 The behaviour / state of a single queue (also called a "queueing node") can be described by a birth-death process, which describes the arrivals and departures from the queue, along with the number of jobs (also called "customers" or "requests", or any number of other things, depending on the field) currently in the system.An arrival increases the number of jobs by 1, and a departure (a job completing its service) decreases "k" by 1 (see Fig.3).