In early 20th century, telephone engineer Agner Erlang from Denmark studied the queueing 
process in telecommunications systems/call-processing.  He formulated the models well 
known as Erlang-B and Erlang-C.  Shown below is the formulation for Erlang-B.

Erlang-B: B(n,x) = (xn/n!)/(1 + x1/1! + x2/2! + .... + xn/n!)
                 = (xn/n!)/Σ(xn/n!)

n = Number of servers
λ = Arrival rate (per hour)
μ = Service rate (per hour)
x = Traffic intensity (Arrival Rate/Service Rate) measured in erlangs
x = λ/μ

Numerator [NR] = xn/n!

Denominator [DR] = Σ(xn/n!)