Calculating BHCA from traffic carried?

Hi there,

I’m confused about the following:
where ever you read about traffic engineering (even in some earlier postings here) you find the following formula:

BHCA=(Traffic in Erlang)*3600/MHT

with
BHCA = Busy Hour Call Attempts
MHT = Mean Holding Time.

As far as I understand ITU-T E.600 says that “Traffic offered” is the superset of all traffic that arrives at a resource, including “Traffic Carried” by the resource and “Overflow traffic”.

What is confusing me now is the exact meaning of “Traffic in Erlang” in the context of the above formula. As far as I understand, it can only be “Traffic Offered” because this is the superset of “Traffic” including both successful and unsuccessful seizures, just like BHCA is defined (BHCA is related to ALL call attempts, no matter whether they are successful or not). If I wanted to take a look at successful seizures, I wouldn’t calculate BHCA but BHCC (Busy Hour Call Completions).

So if I use the Formula at the top of this posting for calculating BHCA, I have to enter “Traffic Offered” as a value for “Traffic in Erlangs”.

Usually, network operators calculate with an average traffic value per subscriber (e.g 0.1 Erl is a typical value for one PSTN subscriber to calculate with) when they do network engineering. But as far as I understand, this per-subscriber-value relates to “Traffic Carried” (because it’s calculated by active connection hours) and includes both outbound AND inbound traffic of a subscriber. So this would not be a value that could be used to calculate BHCA if you don’t divide it by 2 (to separate between incoming and outgoing traffic) and multiply it with a blocking factor.

So how do I calculate a true BHCA if I only have “Traffic Carried”?

Thanks in advance

Frank