# CFLib.org – Common Function Library Project

## ErlangB(Erl, n)

##### Last updated February 14, 2004

Version: 1 | Requires: CF5 | Library: MathLib

Description:
This UDF returns the fraction of incoming calls turned away during the Busy Hour because all lines are busy at the time of the call. The algorithm is based on Qiao, L. and Qiao, S. 1998, A Robust and Efficient Algorithm for Evaluating Erlang's Formula, Manuscript. Web page with algorithm description is available at http://www.dcss.mcmaster.ca/~qiao/publications/erlang/newerlang.html

Return Values:
Returns a number.

Example:

``````<cfset traffic=3456.7>
<cfset lines=2000>

<cfset blocked=ErlangB(traffic,lines)>
<CFOUTPUT>
Percent of blocked calls = #evaluate(100*blocked)#
</CFOUTPUT>
``````

Parameters:

Name Description Required
Erl Busy Hour Traffic (BHT) or the number of hours of call traffic during the busiest hour of operation. Yes
n Number of independent lines available for traffic. Positive integer. Yes

Full UDF Source:

``````/**
* Calculates the Grade of Service (failure rate) based on Busy Hour Traffic (Erlangs) and number of indepenedent lines
*
* @param Erl      Busy Hour Traffic (BHT) or the number of hours of call traffic during the busiest hour of operation. (Required)
* @param n      Number of independent lines available for traffic. Positive integer. (Required)
* @return Returns a number.
* @author Alex Belfor (abelfor@yahoo.com)
* @version 1, February 14, 2004
*/
function ErlangB(erl,n) {
var s=1;
var i=1;
for (i=1; i LTE n; i=i+1) s=s*i/erl+1;
return (1/s);
}
``````

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