Many day-to-day problems in performance analysis of computer systems can be solved by using some simple relationships that do not require any assumption about the distribution of service times or interconnection times. It is worth mentioning the meaning of the term Operational Analysis. The word operational refers to data measured directly from a running system, that is, a functioning system. The concept of a service demand is related to the set of requests a service makes to the system's pool of resources. The items could be people, digital packets, requests, parts to assemble a product, instructions waiting to be executed in a processor, etc. The input flow of items enters the system, joins one or more queues, is somewhat processed, and exits the system in an output stream of departures. In an interactive system, users generate requests to a server or a set of servers such as a cloud-based system that responds to user machines.