Extended Erlang B Calculator
Recall factor (retries)
BHT (Erl.)
Important: Decimal separator is '.' not ','
Brief instructions
Here is a brief introduction to the calculator.  For a running record of the results calculated, press the Results button.  This function open new browser window on your desktop.
  • The Extended Erlang B traffic model is used by telephone system designers to estimate the number of lines required for PSTN connections (CO trunks) or private wire connections and takes into account the additional traffic load caused by blocked callers immediately trying to call again if their calls are blocked.  This traffic model may be used where no overflow facilities are available from the trunk group being designed. The four variables involved are Busy Hour Traffic (BHT), Blocking and Lines:
    • Recall factor is the percentage of calls which immediately retry if their calls are blocked.
    • 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.
  • Of the Busy Hour Traffic, Blocking and Lines values, if you know two of the figures, you can work out the third as follows:
    • Click on the Unknown radio button representing the unknown variable.
    • Enter the 2 known figures into their edit boxes.
    • Press Calc and the third figure will be calculated and displayed in the remaining edit box.
  • For example, if you assume that 40% of blocked calls are immediately retried and, from your call logger, you know that the Busy Hour Traffic is 10 Erlangs, and you want to know how many lines are required in this trunk group if you are prepared to tolerate 2 calls being blocked in every 100 calls attempted then:
    • Select 40% from the Recall Factor drop down box.
    • Press the Lines Unknown radio button (this is selected by default).
    • Enter 10 in the BHT edit box.
    • Enter 0.02 in the Blocking edit box.
    • When you press the Calc button, 17 will be displayed in the Lines edit box indicating that 17 lines would be required in this trunk group.