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Trunk Group Overflow

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    John Bell

    I am oversimplifying this, but here goes…

    I have a PBX and all calls go out through a single trunk group, sized for 1% peak blocking. The trunk group carries “X” CCS of traffic, with “V” CCS being voice and “D” CCS being data. V + D = X I want to split the trunk group into two parts. The first part gets all of the calls, the second part gets only “V” overflow. I want to size these two parts so that the GOS for the voice calls is 1% peak blocking and the GOS on the data calls is 10% peak blocking.

    Any suggestions?

    Luis Martinez


    I’m Luis, Network Planning Manager of a long distance company in México (Avantel)

    I would like to suggest you the following:

    – Does the E1 (2 Mb) is R2orSS7?, If so, then you can split it up into two new trunk groups (this is not possible with PRI). – Size the first trunk group using the traffic you have during busy hour, convert to E0 using gos=0.01 or what ever you want.

    – Trunk group for data: To apply GOS=.1 it should have a behaviour like voice traffic (modem calls) if not, you have to take care of the traffic it is (semiperment connection, simultaneous data sources, then erlang B or C is not applicable and a source to channel assigment could work)

    I hope this brief explanation could help you. This is my email:

    Peter Turner

    First of all I suggest you set up the system so that the voice traffic is first offered to the voice only group overflowing onto the common group. This will bemore efficient than offerring the voice to the common group first.

    If the ammount of data traffic is say 50% or more of the voice traffic we could use ErlangB for sizing ignoring the peakedness of the overflow voice traffic. Otherwise we need amore sophisticated method to estimate blocking performance.

    Therefore I suggest as a first shot to dimension voice only circuits to a blocking of 10% using Erlang B. Then add the volume of overflow voice to the volume of switched data traffic and again dimension to a GOS of 10%. This will get you in the correct ball park but will underestimate the number of circuits required if the volume of overflow voice is high relative to the data traffic. In this case you need to use more sophisticated methods. Wilkinsons equivalent random traffic method comes to mind.

    There are two methods commonly used to analyse such systems (assuming random traffic and exponentially distributed holding times). The first is to use a 2-dimensional Markovian system analysis. The second is to use simulation.

    Send me an Email on if you want further information.

    John Bell

    Thanks, Peter.

    I wanted to publicly acknowledge you for your help. I used an EXCEL add-in to model things.

    The traffic I am modeling is split roughly 50-50 between voice and data. If I look at my current situation, suppose I have 200 Erlangs of traffic ging into a single trunk group engineered for 3% peak blocking (70% Retrial)… I wind up needing 214 circuits. If I split into two trunk groups, one for voice at 3% blocking, one for data at 10% blocking (70% Retrial on both), I wind up needing 112 circuits for voice and 104 circuits for data, or 216 circuits total. If I do what I thought to do originally, namely, route everything into anTrunk Group engineered for 10% blocking and then overflow only the voice calls to another trunk group so that the overall GOS for the vice calls was 3%, I wind up with 201 circuits and 4 circuits in the overflow group, for a total of 205. Finally, if I do what you suggested, route voice into a voice only group and then overfow it into a common group, I wind up with 68 circuits in the first group and 134 circuits in the second group, for a total of 202 circuits. Your suggestion is obvious to me now, since it has me using two more symmetrically-sized trunk groups… but I don’t think I would have thought of it on my own.

    Thanks again!!!

    Rich Kingoli

    Dear John Bell,

    I saw your exchange with Peter and was wondering if can I get a copy of the Excel workbook you refered to? I have a similar problem and could sure use another angle on it.


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