Chapter 12, Problem 7E

Explanation of Solution

Downtime calculation for 99% reliability:

When the carrier is 99% reliable, then it means that it is 1% unreliable.

Formula for computing the downtime is given below:

$Downtime=\left(\begin{array}{l}Numberofdaysinayear\times \\ Unreliablepercentage\end{array}\right)$        (1)

Substitute “1%” for “Unreliable percentage” and “365” for “Number of days in a year” in the Equation (1)

Explanation of Solution

Downtime calculation for 99.5% reliability:

When the carrier is 99.5% reliable, then it means that it is 0.5% unreliable.

Substitute “0.5%” for “Unreliable percentage” and “365” for “Number of days in a year” in the Equation (1).

$\begin{array}{c}Downtime=\left(365\times 0.5\%\right)\\ =365\times \frac{0.5}{100}\\ =1.825\end{array}$

The number of downtime is 1.825 days.

Note: 1 day = 24 hours.

$\begin{array}{c}Numberofdowntimehours=\left(1.825\times 24\right)\\ =43.8\end{array}$

Therefore, the number of downtime hours is “43.8” hours.

Downtime calculation for 99.9% reliability:

When the carrier is 99.9% reliable, then it means that it is 0.1% unreliable.

Substitute “0.1%” for “Unreliable percentage” and “365” for “Number of days in a year” in the Equation (1).

$\begin{array}{c}Downtime=\left(365\times 0.1\%\right)\\ =365\times \frac{0.1}{100}\\ =0.365\end{array}$

The number of downtime is 0

Explanation of Solution

Minimum reliability:

Most firms need the reliability of a common carrier to be “100%”. However, this is not a feasible.

The closest reliability percentage most firms consider would be “99.99%” or “99.9%”. Since, their downtime of these reliabilities is less than 9 hours...