What is Erlang C?

What is Erlang C?
Is It Accurate?
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Erlang Calculators

The Erlang traffic models were established by A.K. Erlang, a Danish scientist who is credited with much of the early work in telephone traffic design.

Erlang calculators are used throughout the world to carry out a variety of statistical estimations associated with telecommunications systems and call centers.

There are several variants of Erlang calculators for specific purposes -

Erlang C

Call Centers, help desks, checkout queues, where customers can wait in a queue.

Extended Erlang B

Used by telephone system designers to estimate the number of lines required for PSTN connections and takes into account the additional traffic load caused by blocked callers immediately trying to call again if their calls are blocked.

Erlang B


The Erlang C calculator models the performance of systems which incorporate queuing (rather than a caller simply getting a busy signal and hanging up). Queuing applications include switchboard operators, call center agents and helpdesks. However the same calculations also apply to supermarket checkout queues, toll booths etc.

The Erlang C model makes the following assumptions -

  •  Calls are offered randomly in a queue (Poisson arrivals)

  •  Users wait if they find the system busy (no account is made of the effect of abandoned calls)

  •  Service times are exponential

  •  Users are served in the order of their arrival

  •  Users are directed to the first-available agent

  •  Queue sizes are unlimited

  •  There are not dramatic variations in call volumes within the period being calculated


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