How many concurrent calls my system can handle?
Nikolay Naychov, 9 Sept 2012
One of my favorite questions in my practice. Note that the “real” measure of performance is the number of call attempts per second (since each request has to be processed by the system), while the number of concurrent calls is derived from it, based on your traffic patterns (average length of call, call success rate, etc.). A system which processes traffic with a lower ASR (average success rate) will handle fewer concurrent calls than an identical system which handles traffic with a better ASR.
Let’s make this real & some numbers to this.
Assuming an aggregated call processing speed of 50 call attempts per second, an average call duration of 5 minutes, and a call success rate of 50% (the industry norms), it means that 50% of the 50 call attempts per second will succeed. So 25 calls will be connected, while the same amount of previously connected calls will be disconnected. Since the average call duration is 300 seconds, this means that at all times approximately 25 *300 = 7,500 calls will be in a “connected” state. If your ASR changes, it will have an immediate impact on the number of concurrent calls, e.g.:
ASR 25%: (50 * 25%) * 300 = 12.5 * 300 = 3,750 concurrent calls
ASR 60%: (50 * 60%) * 300 = 30 * 300 = 9,000 concurrent calls
Thus the value of 7,500 is an estimate based on industry norms (5 minutes ALOC, 50% ASR). The actual number in your system will ultimately be determined by your traffic patterns and other factors, such as the hardware used, network connection quality, and whether or not you proxy the voice stream for a large portion of calls.
To simulate how the expected maximum number of concurrent calls changes based on your traffic patterns, use this VoIP capacity calculator.
How many minutes of monthly traffic that means?
Again, this depends mainly on your traffic patterns (the ratio between the duration of your peak and off-peak hours and the differing amount of calls in peak and off-peak hours) and your call success rate. We can estimate this based on typical traffic patterns (as indicated below) with daily peak hours, medium load hours and off-peak hours. We should also bear in mind that your network must be able to handle all kinds of traffic – off-peak hours, peak hours, and extraordinary traffic splashes (e.g. New Year’s Eve).
Thus the total performance capacity must be greater than the maximum anticipated BHCA (busy hours call attempts). On average, extreme traffic peaks (such as Christmas, New Year’s, etc.) will have 2 to 3 times more call attempts than your normal peak hours, so approximately 50 / 2.5 = 20 call attempts per second can be expected during your peak hours. Using the same “industry norm” figures as above (50% ASR, 5mins ALOC), your switch could process (20 * 50%) * 5 * 3600 = 180,000 minutes during your peak hours. We can estimate that your total amount of daily traffic will be about 6 times the amount of traffic during one peak hour, i.e. 180,000 * 6 = 1,080,000 minutes per day. Then, multiplying the daily amount by the number of “business” days in a month, we get
1,080,000 * 20 = 21,600,000 minutes.