The simplest sort of queuing model describes (for example) customers entering a line, being dealt with by one or more ‘servers’, and then leaving. Such queuing models can be represented using Kendall’s notation:
where:
Often the last members are omitted, so the notation becomes $A/B/S$ and it is assumed that $K = \infty$, $N = \infty$ and $D = FIFO$.
Some of the most popular probability distributions for $A$ and $B$ are:
$M$: Markovian, meaning a Poisson process.
$E^k$: an Erlang $k$-process, which is the convolution of $k$ identical Poisson processes.
$D$: a degenerate distribution, meaning a delta function at some fixed waiting time.