Stationary distribution of the number of demands

Mass Service Theory (MST) studies the processes in which In which demands to perform any type of services, and the servicing of these demands occurs. The objects (TMT) can be manufacturing processes, supply processes, transport, trade, military operations.
The common feature of all tasks (TMO) is the random nature of the phenomena under study:
1) the number of demands arriving for service, and the number of demands coming for servicing, and the time interval between their arrival moments are are random variables;
the duration of service of each request is of random nature.