e sent to single or several exchanges. Newer traffic management systems will allow greater automation so that responses to threshold-triggered alarms can be pre-programmed, thus allowing potential problems to be averted without human intervention. 13.3 Traffic Management in ATM Networks The greatest benefit of ATM is the well-defined way in which it manages traffic. The ATM Forum specification on traffic management is complex, and an explanation of some of the basics will now be given. ATM offers five classes of service, and any ATM connection must satisfy a sus- tainable cell rate peak cell rate and maximum burst size associated with a particular class of service. These three parameters are important in describing the profile of an ATM connection. ATM traffic management mechanisms include Call Admission Control (CAC), Resource Management, Usage Parameter Control, Network Parameter Control, Priority Control, Traffic Shaping and Explicit Forward Congestion Indication. These mechanisms collectively help define the quality of ATM service. Under ATM Forum traffic management specifications, CAC asks for a specific QoS. The network then refers to a specific list of performance requirements for the connection, including QoS objectives for Cell Loss Ratio (CLR), Cell Transfer Delay (CTD) and Cell Delay Variation (CDV). CLR is the ratio of lost cells to total transmitted cells. Cells can be lost when an ATM switch malfunctions, but usually cells are explicitly discarded for non-compliance or in response to network congestion. Buffer management is the strategy for decid- ing how and when to discard packets. ATM network buffers address congestion at the cell level. The peculiarities of ATM networks demand that there be short queues within the network. There should 50 HJ Chao and X-L Guo, Quality of Service Control in High-speed Networks. Wiley, New York, 2002. 13 Traffic Management also be a minimum number of buffers to ensure that the required cell loss rate is achieved for each virtual channel. Cells can be discarded sele