Erlang Library for Excel Function Reference

ErlbTrafFromBlockage

Applies to: Excel 2003, Excel 2007, Excel 2010

Returns the traffic load in erlangs that will produce the specified blockage for the specified number of servers, in the Erlang B queueing model.

Syntax

ErlbTrafFromBlockage( blockageFraction, nsrv)

blockageFraction is the blockage. Must be greater than or equal 0 and less than or equal to 1.

nsrv is number of servers, and can be any non-negative number.

Examples

Formula	Description	Return Value
=ErlbTrafFromBlockage( 0.01, 17)	Returns the number of erlangs of offered traffic that will produce blockage=0.01 (that is, 1%) when there are 17 servers (in the Erlang B queueing model)	9.65160
=ErlbTrafFromBlockage( 0.05, 17)	Same as preceding, except blockage = 5%	12.4613
=ErlbTrafFromBlockage( 0.05, 17.5)	Same as preceding, except number of servers = 17.5. Illustrates the fact that nsrv does not have to be an integer.	12.9225

VBA Function Declaration

Public Function ErlbTrafFromBlockage(blockageFraction 
  As Double, nsrv As Double) As Double

Remarks

If this function is not available, and returns the #NAME? error, then you must install and load the Erlang Library for Excel from Abstract Micro Systems.

How?

Case Study: Computing the Traffic Capacity of a Trunk Group

You can use ErlbTrafFromBlockage to compute the maximum offered traffic load that a trunk group may experience while maintaining a blockage less than a specified value. We might call this number the Traffic Capacity of the trunk group.

For example, suppose we have a group of 25 trunks, and we wish to callers to that trunk group to experience a busy signal (all trunks busy condition) no more than 0.01, or 1% of the time. Suppose further that the calls arrive in a Poisson process, that service times are exponentially distributed, and that blocked callers (those who experience a busy signal) go away and never return. Then the assumptions of the Erlang B queueing system are satisfied, and we may compute the Traffic Capacity x as follows:

x = ErlbTrafFromBlockage(0.01, 25)
= 16.1245 erlangs

This means that we may offer up to 16.1245 erlangs of traffic to our trunk group without driving the blockage above 1%.

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