Engset calculator

This is our free online Engset calculator. Use it to estimate how many lines your trunk groups need when you have a finite number of traffic sources.

Engset Calculator
Number of traffic sources

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Engset Calculator instructions

The Engset traffic model is used to explore the relationship between the traffic offered to a trunk group, the blocking that traffic will experience and the number of lines provided. It assumes a finite number of sources from which the traffic is generated and is used to replace the Erlang B traffic model, which tends to over-estimate blocking when the ratio of the number of sources to the number of lines is less than ten.

To use the Engset calculator, firstly enter the number of traffic sources into the first box. This is the number of points from which traffic originates – for example, the potential callers to a help desk or the points of egress from an adjacent switch.

There are three further variables:

  • Busy Hour Traffic (in Erlangs): the hours of call traffic during the busiest hour of a telephone system’s operation
  • Blocking: the ratio of calls that fail through insufficient lines (e.g. 0.03 mean 3 calls blocked per 100 calls attempted)
  • Lines: is the number of circuits in a trunk group
  • Click on the button representing the variable you want to calculate
  • Enter the two known figures into their edit boxes
  • Press Calculate and the unknown value will be calculated and displayed in the remaining box

If you know two of the figures, you can work out the third.

For example, in a system with 250 traffic sources, if you know that the Busy Hour Traffic is 75 Erlangs and you want to know how many lines you need if you can tolerate 1% of calls being blocked, press the button next to Lines, enter 75 in the BHT box and 0.02 in the Blocking box. When you press Calculate, the answer (88 lines) will be displayed. An equivalent calculation with the Erlang B calculator that assumes infinite traffic sources reports that 91 lines are required.

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