Free Erlang traffic calculators

The world's first online Erlang traffic calculators

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We have compiled a list of questions about Westbay Traffic Calculators which are often put to us. Please contact us if you need more information.

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Index of questions

  1. What is an Erlang?
  2. How is Erlang B useful for trunk dimensioning?
  3. How can I work out how many lines I need?
  4. How can I work out how many call centre agents I need?
  5. Should I include wrap up time when using Erlang C?
  6. How do I work out the Busy Hour Traffic?
  7. Does Westbay Traffic Calculators have reporting capabilities?
  8. Do you sell the source code, or a DLL to which my own programme can connect?
  9. How accurate are the Erlang traffic models?
  10. Why don't you include the Poisson traffic model?
  11. What traffic levels can the calculators work up to?
  12. If I download the software on to my desktop, will I be able to subsequently transfer the software to my laptop?

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What is an Erlang?

An Erlang is a unit of traffic measurement. In practice, it is used to represent one hour of traffic, although strictly speaking, Erlangs are dimensionless.

How is Erlang B useful for trunk dimensioning?

Erlang B is a traffic model which can be used to explore the relationship between the busy hour traffic offered to a trunk group, the required grade of service and the number of lines required. As such, it is used for route dimensioning (determining how many lines are required on a trunk group).

How can I work out how many lines I need?

This is a complex issue, and is the subject of a white paper called Dimensioning Trunk Groups which is on this Web site.

How many call center agents do I need?

This subject is even more complex and is the subject of another white paper called Call Center Staff Modelling and Trunk Design. The white paper will guide you through working out how many agents are required for an hour, and then discusses the means of working out the number of lines required into an call centre.

Should I include wrap-up time with Erlang C?

Yes. Wrap up time (or wrap time) is the time an agent remains unavailable after completing a call. It is usually the time which a call centre agent uses to complete any administrative tasks relating the the call such as entering an order. As wrap up time uses an agents time in the same way as actually handling a call, wrap up time should be included in the average call duration figure.

However, when using Erlang B to estimate the number of lines required into a call centre to deliver inbound calls, wrap up time should not be included, but the average delay to all calls should be included as it queuing time occupies a trunk in the same way as conversation time.

How do I work out the Busy Hour Traffic?

You need to work out the number of hours traffic offered to a trunk group during the busiest hour. You could use the following formula:

Busy Hour Traffic = Number of calls * Average call duration (s) / 3600

If possible, you should extract these figures from a call logger. Otherwise, you will need to make reasonable estimates based on your understanding of your business.

Remember that the busy hour figure should represent the busiest hour you are ever likely to encounter, so use a month or two's data to work it out.

Are there reporting capabilities?

Yes. By pressing the Results button, you can see a list of any calculations you have made during the previous week. You can filter this list so that the results on view are only for the current session, or any day during the past week. This calculation history can also be printed.

Do you sell source code or DLLs?


How accurate are the Erlang models?

They are usually accurate and have been used throughout the industry for decades. But they do make certain assumptions about the nature of offered traffic which may not necessarily apply in your case. For example, they assume random (Poisson) arrivals. This is usually reasonable, but sudden peaks of calls resulting from a radio or television advertisement are not accurately modelled. In these circumstances, you should use the calculators with caution and common sense.

Why don't you include Poisson?

Because we don't think it is very accurate. Poisson (or Molina) can be used to model situations where there are a high number of retries, that is, callers who immediately redial because they have encountered blocking.

We prefer to use the Extended Erlang B traffic model for this purpose because it allows you to specify the ratio of blocked calls which will immediately retry.

What traffic level can the calculator work to?

The Erlang B Calculator can work to 90,000 Erlangs and 90,000 lines.

The following table shows the operational limits of all the calculators:

  Erlang B / Ext. Erlang B Erlang C Call Minutes
Operating system All 32 and 64 bit versions of Microsoft Windows.
Traffic model used Erlang B / Ext. Erlang B Erlang C Erlang B
Operational limits BHT 90,000 (max) Erlangs
Blocking 0.001 - 0.500
Lines 90,000 (max)
Retries 95% (max)
ACD Agents 1 - 10000
Calls per hour 1000000 (max)
Duration 10000 (max) seconds
1 - 30000000 mins. per day
Blocking 0.001 - 0.500
Lines 90,000 (max)

Will I be able to install on my laptop?

If you purchase the software when you are using your desktop computer, then you may transfer the installation file to your laptop computer and install it there. However, you may not install on both computers if you are only covered by one single user licence; you will need to buy two licences.

Has this answered your question?

If not, then please send us an email.

Purchasing Westbay Traffic Calculators

The full working version of Westbay Traffic Calculators is available for 99 US Dollars.

Usually, the software can be working on your PC in under 10 minutes!

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Last modified: 25 May 2017

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