Pn = Probability of n customers in a system (steady state)

Pn(t) = Probability of n customers at time t in a system (transient state)

Ls = Expected number of customers in a system

Lq = Expected number of customers in queue

Ws = Expected waiting time in a system (in queue + in service)

Wq = Expected waiting time in queue

λn = Mean arrival rate when n customers are in the system.

(If λn = Constant for all n, this constant is denoted by λn)

µn = Mean service rate for overall system when n customers are in the system.

(If µn = Constant for all µ, this constant is denoted by µ)

ρ = λ / µ = Traffic intensit:Yiutilization factor

s = Number of servers

N(t) = Number of customers in queueing system at time t.

It has been known that L5 = λ W5, Lq = λ Wq. These relations are called 'Little's formula'. Also we have,