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Rural fiber distribution TAP architectureA more efficient approach for deploying a future-focused FTTH networkSplitter cabinetMNONUONTFiber feederBringing broadband service to rural and underservedexurban areas can pose unique challenges toHECO1:32providers. Deployments must cover great distancesto reach just a few homes. Rural areas have highercosts per home passed, and require high subscriberFiber feederHECOTapTapTapTaptake rates to make fiber deployments economicallypossible. Providers must invest heavily in equipmentand labor, so solutions that can reduce expendituresMNONUONTMNONUONTMNONUONTMNONUONTin either of those key categories can make thedifference between economic success or failure.Figure A: A traditional, centralized, FTTH network architecture (top)compared to a distributed TAP network architecture (below)This white paper will explore the tap networkarchitecture option to create or expand ruralfiber-to-the-home (FTTH) networks.A tap network design is quite different from the design ofa traditional “centralized” FTTH network, which typicallyuses splitters installed in a cabinet configuration to distributeFIBER-OPTIC TAPSdata to subscribers. In this splitter-based architecture, a fiber-In a tap FTTH network architecture, a fiber cable is deployedoptic feeder line runs from the central office or head-endthroughout a service area, and fiber-optic taps divert opticallocation to a cabinet in the street or service area. The feedersignals to subscribers. It’s a simple process: the cable is opened,line terminates on an optical splitter in the cabinet, whichand one of the fibers inside is carefully cut. A fiber-optic tap isdistributes the signal to subscribers with additional fibers. Thisspliced into the line, which siphons off a portion of the signalhub-and-spoke design gives providers great flexibility, as thefor a subscriber. The tap allows the signal to continue downcabinets allow easy management of both fiber connectionsthe line to the next home or business, where the process isand central office equipment, and can also be in proximity torepeated. Multiple taps can be spliced onto the line until theremote central office equipment.signal is exhausted—usually at 32 subscribers. At this pointanother fiber in the cable is cut, and the process continues.Rural fiber distribution TAP architecture: A more efficient approach for deploying a future-focused FTTH network1

DIFFICULT TOPOGRAPHYOf course, network architecture is a crucial decision forproviders embarking on rural installations. These deploymentscan cover great distances of sparsely-populated terrain,with just three or four homes per kilometer. Land canbe mountainous, forested, or desert, with little existinginfrastructure. Providers need solutions with design simplicity,to keep labor and equipment costs as low as possible.Figure C: 1x32 splitterFigure B: Rural terrain with little existing infrastructureEQUIPMENT SAVINGSThe biggest difference between tap network and splitter-basedarchitectures is cabling requirements. For a deployment servingFigure D: FDH 3000 cabinet256 subscribers, the minimum number of fibers required inIn comparison, for a 256-subscriber deployment, tapthe splitter-based architecture is 256. These 256 fibers run inarchitecture needs a minimum of eight fibers. Two four-fiberseveral smaller cables from the equipment cabinet. The cabinetcables are run directly into the serving area, without theis necessary to house the eight 1x32 splitter components,need of a cabinet to house splitters and connections.which route optical signals to subscribers, as well as permitCable savings would depend upon the length of the runsfiber access. For many rural deployments, splitter-basedto the actual drop points; but, since four-fiber cable costsarchitecture is considerably more expensive, as it requires themuch less than 72-fiber cable, savings could easily run touse of much more fiber cable and distribution equipment.thousands of dollars. With tap architecture, providers haveseen large reductions in the number of optical fibers usedTAP ARCHITECTUREin a deployment—some as large as 87 percent. TappedKey benefits:architecture also avoids the need for an equipment cabinet,·· Equipment savings·· Labor savingssplitters, mounting pad, and cabinet installation labor.·· More efficient to deploy·· Easier to maintain·· Easy future expansionRural fiber distribution TAP architecture: A more efficient approach for deploying a future-focused FTTH network2

