Free Erlang traffic calculators

The world's first online Erlang traffic calculators

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Engset Calculator

Engset Calculator
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Number of traffic sources:  
BHT (Erl.)

Brief instructions

Here is a brief introduction to the calculator.  For more detailed information, press the Help Button. For a running record of the results calculated, press the Results button. Both these functions open new browser windows on your desktop.
  • The Engset traffic model is used to explore the relationship between the traffic offered to a trunk group, the blocking that will be experienced by that traffic and the number of lines provided when there are a finite number of sources from which the traffic is generated.  It 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.

     The four variables involved are Sources, Busy Hour Traffic (BHT), Blocking and Lines:

    • Sources is the number of points generating the traffic.  This can represent the number of subscribers served by a line concentrator or the number of PBX extensions with access to the trunk facilities in question.
    • Busy Hour Traffic (in Erlangs) is the number of hours of call traffic there are during the busiest hour of operation of a telephone system.
    • Blocking is the failure of calls due to an insufficient number of lines being available.  E.g. 0.03 mean 3 calls blocked per 100 calls attempted.
    • Lines is the number of lines in a trunk group.
  • If you know the number of sources and two of the other figures, you can work out the fourth as follows:
    • Enter the number of traffic sources in the Sources box.
    • Click on the Unknown radio button representing the unknown variable.
    • Enter the 2 other known figures into their edit boxes.
    • Press Calc and the result will be calculated and displayed in the remaining edit box.
  • For example, imagine that you have 100 telephone extensions on a PBX, generating 35 Erlangs of traffic during the busy hour and that this traffic is offered to a PSTN link used solely for that outbound traffic.  To calculate how many lines are required in that trunk group if 2% blocking is acceptable:
    • Enter 100 in the Sources box.
    • Press the Lines Unknown radio button (this is selected by default).
    • Enter 35 in the BHT edit box.
    • Enter 0.02 in the Blocking edit box.
    • When you press the Calc button, 43 will be displayed in the Lines edit box indicating that 43 lines would be required in this trunk group.  By contrast, the Erlang B Calculator, which does not take the number of sources into account, would report that 45 lines are required.
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Last modified: 14 July 2014

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