Grade of service

1. The probability of a call being blocked or delayed for more than a specified interval, expressed as a decimal fraction. This may be applied to the busy hour or to some other specified period or set of traffic conditions. Grade of service may be viewed independently from the perspective of incoming versus outgoing calls, and is not necessarily equal in each direction.
2. In telephony, the quality of service which a circuit is designed or conditioned to provide, e.g. voice grade[?] or program grade. Criteria for different grades of service may include equalization[?] for amplitude over a specified band of frequencies, or in the case of digital data[?] transported via analog circuits, equalization for phase also.

Source: Federal Standard 1037C and MIL-STD-188