What paper did Hall suggest the queuing rule of thumb $s \geq \max ( 1, \rho + \sqrt{\rho})$?

Question

According to this site:

Hall (1991) cited an argument of his previous paper that operation research profession could and should be more scientific and less mathematical. In fact, Hall also suggested another queueing rule of thumb that queues should be small if the number of servers is $$s \geq \max(1, \rho + \sqrt{\rho})$$

Where $s = $ number of parallel servers

$\rho = \frac{\lambda}{\mu}$ = traffic intensity = ratio of mean arrival rate and mean service rate.

However, a complete reference is not available on the site. I'm not familiar enough with this literature to guess who Hall might be.

Answer 1

In $\rm[I], $ the author states that since in $\rm M/M/\infty,$ $$P_n=\frac{\rho^n}{n!}e^{-\rho},$$

the number of customers in the system should only occasionally exceed the sum of the mean, $\rho,$ and the standard deviation, $\sqrt \rho.$

Then the author proposes the concerned rule of thumb.

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Reference:

$\rm [I]$ Queueing Methods, Randolph W. Hall, $1990, $ sec. $5.4.2, $ p. $146.$