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DSGuest
I have a trunk group with roughly 1200 ccs/hour. I want to equip enough trunks to limit blocked calls to about 1 out of 100. Does anyone know how to determine this?
WebmasterGuestAssuming that your traffic figure is for the busy hour, divide it by 36 to convert to Erlangs (33.333). Then, use our free online Erlang B Calculator, setting the blocking as 0.01.
The answer comes out at 45 lines.
We also sell a Windows version of this calculator for 80 US Dollars for a single user licence. This also includes an addin for Excel that allows you to perform these calculations within a spreadsheet. Click here for more information. I have attached a screenshot of the Windows product to this message.
Please feel free to contact us if you require any further information.
DSGuestI’m making the assumption that 100% of the blocked calls remain within the system. As I understand it, the Erlang B calculator does not retain blocked calls.
WebmasterGuestThat’s true. Erlang B assumes that all blocked calls are immediately cleared from the system.
Which model you use depends on how long the blocked calls remain in the system. If they remain in the system only as long as their holding time (had they been answered), then you should use the Poisson (Molina) model. If you are only looking at 1% blocking, the error introduced by using Erlang B would be almost negligible.
If calls are held in the system until they are answered, then Erlang C is the correct model to use for analysing this form of queuing.
We do not have products that support Poisson (Molina), because this situation is not often found in the real world (blocked calls are usually cleared or queued). We do have Erlang C Calculators though.
You may also wish to consider the Extended Erlang B model. There is a free calculator on this site. It is used to model the situation where a percentage of blocked calls immediately retry. Again, for 1% blocking, you probably wouldn’t notice a difference in the results anyway.
DSGuestI’ve come accross a copy of the Poisson table, but it has a limited number of trunks listed. Is there a formula to calculate the Poisson value? Or, is there an expanded Poisson table online?
WebmasterGuestThere’s a good book called Traffic System Design Handbook by James R. Boucher and published by IEEE Press. It is easy to read and has a Poisson table up to 200 lines.
I’ve never seen any tables with higher values.

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