|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
- The 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. The three variables involved
are Busy Hour Traffic (BHT), Blocking and Lines:
- 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 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 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:
- Press the Lines Unknown
radio button (this is selected by default).
- Enter 10 in the BHT edit
- Enter 0.02 in the Blocking
- 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.