Queuing Theory:

Queuing theory is the mathematical study of waiting lines. The theory enables mathematical analysis of several related processes, including arriving at the (back of the) queue, waiting in the queue (essentially a storage process), and being served by the server(s) at the front of the queue. The theory permits the derivation and calculation of several performance measures including the average waiting time in the queue or the system, the expected number waiting or receiving service and the probability of encountering the system in certain states, such as empty, full, having an available server or having to wait a certain time to be served.
Here are details of four queuing disciplines:
First in first out
This principle states that customers are served one at a time and that the customer that has been waiting the longest is served first
Last in first out
This principle also serves customers one at a time; however the customer with the shortest waiting time will be served first.
Processor sharing
Customers are served equally. Network capacity is shared between customers and they all effectively experience the same delay.
Priority
Customers with high priority are served first