Bandwidth ?
I assume you mean the number of channels compared to the number of erlangs?
6 channels can carry only 2.3 erlangs (which means that only 2.3 channels are busy during one hour, in average, but that’s enough to get 2% congestion), and so on?
Well, the reason is because the traffic has a random rate of arrivals, and you want to keep the probability of “1 call incoming while 6 are already engaged” as low as possible (below 2%).
Regards,
Pix