||This article provides insufficient context for those unfamiliar with the subject. (October 2009)|
When Agner Krarup Erlang developed the Erlang-B and Erlang-C traffic equations, they were developed on a set of assumptions (see grade of service). These assumptions are accurate under most conditions; however in the event of extremely high traffic congestion, Erlang's equations fail to accurately predict the correct number of circuits required because of re-entrant traffic. This is termed a high-loss system, where congestion breeds further congestion at peak times. In such cases, it is first necessary for many additional circuits to be made available so that the high loss can be alleviated. Once this action has been taken, congestion will return to reasonable levels and Erlang's equations can then be used to determine how exactly many circuits are really required .
An example of an instance which would cause such a High Loss System to develop would be if a TV-based competition were to announce a particular telephone number to call at a specific time. In this case a large number of people would simultaneously phone the number provided. If the service provider had not catered for this sudden unanticipated peak demand, extreme traffic congestion will develop and Erlang's equations cannot be used .
 Kennedy I., School of Electrical and Information Engineering, University of the Witwatersrand, Personal Communication.