Working with telephone lines, Erlang showed in 1917 [8] that traffic requests follow a Poisson process, that is, calls are
independent and the activity of a channel is exponentially distributed (the longer the duration of a call, the likelier for the
user to finish the connection), and the probability of a user to reach a situation where no more channels are available can
be determined by some characteristics of the system, such as the number of channels and the average duration of a call.
Erlang’s work prompted the development of queuing theory [9]