the GCRA, the network can either drop the cells or set the Cell Loss Priority (CLP) bit, allowing the cells to be dropped at a congestion point. Basic policing works on a cell-by-cell basis, but this is sub-optimal for encapsulated packet traffic as discarding a single cell will invalidate a packet's worth of cells. As a result, schemes such as partial packet discard (PPD) and early packet discard (EPD) have been developed to discard a whole packet's cells. This reduces the number of useless cells in the network, saving bandwidth for full packets. EPD and PPD work with AAL5 connections as they use the end of packet marker: the ATM user-to-ATM user (AUU) indication bit in the payload-type field of the header, which is set in the last cell of a SAR-SDU.