Erlang-B is a formula derived from the Erlang distribution to describe the propability of call loss on a group of circuits or trunks. The formula is used in planning telephone networks. The formula was derived by Agner Krarup Erlang.

The formula applies under the condition that an unsuccesful call, because the line is busy, is not queued or retried, but instead really lost forever. It is assumed that call attempts arrive following a poisson distribution. Further it is assumed that call arrivals are independent, and message length (holding times) are exponentially distributed (Markovian system) although the formula turns out to apply under general holding time distributions.

The formula provides the GoS (grade of service) which is the propability that a new call arriving at the circuit group is rejected because all circuits are busy.

From wikipedia

Erlang B calculation is available as part of Shadow CMS. Contact RSI for details.