When you are defining an ATM vpi/vci pair on a virtual path, regardless of whether you are talking about IMA or DS3, OCx, etc, class of service can usually be defined UBR, VBR-RT, VBR-NRT, or CBR. If we are talking about VoIP over ATM, and it sounds like we are, you can implement QoS at layer 3 with atm and/or frame-relay traffic shaping and policy based routing. Peak cell rate, sustained cell rate, and burst size in cells are determined by the provisioning of the virtual path in the service provider network. You will get these from the carrier/telco and shape your paths accordingly in the router. You will want to make sure you can get large packet pings through the network without packet loss. Ping with a 1500 byte packet, and if you are 'out of shape' you will see intermittent packet loss. If SCR, PCR, and burst are not set properly to match the service provider network, you will not be able to reliably pass pings (you will get long ACKs in TCP) like the one described above through the network and voice (as well as data) will subsequently suffer.
Pat
