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Performing a calculation

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You can use the Call Minutes Calculator to work out how many lines you need if you know the daily traffic offered to those lines, or vice versa. To perform a calculation, you should firstly enter figures into the Assumptions section of the calculator, or leave them at their defaults (remembered from last time you ran the calculator). There are two figures in this section.


Blocking target


Use this section to specify the Blocking target which should be used when making calculations. This specifies the acceptable grade of service in terms of calls blocked due to insufficient lines having been provided.


Busy Hour Factor


The Busy Hour Factor is the percentage of one day's calls which are made during the busy hour. This is used to work out the peak traffic offered to the trunk group which is required to work out the number of lines required.



Next, you should click on one of the radio buttons to indicate whether you want to work out the number of lines required or the traffic level which a trunk group can be offered. The two radio buttons are in the top section of the calculator, and you should click the button next to the quantity which is unknown. One of the two edit boxes in the top section will be greyed. The other box should be used to enter the figure which you know. This will be one of the following:


Total traffic per day


This is the total traffic offered to the trunk group in one day. It is expressed in minutes. For example, if one hour of traffic is offered to the trunk group in one day, then the figure 60 should be entered.


Number of lines required


You should enter the number of lines in a trunk group into this box.



The final section contains an box which shows the Busy Hour Traffic corresponding to the daily traffic and Busy Hour Factor shown. You can switch this figure between Erlangs and C.C.S. using the radio buttons at the bottom right of the window.


There is a small button to the right of this box. Pressing this button transfers the displayed busy hour traffic figure to the corresponding boxes in the Erlang B and Extended Erlang B calculators and helps to integrate the calculators.


When entering figures into the edit boxes, a full stop or period should be used as a decimal mark, NOT a comma as is the convention in some European countries.


To start the calculation, press the Calculate button. After a short time, the unknown figures will be displayed in the relevant edit boxes. If you want to stop a calculation before it has been completed, press the Stop button.


See also


Displaying previous results

Operational limits

Example calculations