We define the discrete$-$valued functions M$($t$)$ and B$($t$)$ for a given resource as follows: $-$ M$($t$)$: Scheduled number of units $($capacity$)$ of the resource at time t$,$

$-$ B$($t$)$: Number of units of the resource that are actually busy at time t$.$

Imagine Ali is the only person working at a call centre. Ali is scheduled to take a one$-$hour lunch break at $12\mathrm{.}00.$ Ali is idle from $11\mathrm{.}30$ to $11\mathrm{.}40,$ and at $11\mathrm{.}40$ he receives a call that lasts $40$ minutes. Ali starts the break when the call is finished and returns to work at $13\mathrm{.}00.$ There are no more calls before $13\mathrm{.}30.$