The Erlang B formula determines the probability that a call is blocked and is a measure of the Grade_of_Service for a Telecommunication environment. Traditionally, it was used for “trunked” systems.
The primary driving element with Erlang B is that it estimates, statistically, the probability that all circuits will be busy when a call is attempted.
The Erlang C model is more focused on delays. It is a delay model as opposed to a blocking model
VBR/ Wallis Dudhnath