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The Erlang C model is the standard mathematical model for call centers. Delayed customers arrive in the queue and stay there until they are served. There is an infinite number of waiting spots. The order of service is first-come-first-served (FCFS).

There are two performance measures that are often used:

  • the average waiting time, or the average speed of answer (ASA);
  • the service level, i.e., the percentage calls that is served within a certain time (called acceptable waiting time (AWT)).

    While it is a standard model for a single-skill call center, the use of the Erlang C model has a number of disadvantages. The most important are:

  • abandonments are not modeled, in an understaffed call center the Erlang C formula predicts infinite long waiting times;
  • the number of lines is infinite.