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By: Aivis Olsteins

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Voice Call Capacity Calculation

How many ports do you need to process 50,000 calls per day? 1000? More? How to estimate it?

One of the most frequently asked question from people beginning in the field of VoIP and other telecom services is about number of ports (or simultaneous calls, or CC) and how to get the right number. I am frequently asked about systems able to process 1000 or 5000 or 10,000 simultaneous calls. While on one hand these numbers do not seem to be big, let's have some mathematics help us to find an estimate.

Getting number of ports from total daily minutes.

This part is relatively easy if you think what the total number of daily minutes mean. Since the number of real minutes in a day (24 hours) is exact and limited number (24h x 60min = 1440), then one port or one simultaneous call can support exactly 1440 minutes in a day, if fully loaded at 100%. That is of course not possible, but that is where the calculation starts from. Now, 2 ports can support 2x1440=2880 minutes, 3ports = 3x1440=4320 and so on. That is in the case if traffic flow does not fluctuate during day, i.e. is always the same regardless day or night. Again, that is not what happens in reality. If you provide your services within limits of a country or region, most of businesses are open perhaps in a 8...12 h time window. That means that most if not all traffic falls into these hours. So, to squeeze all traffic, say in 8 hour time period, you would actually need 3 times more ports, since 24h / 8 h = 3. This is known as peak ratio, and in most cases is around 2...3. The operators which cover larger geographic areas, have this number lower, because their traffic is spread across more timezones.

So general formula to get number of ports is: 

Number of ports = Daily minutes * Peak Ratio / 1440.

Check out our Online Tool which can do it for you and even recommend some default values.

The second part of the question usually is about maximum calls per second, or CPS. Here, we have several more parameters in play:

ASR - Answer / Seizure Ratio - a number of successfully connected calls versus all call attempts. Very typical values are somewhere between 30...60% (or 0.3 ... 0.6)

ACD - Average Call Duration - a duration of average successful call (non connected do not count here) , expressed in minutes (important!). It may vary very significantly. Call center traffic may have ACD as low as 30sec - 2minutes), versus Callshop services which may have it as high as 6 minutes or more. Again, you can look default suggested values in our Online Tool.

Lets do it step by step:

ACD = Total minutes / Good calls, i.e. number of good calls = Total minutes / ACD.  Next, we know that ASR represents relation between all call attempts and good calls, so: All calls = Good calls / ASR. Now when we have a total number of calls per day and we know number of seconds in a day, we get CPS = All calls / 24 / 60 / 60.

Putting all together, here is what we got:

Average CPS = Minutes  /  86400 / ASR / ACD.

This is averaged value. Keep in mind we have peaks during time of the day, so we will need to multiply the average with peak ratio to get Max CPS value:

Max CPS = Minutes * Peak Ratio / ASR / ACD / 86400

Again, check out our Online Calculator with the recommendations of some of the default values.


Update 2019-02-09: Please see also my latest post about application of queueing theory in call capacity calculation.


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About Author
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My name is Aivis Olsteins and I am owner of DataTechLabs. My experience in Telecoms started in early 1990's and I have worked in multiple technical positions in mobile, messaging and data networks. My expertise lies in telecom networks, database systems, distributed processing and large data analysis. These posts are my attempt to share my knowledge with everyone who might find it useful.

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