nd image transfer, messaging, and distribution of noncritical information, where there is no need for a set response time or service guarantee. ATM provides a very well-planned approach to providing QoS. Table 10.3 shows how each service class allows you to define or not define certain parameters. The parameters boil down to two major categories: QoS parameters-Thes parameters include cell error rate (CER; the per- centage of errored cells), cell loss ratio (CLR; the percentage of lost cells), cell transfer delay (CTD; the delay between the network entry and exit points), cell delay variation (CDV; the jitter), and cell misinsertion rate (CMR; the number of cells inserted on the wrong connection). Traffic parameters-These parameters include peak cell rate (PCR; allows you to specify the maximum amount of bandwidth allowed on a connec- tion), sustainable cell rate (SCR; allows you to specify guaranteed band- width during the variable transmissions-used only by VBR), maximum burst size (MBS; allows you to specify the maximum number of cells that will be transmitted at PCR-used only by VBR), cell delay variation toler- ance (CDVT; allows you to specify the maximum allowable jitter), mini- mum cell rate (MCR; allows you to specify the rate in cells per second that the source can transmit-used only in ABR), and allowed cell rate (ACR; works with ABR's feedback mechanism that determines cell rate). As Table 10.3 shows, UBR allows you to define very little, whereas CBR allows you to tightly control most of these parameters. Depending on the service class, you have the option of defining or not defining certain parameters, and that gives you control over the performance of an applica- tion within a service level. The transmission path in a virtual circuit with ATM is composed of virtual paths and its virtual channels (refer to Figure 7.19 in Chapter 7, "Wide Area Networking"). You can think of the virtual channel as an individual conversation path and the virtual path as a grouping of virtual channels Chapter 10 Next-Gen