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Reply To: MilliErlangs per sub

John Bell

There is a white paper on this site that talks about using call logs to dimension trunks. After I quote it, I’ll tell you what I did for a network I had to design. Here’s an excerpt:

“It is also common for traffic figures for calls carried over particular trunk groups to be retrieved from loggers. With existing networks, these figures are often used to calculate the optimum number of trunks for a group. It is the view of Westbay Engineers Limited that this practice is not a sound basis for network design for the following reasons:

Call loggers can give a distorted view of the traffic on a link as they measure carried traffic rather than offered traffic. Changes to link capacities based on these figures can often be under-estimated because traffic models base their results on offered traffic. In other words, increasing the number of lines generates more traffic which needs more lines! Link traffic does not include those calls from callers who do not use the direct links because the grade of service is bad. Tweaking links as necessary avoids the central issue of network design which is to produce the most economical network layout. To design a network, consideration should be given to locations between which links should be introduced or removed rather than changing the size of existing links.”

Now for some less depressing information. I had a similar problem to the one you are facing. To solve it, I used the *traffic reports* (not the call logs) in the switch I was studying, along with information about the number of people who had signed up for service that could be served by my switch.

Over time, I came up with a model of how many CCS (or Erlangs) a typical subscriber generated for several different types of calls (local, long distance, incoming, etc.) Then I used information from my company’s marketing people where they estimated the number of people who would sign up for service at each node in our network. For each site, I took this number of “subscribers” and multiplied that by the number of peak CCS per subscriber for each different type of call. Then I used those numbers to size the trunk groups. You can email me at if you want to discuss further.