The Maximum Utilisation (MU) input tells STEM to install an additional variable amount of slack capacity (usually bandwidth) in proportion to the nominal total demand to allow for transient peaks in usage.
From STEM 7.4 onwards, introduction of a Deploy With Utilisation input allows consideration of geographical deployment when a resource specifies a Maximum Utilisation constraint.
The range of notational utilisation at each site is limited to 0–MU when Deploy With Utilisation = Yes (default). Utilisation at the ‘hottest’ site wll not exceed MU, and will be proportionately lower at other sites, resulting in lower overall utilisation than without the MU constraint.
Figure 1: Notational utilisation at each site when Deploy With Utilisation = No
Figure 2: Notational utilisation at each site when Deploy With Utilisation = Yes
Thus, the Maximum Utilisation constraint will impact the Installed Units result directly. If Deploy With Utilisation = No, the Installed Units result will only be affected if the specified overall Maximum Utilisation is lower than that which would already result from an existing allowance for deployment.
The installation result matches that of an unconstrained resource with equivalent input Capacity = c . MU (where c is the Capacity of the original resource).
Legacy behaviour
STEM 7.4 will set the input Deploy With Utilisation = No for all resources when loading an older model in order to maintain the original results. However, you may wish to review the results (and this setting going forward) if any resources in your model combine the Maximum Utilisation and Deployment features.
Example
Consider a resource with the following input characteristics and for which the overall demand gradually increases from zero to 1000 subscribers over ten years:
-
Capacity = 50 Ports
-
Maximum Utilisation = 80%
-
Sites = 10
-
Distribution = Monte Carlo.
Figure 1 below compares the results for two scenario variants,
Lean
and
Variable
, which switch the Maximum Utilisation input between 100% and 80%, respectively, and for which Deploy With Utilisation = No.
Figure 1: No difference with
Maximum Utilisation
when
Deploy With Utilisation
= No
The chart on the right (where Maximum Utilisation = 80%) looks exactly the same! Indeed, there is no difference at all because the Deployment calculation already results in sufficient slack capacity for the Utilisation Ratio result never to exceed 78%.
Figure 2 below shows the results for a third scenario variant, Variable NEW, which in contrast sets the input Deploy With Utilisation = Yes.
Figure 2: Increased overhead with Maximum Utilisation when Deploy With Utilisation = Yes
You can see that a greater overhead of slack capacity is installed; in addition to the more or less fixed geographical overhead (roughly 250 ports), now there is also a variable component (in proportion to the demand) which amounts to another 250 ports by the end of the model run (equivalent to the 25% of demand required to ensure that utilisation would not exceed 80% at any one site). The combined effect results in an overall utilisation which never exceeds 66% within the model run.
(You may find it instructive to consider how the results would compare for a resource with inputs Capacity = 40 Ports and Maximum Utilisation = 100%; i.e., the same effective capacity of the original resource.)
See also 10.3.25.1 Maximum Utilisation.