Industry veterans recall, too well, optimistic campaigns proclaiming “This is the year -- fiber-to-the-premises (FTTP) will take off!” The right mix of technological advancement, regulatory environment, and customer receptivity has finally brought truth to that promise. Now, every service provider asks not “Should we try a pilot?” but “Where and when does FTTP make sense for our network?”

Telecommunications is a business, after all, and beyond the surplus of marketing statements about great service, enriching communities, and enabling new businesses, a service provider must turn a profit. With customers in every application space now clamoring for FTTP, how to extend this advanced service to more of them is an urgent question for everyone in the telecom pyramid, not just the executives with the nice view at the top!

Serving More Than Just Suburbia
Millions of potential FTTP customers in the U.S., but especially outside the U.S., live in closely-packed multi-dwelling units (MDUs). In the U.S., most MDUs were not built to accommodate future communications / multimedia services or infrastructure additions. This article examines a variety of brownfield MDU environments, and offers solutions that may have the potential to reduce provisioning to a matter of hours -- not days.

FTTP in existing MDUs challenges traditional provisioning schemes on several fronts. Products and techniques devised and meant for detached, single-family units (SFUs) are not well-suited for use in the MDU environment. Service drop cables suitable for direct burial or aerial span are overly robust for indoor routing. In addition, unless an MDU is very large, the optical splitter cabinet is usually several blocks away, requiring a cable suitable for both the outside plant (OSP) and inside the building. What’s more, OSP cables are not typically flame-rated, so indoor routing requires placement of new conduit or creation of a transition point from the OSP. All of these challenges add costs: for overly robust products, for additional materials within the MDU, and for greatly increased installation time per customer.

Likewise, products and techniques devised for enormous office buildings are cost-prohibitive for most MDUs. Wiring closets, zoned cabling, aggregated trunks, and multiple interconnects are sensible options for office buildings, but they require complex bills of materials and a great deal of skilled installers’ time. MDU service does not cost-justify a solution built to allow periodic reconfiguration of an entire premises network or to allow “forklift upgrades” of active electronics in the building.

Building modification and network provisioning costs dictate whether a network operator can profitably offer FTTP service. Innovative product solutions minimize these costs. Similarly, waiting time and construction inconvenience are key determinants of whether potential subscribers will welcome the service. Innovative product solutions can help minimize these deterrents.

Walkups
MDU solutions that do make sense are enabled by the recent innovation of bend-insensitive optical fibers and by novel cable assemblies incorporating those fibers. Figure 1 illustrates a very common building type in the northeastern U.S. “Walkups” typically house 1 tenant per floor and rise 6 stories or fewer. Many were constructed with just basic utilities, water, and electricity, and almost none incorporate pathways such as vertical or riser spaces for new services.

Figure 1. This illustrates a very common building type in the northeastern U.S.: a walkup. Walkups typically house 1 tenant per floor and rise 6 stories or fewer. This figure shows several service cables bundled around a structural messenger, which is anchored to the building; illustrates a building service cable feeding an MDU terminal; and shows a factory-installed connector (with cable slack) secured to the subscriber end of the tenant service cable.

In these situations, new service means building a new pathway. Fabricating such a space requires scheduling several trades to build it (carpenters, drywallers, painters) and others to install the new service (pull and terminate cables, either with connectors or fusion splices). In short, this installation option brings many intrusions into both common areas as well as the homes of tenants.

However, with the introduction of cable assemblies that use bend-insensitive fiber, running service cables on the outside of a building becomes possible, eliminating nearly all of those intrusions. Figure 1 shows several service cables bundled around a structural messenger, which is anchored to the building. It also illustrates a building service cable feeding an MDU terminal, which in turn feeds the tenant service cables. In this example, both the building and tenant cables are preterminated, requiring only mounting and plugging in the cables.

Figure 1 also shows a factory-installed connector (with cable slack) secured to the subscriber end of the tenant service cable. The slack allows a technician to route an individual service cable through the exterior wall and to plug it into the optical network terminal (ONT). The keys to this installation method are prepositioning and preterminating the service cable. Doing so allows service provisioning during a single service call. All the installer has to do is drill an entry hole, push the connector through, seal the entry hole, mount the ONT, and plug in the cables.

