Editing Asynchronous Transfer Mode (section)

== Traffic engineering ==
Another key ATM concept involves the [[traffic contract]]. When an ATM circuit is set up each switch on the circuit is informed of the traffic class of the connection. ATM traffic contracts form part of the mechanism by which [[quality of service]] (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.
# CBR {{En dash}} Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.
# VBR {{En dash}} Variable bit rate: an average or Sustainable Cell Rate (SCR) is specified, which can peak at a certain level, a PCR, for a maximum interval before being problematic.
# ABR {{En dash}} Available bit rate: a minimum guaranteed rate is specified.
# UBR {{En dash}} Unspecified bit rate: traffic is allocated to all remaining transmission capacity.

VBR has real-time and non-real-time variants, and serves for [[Burstiness|bursty]] traffic. Non-real-time is sometimes abbreviated to vbr-nrt. Most traffic classes also introduce the concept of cell-delay variation tolerance (CDVT), which defines the ''clumping'' of cells in time.

=== 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.

=== Traffic shaping ===
[[Traffic shaping]] usually takes place in the [[network interface controller]] (NIC) in user equipment, and attempts to ensure that the cell flow on a VC will meet its traffic contract, i.e. cells will not be dropped or reduced in priority at the UNI. Since the reference model given for traffic policing in the network is the GCRA, this algorithm is normally used for shaping as well, and single and dual [[leaky bucket]] implementations may be used as appropriate.