There are a number of questions that have popped up in this thread that I may be able to answer. The first is the difference between a BHCA (Busy Hour Call Attempt) and BHCC (Busy Hour Call Completion) BHCA is the total number of attempts that a device (i.e. a switch, BTS, BSC, etc.) receives in the busy hour. This includes successful calls (the two parties talk to each other) and unsuccessful calls (the two parties are not connected for reasons like line busy, no resources, missed pages, etc.) So BHCA should be bigger than BHCC. You typically worry about BHCA (or BHCC) if you are looking at the capacity of a processor (i.e. the CPU of a switch or BSC) where the devise just processes messages and does not need to dedicate a resource for the duration of a call.
The Erlang is used when you need to provision resources that need to be dedicated for the duration of the call (i.e. trunks, radio channels, etc.) The Erlang is defined (as others on this thread have pointed out) as the calls per hour (or BHCA) * call holding time (in seconds) / 3600 (seconds in an hour.) It is also referred to as the traffic on a link. You would then the Erlang-b formula to determine the number of resources that would be required with only a certain amount of blocking. Some of the standard blocking values are 1% for trunks and 2% for radios in a cell. You would then consult an Erlang-B table to determine the number of channels (links, trunks, etc.) that you would need.
To make sure this is clear, let’s assume that we have a BHCA of 100 and a call holding time of 90 seconds. This would give us 2.5 Erlangs (100*90/3600). If we consult an Erlang-B table or an Erlang-B formula (see this web site for a program or John Belamy’s Digital Telephony book for an Erlang-B table.) and we assume a 1% blocking we would need 8 channels.
One other data point, we can use the Erlang-B table to estimate the traffic a link can support. A T1 (24 channels) can support 15.3 Erlangs with a 1% blocking.
Please email me (firstname.lastname@example.org) if you want to discuss this more.