This deployment method yields several advantages. The first is maximizing a very scarce resource: skilled technicians’ time. In a race to serve potentially millions of subscribers, significant advantage goes to the swift. The method serves the network operator, too, as service revenue begins in days or even hours, not weeks, after starting a build. Finally, the significant cost of an ONT and the intrusion of installing it are deferred until a resident is ready to subscribe.

These products and techniques may be extended to MDUs with multi-tenant floors as well. The treatment shown in Figure 1 works equally well for 2 to 4 tenants per floor: simply install additional MDU terminals (or 1 with more ports) and additional bundles of tenant drop cables. The advantages of speed and cost are identical to the single-tenant-per-floor application.

Poor Access to Outside Walls
As it will, reality sometimes intrudes upon this elegantly simple solution. It is not always possible to run cable assemblies up a building exterior, or the desired route is inaccessible; for example, existing building bylaws may forbid exterior modifications. Similarly, tenants might occupy 4 corners of a floor, but only 1 exterior wall is available for installing service cables. In such scenarios, bend-insensitive fibers enable new indoor solutions. Figure 2 illustrates a solution completely inside a building. Unlike the outdoor solution, it requires inside pathways. Where none exist, they must be created. Surface raceways such as latch ducts are simple and quick to install, and designs are available which virtually disappear or complement ornate building details.

Figure 2. This illustrates a solution completely inside a building. Unlike the outdoor solution, it requires inside pathways. Where none exist, they must be created.

Detail views show a technician installing a service cable in a newly added raceway. The cable is field-terminated and routed inside the premises. While not as simple to connect as a preterminated cable, this method accommodates the plethora of unique drop cable lengths for this application. Preterminated drops are possible, but field-termination is preferred to hundreds of different length cable assemblies. How many assemblies can fit in a service truck is more limiting than how many fit in a warehouse.

The middle detail shows a quickly installed junction that accommodates a small number of interconnections. The junction "collects" fibers from several drop cables into a single cable running from the junction box to the basement MDU terminal. That cable may also be preterminated on the MDU terminal end to save additional time.

Flexible Solutions
Of course, real-world solutions also fall along a continuum between these examples. Variations of the products already shown enable designers to accommodate unique requirements or enable technicians to adjust to unexpected field conditions.

An additional consideration is whether junctions "collect" multiple fibers from several cables into one cable, or whether junctions "collect" signals from multiple fibers into a single fiber (optical splitting at the "collector"). Figure 3 shows an example of the latter. The example structure houses multiple tenants per floor on several floors. The MDU terminal on the bottom floor splits the optical signal of the building service drop, but at a reduced ratio, for example 1x8. Some of these "higher-share" fibers might be allocated to businesses in or near the building, and the remainder would feed an outdoor bundled assembly identical to that shown in Figure 1. The tenant drops, instead of serving individual subscribers, would feed an additional terminal on each floor. The optical signal is further split (1x4 to complete the 1x32 split in this example) and then fed into an indoor bundled assembly. From this point, the installation mirrors the scenario of Figure 2.

Figure 3. An additional consideration is whether junctions “collect” multiple fibers from several cables into one cable, or whether junctions “collect” signals from multiple fibers into a single fiber (optical splitting at the “collector”). This figure shows an example of the latter. The example structure houses multiple tenants per floor on several floors. The MDU terminal on the bottom floor splits the optical signal of the building service drop, but at a reduced ratio, for example 1x8.

The immense variety of brownfield MDUs will present technicians a near-boundless set of scenarios. However, 5 basic components equip them with solutions to virtually all:
1. Outdoor bundled assembly.
2. Indoor bundled assembly.
3. MDU terminal.
4. On-floor collector (junction box).
5. On-floor terminal (includes optical splitter).

This limited inventory ensures technicians have what they need on the job: it fits on a truck. Furthermore, it turns formerly challenging installations into routine work and does so using the existing field craft.

About the Authors

Ken Dunn is Product Tech Support Manager, Corning Cable Systems, and has more than 16 years of experience in engineering, product management, and operations.

Daniel Cassell is Applications Engineer, Corning Cable Systems, and has more than 3 years of experience in field and applications engineering.

 Kevin Ball is Project Manager, Corning Cable Systems, and has more than 2 years of experience in field engineering and project management.

For more information, visit www.corning.com.

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