Four of the most widely used waiting line models–M/M/1 or A, M/M/S or B, M/D/1 or C, and Limited population or D—all share three characteristics: Poisson arrivals, FIFO discipline, and exponential service times.

A queueing model is constructed so that queue lengths and waiting time can be predicted. is generally considered a branch of operations research because the results are often used when making business decisions about the resources needed to provide a service.
A queuing system is characterised by three components: Arrival process - Service mechanism - Queue discipline.

Related Test: Operations Management 101 Practice Test: Transportation Modeling