A number of parameters are used to define the QOS. These include the Cell Error Ratio, Cell Loss Ratio, Cell Misinsertion Rate, Cell Transfer Delay, Mean Cell Transfer Delay, Cell Delay Variation and the Severely Errored Cell Block Ratio. A number of classes of QOS are supported by the network and fall into either a Specified QOS or an Unspecified QOS class. Among all the classes of QOS the network must only support the Specified QOS class 1 which is the circuit emulation service and constant bit rate video. A specified QOS may have two cell loss objectives, for the high and low priority traffic. The unspecified QOS has no specified objectives given to the user but may have internal parameters for the network but these may change during a call. Even though there are no QOS the call may have specified traffic parameters, in fact that would be desirable for the network. This type of traffic could be the so called best-effort traffic. This allows the network to respond to time variable resources. The unspecified QOS is optional for the network to support.
Degradation of QOS may arise for many different reasons and one of these is the ATM switch. The buffer capacity could be a complex multiple queue system with an algorithmicaly defined service rule that could be based on priorities. The switch may thus introduce loss under heavy load. Of particular interest to JPL is that the buffering strategies for wide area low speed networks may be more complex than for high speed local area networks due to the detection of the PDU by a higher level. For compliant connections the QOS will be supported for at least the number of conforming cells as specified in the conformance definition. For non-compliant connections the network does not need to support and QOS. The QOS of a VPC will be the combination of the strictest set of QOS's of the underlying VCC's.