TAPER SPLICINGA common practice in a splitter-based approach is for a72-fiber cable to be initially used in the serving area.Once 24 of the fibers are used, it’s cost effective to splicethe larger cable to a smaller 48-fiber cable. This is called“taper-splicing.” Here, tap architecture enjoys another costsavings. Because the commonly-used four-fiber cable is sosmall, it does not need to be taper spliced. Tap architectureeliminates the cost of splicing the larger fiber cable to theFigure E: Mini-OTE 300 with eight-port tapssmaller fiber cable, as well as the cost to install a spliceclosure. An additional management benefit: an operator needonly stock a single distribution fiber cable type, as they canuse the same four-fiber cable throughout the serving area.LABOR SAVINGSTap architecture commonly uses four-fiber cables, comparedFigure F: FOSC 450 with TAP Traysto the 72- or 48-fiber cables used in splitter architecture, whichSPLICE MAPPINGcreates considerable savings in splicing and material labor.In most deployments, a special map must be produced toThe number of splices required is further reduced becausechart splice input and output locations, and to indicate whichtap architecture requires no splitters at the entrance to thefibers are connected. This is especially true with a splitter-basedservice area. In a typical tap deployment, just two splices willarchitecture, as technicians need to keep careful track of whichbe necessary for each two to eight homes: one for the input tofibers are joined when working with 72-fiber cables. Thisthe module, and one for the through-leg. This compares to asinformation must also be created for each taper and branchmany as 72 splices required with a centralized architecture.point in the distribution network, as well as for splitters inWORKS WITH PLUG-AND-PLAY EQUIPMENTserving area cabinets. Splice maps can add significant cost tosystem designs, as a typical service area could have hundredsOperators can further reduce the need for splicing byof splices, and maps must be maintained and updated to showusing an optical terminal enclosure, or OTE, equippedany field or customer changes.with preconnectorized adapters. These fiber enclosureseliminate the need for drop cable splices, as techniciansSheath 10112 fibersSheath 10512 fiberscan simply plug preterminated drop cables into ports onthe outside of the terminal. Because splices at residencesand businesses are eliminated, this type of OTE will saveeven more labor time.Sheath 1106 fibersSplitter-basedSplice: SP01Figure G: Splice mapRural fiber distribution TAP architecture: A more efficient approach for deploying a future-focused FTTH networkSplice3

In tap architecture, no splice map is needed, as only a singletime domain reflectometer, or OTDR. In splitter architecture, thefiber is used on each run, and the indication of which fiberOTDR signal can’t pass through the distributed splitters, makingis spliced is readily done on the standard design map. Thisit more difficult to check connections.simplicity helps avoid both expensive connection errors andthe production time for the splice map.FUTURE EXPANSIONSome operators believe that, compared to splitter architecture,DEPLOYMENT EFFICIENCIEStap architecture networks are difficult to expand. While it’sThe high fiber-count cables used in traditional centralizedtrue tap systems are often designed with minimal fiber use,architecture deployments are considerably larger than theto save as much upfront cost as possible, designers can usefour-fiber cables used in tap architecture. For technicians ona 1:2 split at launch to increase optic use efficiency. Ifa tap deployment, a smaller cable allows more cable to beexpansion is required later, this 1:2 split can be removedwound onto truck reels, which further reduces the number ofto add capacity, and additional fiber-optic taps added tosplices they must make when they reach the end of a reel ofchange to a higher port count. And, in another expansioncable. Deploying smaller cable creates additional efficiencies,strategy, many operators purchase dark fibers along withas technicians save time because fiber closures and otherthe initial four-fiber cables, as the economics are best with aequipment can be smaller; and, if they must dig hand holes,full buffer tube of 12 fibers. The cable size doesn’t change,they can be smaller.and the additional dark fibers provide the highest utility ofall solutions, with each fiber good for another 32 homes.Rural electric co-ops have often deployed taparchitecture networks, because they already haveconsiderable infrastructure in place. Additionally,electric co-ops often find deployment efficienciesSUMMARYWhile several fiber architectures have been developed to supportwith tap network architecture, as there are manyFTTH deployments, tap network architecture is optimal forsimilarities to electrical system deployments.rural broadband networks. A major benefit of this design is thesignificant reduction in fiber required to serve a rural area. Withthe long distances typically involved in rural FTTH deployments,this reduction in fiber count can dramatically reduce up-frontnetwork costs, and allow providers to serve areas where deployinga fiber-optic network would have been cost-prohibitive.Tap network architecture: simplified design with lesscable, less equipment, and less splicing means fasterinstallations at lower cost·· Fewer fiber cables required·· Design simplicity—one type of small fiber count cable canbe used·· Less splicing required, saving skilled laborMAINTENANCE EFFICIENCIESTroubleshooting is efficient in tap architecture, as technicians can·· No need for complicated splice maps·· Large equipment savings—distribution cabinets and splittersgenerally not neededeasily check connections through fiber-optic taps with an opticalRural fiber distribution TAP architecture: A more efficient approach for deploying a future-focused FTTH network4

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A tap network design is quite different from the design of a traditional “centralized” FTTH network, which typically uses splitters installed in a cabinet configuration to distribute data to subscribe