Technology
Fundamental FTTH
Planning and Design: Part 1
Placement of network elements in the field can have a major effect on
deployment costs. Here are some principles to follow.
By David Stallworth ■ OFS
C
hoosing the right fiber-to-thehome design strategy is very
important, but the best strategy
in the world may not achieve the desired results if network elements are not
placed economically in the field. Years
ago, OFS initiated a Fundamental Planning research project to address this issue. The project was later expanded to
include the locations of central offices
(COs) and nodes. This massive effort led
to the development of ideal configurations for outside-plant networks and optimal locations for COs and nodes.
In the study, we developed an ideal
solution based on an ideal service area –
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locations studied. We studied virtually
every possible configuration, evaluating
multiple feeder routes; different routing
techniques, such as parallel routes, perpendicular routes and circular routes;
and different cable sizes or route capaci-
By understanding what an ideal network should
look like, a planner can configure a real network
to conform to the ideal as much as possible.
a square with streets arranged in square
blocks (see Figure 1).
The initial study consisted of moving the CO or node around the area to
study the effects of its position on total
network costs (see Figure 2; the CO is
the red point). Most of the variation
was in outside-plant cost, as other costs
(switches, terminations and so forth)
remained constant. The CO or node
was placed in various positions in the
ideal area, and various configurations
of feeder routes were studied along with
the associated support structures.
Figure 2 shows just a few of the
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ties. As the studies progressed, patterns
began to emerge that were both intuitive
and informative.
The node goes in the middle
From this study, we reached several
general conclusions that have proven
over time to define the most economical method for planning CO or node
locations and outside-plant network
configurations. Some of the conclusions became obvious once they were
discovered, and others were revelations
that have been further refined over the
years. By understanding what an ideal
network should look like, a planner can
configure a real network to conform to
the ideal as much as possible.
The study showed that the most economical location for a CO or node is exactly in the middle of the area it serves
(see Figure 3). This position not only
yielded the lowest cost for all customers in the serving area but also provided
the lowest optical loss to all customers.
Most designers consider this an intuitive
finding.
For a square area with uniform density throughout, the ideal node location
is in the middle of the area it serves. Let’s
call this the geographic economical location (GEL). However, because not all
areas have this characteristic, we should
About the Author
David Stallworth is the design and product manager at OFS, a manufacturer of optical fiber and connectivity solutions. You can reach him at 770-798-2423 or by e-mail
at dstallworth@ofsoptics.com.
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Technology
Figure 1: We began with an ideal study area that was completely uniform.
Figure 2: Within the ideal study area, we tried out many possible network configurations.
Figure 3: This is the most cost-effective location for the central office.
consider what happens when the density
is not uniform. Consider the situation in
Figure 4, in which the lower left quadrant, outlined in blue, has a density of 64
homes instead of 32. How would this affect the ideal location for a CO or node?
With uneven density, the ideal location moves from the center of the serving
area closer to the denser area, but not all
the way there. Clearly, the denser area has
more customers and requires more facilities; at the same time, other areas’ costs
increase as the CO or node is moved further away. Let’s call the new ideal point
the density economical location (DEL).
Of course, the denser the area, the
more the ideal will drift toward it, so
density plays a role in CO or node location. Figure 5 indicates the effect of
the denser area on CO or node location.
This is a conceptual drawing that shows
the ideal location moving closer to the
denser area.
Notice that the DEL is now offset
from the GEL. One can develop intricate
formulas to calculate the relationship between the two points for an area. In the
real world, it is better to understand and
apply the relationship than to spend time
on detailed calculations because the real
world poses still more problems.
Because not many towns or cities
are ideal, the planner must make adjustments to fit actual city or area layouts.
Experience is important in meshing the
Figure 4: We then made the study area less ideal by varying its density.
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Technology
Cost rises exponentially the farther
away the central office or node is
located from the ideal point (that
is, the center of the service area
adjusted for density variation).
Figure 5: Density affects the proper placement of the CO or node.
Figure 6: Cost rises exponentially with the distance from ideal location.
Figure 7: Splitter cabinet location (cabinet is red dot)
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ideal with the actual. For example, on the coast of South Carolina, where I live, we do not have many eastern routes – the
Atlantic Ocean is in the way! Valleys are usually oblong and not
square, yielding only two possible routes. However, the principles of the ideal world still apply, and planners should make
every effort to come as close as possible to the ideal.
There is a very good reason to stay as close as possible to the
ideal: As shown in Figure 6, cost rises exponentially as
the location moves away from the ideal point.
In building a network, the first step is to locate the
GEL point and then adjust it for density. After establishing the DEL, the designer can examine the area to locate
appropriate land or rights-of-way. This requires good engineering judgment. Access to a road network is needed
for placing cables. In addition, the location should be
blended in with the surrounding structures and should
be as “green” as practical.
the principle applies at any scale
Though this model was initially developed for CO and
node locations, it has additional applications. If optical splitters for a PON are placed in a cabinet, the same
question arises: Where should the cabinet be located?
The analysis is very similar because the cabinet serves a
defined area, and cables need to be laid to all customers
in the area. We should expect the findings to be similar if
the premises and conclusions are correct.
Figure 8: Smaller cabinet locations (cabinets are red dots)
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Technology
Figure 7 shows where to place a cabinet to serve a subarea that the designer
has identified as having consistent requirements (the subarea may be all
residential or all business, for example).
Note that the subarea is carved up into
even smaller 32-home areas.
The same area is shown in Figure 8,
except that a smaller cabinet size is used
(160 instead of 320 homes). Even the
smaller cabinets are placed in the middle
of the areas they serve. Documenting
service area boundaries is essential so
that facilities can be planned and placed
to serve a defined area and not extended
past any boundary line.
Even in very small areas, costs rise as
the cabinet location moves away from
the ideal central location. This principle
applies in an active Ethernet deployment where remote nodes are placed
in the field: Such nodes should also be
in the middle of the areas they serve.
The same holds true for optical splitters
placed in a distributed manner (distributed splitting means placing a splitter in
The principle of locating equipment in the center
of its service area is valid no matter what the
scale. Central offices, field cabinets, active nodes,
splitters and even drop closures are all most
economically placed at the center.
each 32-home area, typically in a drop
closure and spliced to the fibers entering
and leaving the closure).
distributed architecture
In a distributed 1 x 32 architecture, it is
possible to serve at least 256 customers
with a single 24-fiber cable by using the
optical splitter to full advantage to eliminate dead fibers. The same is true with
a cascaded splitter deployment, where
a 1 x 4 splitter serves four 1 x 8 splitters spread over the 32-home area: Each
splitter should be placed in the middle of
the customers or area it serves.
These concepts can be extended still
deeper into the network: A drop closure,
too, should be placed in the middle of the
area or customers it serves. Even though
the potential saving from positioning
a single drop closure is not large, many
drop closures are placed in a network.
Adhering to these concepts for all of them
may result in an overall smaller cost.
In Part 2 of Fundamental FTTH
Planning and Design, we will turn our
attention to configuring the fiber routes
out of these facilities. BBP
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