To see the Little's law triplet, consider the line of customers at the grocery store checkout lane shown in Figure 1. Following the usual queueing theory convention, the queue includes not only the customers waiting but also the customer currently in service.
As an aside, it is useful to keep in mind that there are only three types of performance metric:
- Time $T$ (the fundamental performance metric), e.g., minutes
- Count or a number $N$ (no formal dimensions), e.g., transactions
- Rate $N/T$ (inverse time dimension), e.g., transactions per minute
Shoppers in Figure 1 arrive from the left into the checkout lane with arrival rate $\lambda$ customers per minute, for example. $\lambda$ is a type-C metric. The total time they spend at the checkout is the sum of their waiting time, $W$ (due to the customers already ahead of them), and their service time, $S$ (the average time it takes to ring up their groceries). This time is called their residence time \begin{equation} R = W + S \label{eqn:Rtime} \end{equation} $R$ is a type-A metric. It gives us the temporal view of the queue. After paying the cashier each customer departs to the right.
Example
For the purpose of simple numerical calculations, let's assume the following average values for the queueing parameters: \begin{align} \lambda &= 0.5 ~\text{customer per minute} \label{eqn:arate}\\ S &= 1.0 ~\text{minute} \label{eqn:Smins}\\ W &= 1.0 ~\text{minute} \label{eqn:Wmins} \end{align} From eqn.\eqref{eqn:Rtime} it follows that \begin{equation} R = 1 + 1 = 2 ~\text{minutes} \label{eqn:Rmins} \end{equation} The residence time tells us that the average time a customer can expect to spend getting through the checkout lane is two minutes.
Little's Law Version 1
The corresponding average queue length, $Q$, in Figure 1 tells us how much space customers are expected to occupy and it can be determined from this version of Little's law \begin{equation} Q = \lambda R \label{eqn:LLQ} \end{equation} Clearly, \eqref{eqn:LLQ} has the form \eqref{eqn:dims}: a rate multiplied by a time. $Q$ is a number (an average value), so it's a type-B metric. Equation \eqref{eqn:LLQ} is probably the most common form of Little's law arising in performance calculations.
Example
Using the values in \eqref{eqn:arate} and \eqref{eqn:Rmins} it follows that \begin{equation} Q = \dfrac{1}{2} \times 2 = 1 ~\text{customer} \label{eqn:LLQval} \end{equation} The average queue length is 1 customer (i.e., both waiting and in service). How a single customer can be simultaneously waiting and in service will become clearer shortly. In the meantime, that's count one for Little.
Little's Law Version 2
Figure 1 reveals that there's another way to express the queue length, $Q$. Like the residence time, we can also decompose the queue length into the length of the waiting line, $L$, and the average number of customers in service \begin{equation} Q = L + m \rho \label{eqn:Qdecomp} \end{equation} This gives us the spatial view of the queue, where $m$ is the number of servers and $m\rho$ is the average occupancy of the servers. In Figure 1 $m = 1$, since there is only a single cashier, and $0 \leq \rho < 1$ is the fraction of time for which the cashier is busy. The value $\rho = 1$ would correspond to 100% busy. Clearly, the cashier can't be busier than 100% of the time.Substituting \eqref{eqn:Qdecomp} and \eqref{eqn:Rtime} respectively into the left side and right side of \eqref{eqn:LLQ} produces \begin{equation} L + m \rho = \lambda W + \lambda S \label{eqn:decompAll} \end{equation} The first terms on each side of \eqref{eqn:decompAll} can be equated separately as \begin{equation} L = \lambda W \label{eqn:LLW} \end{equation} Once again \eqref{eqn:LLW} has the form \eqref{eqn:dims}. Equation \eqref{eqn:LLW} tells us that the waiting line length, $L$, is directly proportional to the waiting time, $W$. You can think of it as an estimate of buffer size, if you prefer. Remarkably, this is the form of Little's law that appears in the title of his 1961 paper.
Example
Using the values in \eqref{eqn:arate} and \eqref{eqn:Wmins} it follows that \begin{equation} L = \dfrac{1}{2} \times 1 = \dfrac{1}{2} ~\text{customer} \label{eqn:LLLval} \end{equation} Half a customer may look strange but remember, these are average values. That's count two for Little.
Little's Law Version 3
Repeating the previous step for the corresponding second terms in \eqref{eqn:decompAll}, we have \begin{equation} \rho = \dfrac{\lambda S}{m} \label{eqn:LLS} \end{equation} Again, \eqref{eqn:LLS} has the form \eqref{eqn:dims}: division by the constant $m$ doesn't alter anything. Equation \eqref{eqn:LLS} tells us that the per-server utilization (a number and therefore a type-B metric) is directly proportional to the service time ($S$). In Figure 1, $m=1$ since there is a single server (the cashier).This version of Little's law is often referred to as the Utilization law, but that doesn't help to make useful connections between performance metrics. It is common practice to use a notation like $U = m \rho$ to represent the total utilization of $m$-servers such that $0 < U < m$.
Example
Using the values in \eqref{eqn:arate} and \eqref{eqn:Smins} it follows that the total utilization is \begin{equation} U = \dfrac{1}{2} \times 1 = 50\% \end{equation} Notice also that \begin{equation} Q = L + U = \dfrac{1}{2} + \dfrac{1}{2} = 1 ~\text{customer} \end{equation} in agreement with the Little's law calculation in \eqref{eqn:LLQval}. And that's count three for Little.
Triplet of Interpretations
The Little's law triplet can be summarized as:\begin{align*} Q &= \lambda R: \; \text{average number of customers in residence}\\ L &= \lambda W: \; \text{average number of customers in waiting}\\ U &= \lambda S: \; \text{average number of customers in service} \end{align*} each of which is consistent with \eqref{eqn:dims}. Knowing these word interpretations might be the key to solving an otherwise obscure performance analysis problem.