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Erlang Library for Excel Function Reference |
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Erlang Library for Excel Function Reference |
Returns the erlangs of traffic that can be carried by a specified number of servers, at a specified service level in the Erlang C queueing model.
ErlcTrafFromFractionOk(nsrv, ahtSeconds, okWaitSeconds, fractionOk)
nsrv is the number of servers.
ahtSeconds is the average handle time (average duration of service) in seconds.
okWaitSeconds is the minimum acceptable wait duration in seconds.
fractionOk is the fraction of callers who begin service after a wait time that is less than or equal to okWaitSeconds. Note: fractionOk should be between 0 and 1.
| Formula | Description | Return Value |
|---|---|---|
| =ErlcTrafFromFractionOk( 25, 180, 10, 0.90) | Returns the number of erlangs of traffic that can be carried by 25 servers, given that the average handle time is 180 seconds, and that 90% of customers begin service after waiting no more than 10 seconds. | 19.0565 |
| =ErlcTrafFromFractionOk( 25, 180, 10, 0.95) | Same as preceding, except 95% of customers begin service after waiting no more than 10 seconds. | 17.7949 |
| =ErlcTrafFromFractionOk( 25.33, 180, 10, 0.95) | Same as preceding, except number of servers=25.33. Illustrates the fact than nsrv does not have to be an integer.. | 18.0783 |
Public Function ErlcTrafFromFractionOk(nsrv As Double,
ahtSeconds As Double, okWaitSeconds As Double, fractionOk As Double) As Double
ErlcTrafFromFractionOk's return value will be less than the value supplied for nsrv, because in the Erlang C model, the traffic in erlangs must always be less than the number of servers.
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?
Suppose that during a certain period of the day, a call center has these characteristics:
| Maximum number of calls that can be waiting: | Very large |
| Period length: | 30 minutes |
| Number of Agents | 102 |
| Average handle time: | 323 seconds |
| Service Level Goal | 95% answered within 10 seconds |
How many erlangs of traffic can this call center carry while meeting the specified service level goal? We might call this number the Traffic Capacity of the call center during this period of time.
Suppose further that calls arrive in a Poisson process, that service times are exponential, and that callers who must wait are infinitely patient and will stay on the line until their call is answered (i.e., that no callers abandon). Then the call center satisfies the assumptions of the Erlang C queueing model, so we can compute the traffic capacity x by means of ErlcTrafFromFractionOk as follows.
x = ErlcTrafFromFractionOk( 102, 323,
10, 0.95)
= 87.0927
We conclude that the traffic capacity of the call center during this period is 87.0927 erlangs.