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Choosing the right fiber-to-the-home design strategy is veryimportant, but the best strategy

in the world may not achieve the de-sired results if network elements are notplaced economically in the field. Yearsago, OFS initiated a Fundamental Plan-ning research project to address this is-sue. Te project was later expanded toinclude the locations of central offices(COs) and nodes. Tis massive effort ledto the development of ideal configura-tions for outside-plant networks and op-timal locations for COs and nodes.

In the study, we developed an idealsolution based on an ideal service area

Fundamental FTTHPlanning and Design: Part 1

locations studied. We studied virtually

every possible configuration, evaluating

multiple feeder routes; different routing

techniques, such as parallel routes, per-

pendicular routes and circular routes;

and different cable sizes or route capaci-

locations and outside-plant networkconfigurations. Some of the conclu-sions became obvious once they werediscovered, and others were revelationsthat have been further refined over the

years. By understanding what an idealnetwork should look like, a planner canconfigure a real network to conform tothe ideal as much as possible.

Te study showed that the most eco-nomical location for a CO or node is ex-actly in the middle of the area it serves(see Figure 3). Tis position not onlyyielded the lowest cost for all custom-ers in the serving area but also providedthe lowest optical loss to all customers.Most designers consider this an intuitivefinding.

For a square area with uniform den-sity throughout, the ideal node locationis in the middle of the area it serves. Letscall this the geographic economical lo-cation (GEL). However, because not allareas have this characteristic, we should

By David StallworthOFS

Placement of network elements in the field can have a major effect ondeployment costs. Here are some principles to follow.

About the Author

David Stallworth is the design and product manager at OFS, a manufacturer of opti-

cal fiber and connectivity solutions. You can reach him at 770-798-2423 or by e-mailat dstallworth@ofsoptics.com.

Learn about the challenges

of FTTH deployment at theBroadband Properties Summit,

April 2628 in Dallas.

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).Te initial study consisted of mov-

ing the CO or node around the area tostudy the effects of its position on total

network costs (see Figure 2; the CO is

the red point). Most of the variationwas in outside-plant cost, as other costs

(switches, terminations and so forth)remained constant. Te CO or node

was placed in various positions in the

ideal area, and various configurationsof feeder routes were studied along with

the associated support structures.

Figure 2 shows just a few of the

ties. As the studies progressed, patterns

began to emerge that were both intuitiveand informative.

THE NODE GOES IN THE MIDDLE

From this study, we reached severalgeneral conclusions that have provenover time to define the most economi-cal method for planning CO or node

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consider what happens when the densityis not uniform. Consider the situation inFigure 4, in which the lower left quad-rant, outlined in blue, has a density of 64homes instead of 32. How would this af-fect the ideal location for a CO or node?

With uneven density, the ideal loca-tion moves from the center of the servingarea closer to the denser area, but not allthe way there. Clearly, the denser area hasmore customers and requires more facili-ties; at the same time, other areas costsincrease as the CO or node is moved fur-ther away. Lets call the new ideal pointthe density economical location (DEL).

Of course, the denser the area, themore the ideal will drift toward it, sodensity plays a role in CO or node lo-cation. Figure 5 indicates the effect ofthe denser area on CO or node location.Tis is a conceptual drawing that showsthe ideal location moving closer to thedenser area.

Notice that the DEL is now offsetfrom the GEL. One can develop intricateformulas to calculate the relationship be-tween the two points for an area. In thereal world, it is better to understand andapply the relationship than to spend time

on detailed calculations because the realworld poses still more problems.

Because not many towns or citiesare ideal, the planner must make adjust-ments to fit actual city or area layouts.Experience is important in meshing the

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. Figure 4: We then made the study area less ideal by varying its density.

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Figure 8: Smaller cabinet locations (cabinets are red dots)

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)

Cost rises exponentially the farther

away the central office or node is

located from the ideal point (that

is, the center of the service areaadjusted for density variation).

ideal with the actual. For example, on the coast of South Caro-lina, where I live, we do not have many eastern routes theAtlantic Ocean is in the way! Valleys are usually oblong and notsquare, yielding only two possible routes. However, the prin-ciples of the ideal world still apply, and planners should makeevery effort to come as close as possible to the ideal.

Tere is a very good reason to stay as close as possible to theideal: As shown in Figure 6, cost rises exponentially asthe location moves away from the ideal point.

In building a network, the first step is to locate theGEL point and then adjust it for density. After establish-ing the DEL, the designer can examine the area to locateappropriate land or rights-of-way. Tis requires good en-gineering judgment. Access to a road network is neededfor placing cables. In addition, the location should beblended in with the surrounding structures and shouldbe as green as practical.

THE PRINCIPLE APPLIES AT ANY SCALE

Tough this model was initially developed for CO andnode locations, it has additional applications. If opti-cal splitters for a PON are placed in a cabinet, the samequestion arises: Where should the cabinet be located?Te analysis is very similar because the cabinet serves adefined area, and cables need to be laid to all customersin the area. We should expect the findings to be similar ifthe premises and conclusions are correct.

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Figure 7 shows where to place a cabi-net to serve a subarea that the designerhas identified as having consistent re-quirements (the subarea may be allresidential or all business, for example).Note that the subarea is carved up into

even smaller 32-home areas.Te same area is shown in Figure 8,

except that a smaller cabinet size is used(160 instead of 320 homes). Even thesmaller cabinets are placed in the middleof the areas they serve. Documentingservice area boundaries is essential sothat facilities can be planned and placedto serve a defined area and not extendedpast any boundary line.

Even in very small areas, costs rise asthe cabinet location moves away from

the ideal central location. Tis principleapplies in an active Ethernet deploy-ment where remote nodes are placedin the field: Such nodes should also bein the middle of the areas they serve.Te same holds true for optical splittersplaced in a distributed manner (distrib-uted splitting means placing a splitter in

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 elim-

inate dead fibers. Te same is true with

a cascaded splitter deployment, where

a 1 x 4 splitter serves four 1 x 8 split-

ters spread over the 32-home area: Each

splitter should be placed in the middle of

the customers or area it serves.

Tese 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, manydrop closures are placed in a network.

Adhering to these concepts for allof them

may result in an overall smaller cost.

In Part 2 of Fundamental FH

Planning and Design, we will turn our

attention to configuring the fiber routes

out of these facilities. BBP

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 mosteconomically placed at the center.