The following standard notation is used to characterize systems with a single queue in equilibrium, identical parallel servers, unlimited waiting room, and with rst-come rst-served (FCFS) queueing discipline. Each system is de ned by


where A denotes the distribution of inter-arrival times B denotes the distribution of service times m denotes the number of servers in parallel

Commonly used symbols for both the A and B positions in this notation are M, D, and G. The symbol M is used for the exponential distribution to denote its Markovian (“memoryless”) property. The symbol D is used for deterministic times. The symbol G is used for a general distribution of independent and identically distributed random variables. Examples:

M/M/1 queue: Exponential distribution for inter-arrival times, exponential distribution for service times, and one server.