Erlang B

Erlang B is a modeling formula that is widely used in call center scheduling. The formula can be used to calculate any one of the following three factors if you know or predict the other two:

  • Busy Hour Traffic (BHT): the number of hours of call traffic during the busiest hour of operation
  • Blocking: the percentage of calls that are blocked because not enough lines are available
  • Lines: the number of lines in a trunk group.

Erlang B can determine the number of trunks, or lines, needed to handle a calling load during a one-hour period. However, the formula assumes that lost calls are cleared; i.e., if callers get a busy signal, they will never retry. This assumption means that Erlang B can underestimate the number of trunks needed. For this reason, it is best used in situations with few busy signals. The Erlang B Extended formula takes into account the callers who will immediately retry if their calls do not go through.

Erlang calculators can be found online or purchased. There is also an add-in available for Microsoft Excel that will include Erlang calculations in a spreadsheet.

Erlang B was developed by A.K. Erlang, for whom the erlang (a unit of traffic density in a telecommunications system) was named.

See also: Erlang C

This was first published in July 2007

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