Grade Of Service is a target value selected by the network engineer as a Service Level to give to the customers (callers). It defines the average percentage of calls that may fail because there are insufficient lines to carry the traffic load. It may be expressed as a percentage (1%) or a probability (0.01)
It is measured (to ensure the real failure rate is near the target failure) by calculating how many calls are offered to the trunk route (Offered Traffic, OT) and how many are acually carried (Carrier Traffic CT). THe difference (OT-CT) are assumed to have failed because there were no lines/trunks/channels available.
THe percentage or GOS is then a simple arithmetic, GOS=(OT-CT)/OT