The Erlang programming language is not the same thing as the erlang, a unit of traffic density.

The erlang is a unit of traffic density in a telecommunications system. One erlang is the equivalent of one call (including call attempts and holding time) in a specific channel for 3600 seconds in an hour. The 3600 seconds need not be, and generally are not, in a contiguous block.

In digital telecommunications, the voice signals are compressed. This makes it possible for one channel to carry numerous calls simultaneously by means of multiplexing. In theory, there are many ways in which a channel can carry a certain number of erlangs. For example, a traffic density of 3 erlangs can consist of three simultaneous calls, each lasting for an hour (a total of 10,800 seconds); it can consist of six calls, each of which are allocated 30 minutes (1800 seconds) of time during the hour; it might consist of 180 calls, each of which occupy one minute (60 seconds) of time during an hour.

Smaller units of traffic density are sometimes used. The hundred or centum call second or CCS is the equivalent of one call for 100 seconds out of an hour. A traffic density of 1 CCS is equal to 1/36 erlang. An erlang can be applied to the group of lines in a telephone trunk line or to the traffic in a telephone call center.

The term is named after the Danish telephone engineer, A. K. Erlang, the originator of queueing theory.

Erlang B is a calculation for any one of these three factors if you know or predict the other two:

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

An extended version of Erlang B lets you add the factor of how many people who are blocked retry their calls immediately.

Erlang C is a calculation for how many call agents (answerers) you'll need in a call center that has a given number of calls per hour, a given average duration of call, and an acceptable level of delay in answering the call.

This was last updated in July 2007

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