STEM is and has always been a demand-driven model. The direct cause and effect between
service provision and network roll-out is precisely what enables STEM to deliver
a faithful set of direct and fully-allocated per-connection (or minute or Mbyte)
cost results.
For
any requirement between Service and Resource, a constant basis is selected, so that
for the purposes of the given requirement, a single measure of demand is considered,
be it connections, annual traffic, busy-hour traffic or even annual revenue. (For
data services, annual and busy-hour traffic are typically interpreted as data volume
and peak bandwidth.)
The heart of network dimensioning in STEM is very simple: it has to be for the sake
of ready verification, and in order to be applicable to a broad range of technologies
such that strategic comparisons can be made.
Incremental demand
For a new Service, we consider its demand, divided by the capacity of a Resource,
and install a whole number of units of that Resource. Central to the proposition
is that STEM should analyse the incremental demand in the following year (or period,
if running with quarters or months). So STEM stores two pertinent data:
- the number of units of the Resource installed in each period
- the capacity of the Resource used by the Service (of which there may be several).
In a subsequent period, STEM first examines the installed base of the Resource to
see how many units (if any) have reached the end of their physical life and must
be replaced if still required. If a Service were using some old capacity which has
now expired, then its residual usage of the Resource will have decreased. The current
demand from the Service is then compared with the residual use in order to calculate
the incremental demand.
If there is existing (possibly newer) slack capacity, then STEM will use this before
installing new units. The Resource Other Details dialog contains an input, Use Slack,
which governs the age priority for using existing slack. Conversely, if Service
demand decreases such that residual use of a Resource exceeds the new demand, then
a second input, Make Slack, governs the age priority for making used capacity slack.
Once the Service-driven demand dimensioning is complete for all Services with a
requirement for the given Resource, then a number of supply-side factors are considered.
Maximum utilisation
Network planners usually prefer to avoid running the network at full capacity for
reasons of both resilience and future provisioning. The Maximum Utilisation input
enables you to specify that a given Resource should never achieve a utilisation
higher than the specified input. Thus, after the demand from all Services is considered,
STEM installs additional units of the Resource if necessary to reduce the overall
utilisation.
Deployment
The Resource element may represent a generic box – say a switch – which must be
installed at a number of distinct and geographically separate sites to support,
collectively, the total Service demand as distributed over those sites. Depending
on how the demand is actually distributed, the impact is an increase in the likely
slack capacity required in the network, up to the theoretical maximum of a whole
unit in each separate location.
The Deployment Sites and Distribution inputs for a Resource allow you to describe
the scope of the geographical diversity – typically in reference to a separate Location
element – and the nature of the distribution. The net result is another prescription
for minimum slack requirements, which STEM will satisfy by installing further units
if necessary.
Planned units
In reality, there may be other economic or pragmatic factors which drive the roll-out
of equipment, such as a fixed-rate programme defined by manpower constraints. The
Resource Other Details dialog contains an input, Planned Units, which is designed
to capture such an external prescription for cumulative installed units, and STEM
will meet this target by installing further units if necessary on a year-by-year
basis.
However, this input is currently handled only as a minimum planning constraint,
as we have not yet implemented a supply-constrained STEM model. The actual Installed
Units result can be compared with the Planned Units input to see where planned roll-out
must be reviewed.
Supply Ratio
Within each individual requirement from a Service for a particular Resource, there
is a Supply Ratio input which multiplies the demand that the Service places on the
Resource, e.g. to represent the routeing of traffic through a network. If only 25%
of the busy-hour traffic is international, then a Supply Ratio of 0.25 would be
appropriate for Resources providing international transmission.
The impact of the Supply Ratio is two-fold. Firstly, there is the obvious multiplication
on the effective demand from the Service as mapped onto the Resource when calculating
new demand. However, in order to calculate incremental demand, the Loading
is also used to divide existing used capacity appropriately, to facilitate a consistent
comparison of new demand with residual used capacity.
Functions
If several Resources have the same logical role in a network, then they may be grouped
together with a Function element so that STEM can effect a migration of demand from
one Resource to another, e.g. to model a technology update. Such a substitution
is effected through a time-series shift in the Mapping inputs for each of the requirements
for the respective Resources in the Function.
STEM will aggregate the residual used capacity of all Resources in the Function
which have been used by a particular Service when calculating the incremental demand
from that Service within the Function. That incremental demand is then split between
Resources according to the current values of the Mapping input for each
requirement, thus allowing expiring capacity of one Resource to be replaced by new
capacity of another.
If a migration must be accelerated beyond the pace of natural replacement, then
the Churn Proportion input for each requirement enables residual used capacity of
a Resource to be recycled as new incremental demand.
Pre-run installation
Resources installed in the first year of a model run – either Y0 or Y1 according
to the Include Year Zero input – may be modelled as a historical installation, governed
by the Max. Age of Installed Units input in the Resource Other Details dialog. This
feature enables a proper distinction between new and existing equipment and the
calculation of suitable Capital Expenditure and Depreciation results.
A by-product of this feature is that equipment will come up for replacement sooner
than if all the equipment were installed brand-new, as the default inputs have an
even installation of all possible installation ages, the oldest of which will be
due for replacement one year into the model run.