Here´s a goog way to think about it. Let´s assume that you are a softswitch vendor and you´re trying to size a VoIP package. All you know is that he currently has 50 E1´s, or 31*50 = 1550 channels, into country X, he has measured ASR of 35%, and a mean holding time of 110 sec. You´re softswitch can handle 200k BHCA and you need to find out if it can handle the above requirement. For the purposes of this calculation we are going to assume max efficiency on the channels during the (i.e. each channel is running 1 Erlang during the busy hour), and that failed calls take 0 seconds to clear. If this is the case then:
BHCC (Busy Hour Call Completions) per channel = 3600/110 = 33 cc/hr./channel.
BHCC for 50 E1s = 33 * 1550 = 51,150 cc/hr.
Because the ASR is .35, and the calls above completed, we can assume that for every 35 successful calls, there were 100 attempts, 65 of which were unsuccessful. Dividing 51,150 by the ASR will then give us the BHCA. So:
51,150/.35 = (51,150/35)*100 = 146,142 BHCA.
So we can breathe easy because our platform can support the customer specs as outlined above.