A Craft Friendly Cable and Enclosure for Fiber to the Curb

Authors:
Jonathan G. Fitz
Pirelli Cables & Systems, LLC
700 Industrial Drive
Lexington, SC 29072
Phone: (803) 951-4037

Jim Fasano
Keptel Antec Products
56 Park Road
Tinton Falls, NJ 07724

Technical Contributors:
David L. Jones
Dr. Jin Liu
Ben H. Wells

All of Pirelli Cables & Systems, LLC

Aaron M. Anderson
BellSouth
(Formerly of Pirelli Cables & Systems North America)
aaron.anderson@bridge.bellsouth.com

Introduction
Demand for bandwidth and new services continue to grow at an exponential pace. Fiber has an obvious advantage in terms of bandwidth for present and future use. Fiber has also become more economical. Thus, the costs and benefits of fiber have become more attractive. As a result, Fiber To The Curb (FTTC) has become an increasingly popular deployment scenario.

Pirelli Cables and Systems has developed a copper and fiber "composite" cable called CurbLink, which addresses a wide range of outside plant applications. This new design includes up to 12 fibers and 24 pairs of conductors under the same sheath. The cable is suitable for aerial, duct and direct-buried applications. Access to any number of conductors and fibers is streamlined for a variety of applications including taut-sheath access. Service to multiple Optical Network Units (ONUs) can be delivered quickly and cost-effectively with a single cable.

In addition, Pirelli has worked closely with Keptel to develop splice enclosures that maximize reliability and deployment efficiency. Aerial enclosures suitable for taut-sheath applications are available, as well as enclosures for pedestal applications. All enclosures include user friendly, Bellcore compliant cable retention hardware and self-sizing cable port grommets. Conductors are separately organized and the fibers and buffer tube are positioned to maximize protection and ease of access during splicing.

The outcome of these development efforts is presented below. The flexibility and cost savings of the present system is demonstrated, as well as some useful features made possible by the combination of the cable and special splice enclosures.

The FTTC Environment
As the name implies, FTTC involves delivering fiber within the proximity of homes or businesses, but not directly to them. It is possible to use the cables and enclosures described below for Fiber To The Home (FTTH), but those cases are beyond the scope of this paper.

The fiber portion of an FTTC network typically terminates at an electronic interface. The interface is often referred to as a Broadband Network Unit (BNU) or Optical Network Unit (ONU). The interface handles multiple users who are connected to it via coax or twisted pair. Individual users have access to data, telephony and video, or some subset of these.

Each ONU requires electrical power. Thus, copper conductors are still needed in some form to supply this power. As we will see, conductors can be supplied in a variety of cable configurations. Cable design options must be considered carefully, since the conductor arrangement can strongly influence installation labor, cable, and network deployment costs.

System Requirements
Let's consider some key factors that determine the usefulness of new cable and enclosure designs for FTTC. Collectively, we will refer to the cable and enclosures as the "system."

Above all, the cable should be easy to deploy. Although bandwidth requirements are easily met with fiber optic cables, there are practical barriers to efficient deployment. These include delivery of electrical power, ease of fiber access and ease of cable and hardware deployment. If these factors are not adequately addressed, the network can become too costly to deploy. The authors believe the best way to address deployment efficiency is to design a system and not just a cable. A system takes the complete installation into consideration and supplies hardware that is critical to the installation, but may not be readily available.

An efficient system also needs to be flexible and backward compatible. A system may work very well in a particular setting, but not well in others. What begins as a well-intentioned effort can result in a solution which is cumbersome for both the manufacturer and the user. The design may lock the user into particular deployment strategies, or force the user to carry a wider range of inventory and provide additional training for craft personnel. Likewise, the cable manufacturer may be expected to manufacture a wider range of products at the expense of efficiency, and ultimately, product cost.

The system must meet all applicable industry performance standards, including those for the optical cable (GR-20), the conductors (GR-421), cable pullout, and water blocking (GR-771) without need for special tools, parts or time-consuming procedures.

Finally, a system should be expandable. Expandability allows new customers to be added with minimal expense and delay. Preferably, a system should be expandable with minimal investment prior to the expansion. This allows the costs of additional service to be deferred until it can generate sufficient revenue.

It is tempting to assume that expandability is unnecessary for rehabilitation projects. The most cost-effective design for a truly fixed customer base would use minimal fiber counts. But, enhanced service often stimulates new demand—particularly for data. If another complete phase of installation becomes necessary to meet unanticipated demand, a non-expandable design is proven to be a false economy.

Evolution of Cables and Deployment Strategies

Twice the Cable, Half the Fun

The first and most obvious solution to the requirements of the FTTC market was to install two completely separate cables. Copper communications cables could be used for powering electronics, which often were built around standard gage telephony wire in the first place. Low fiber count cables were also available.

The two cables might have to be installed in completely separate passes. This was often the case with larger copper cables, and with installation methods ill suited to separate cables. For example, some customers have installed copper and fiber separately when lashing to aerial strand. In some cases, the cables were even installed on separate strands. The cables might also be installed simultaneously. This was typical of direct burial, which can easily accommodate many cables.

Over-Sheathing

In order to eliminate problems with separate installations, the next phase of evolution was to combine the two separate cables under a common sheath. This eliminated any extra steps associated with installing a second cable. However, it added new labor costs, since another sheath had to be removed. These cables could also be bulky, stiff, and relatively inflexible in some planes.

Single-Sheath

The next two steps involved truly integrating the fiber and copper into a single cable. This eliminated extra steps for pulling a second cable without the penalty of an extra sheath. Thus, labor costs were saved on pulling the cable, and on accessing the cable elements.

These designs typically contained fewer fibers and conductors than before. This greatly reduced the cross-section, making the cable lighter and easier to handle.

An initial design put the fiber buffer tube and conductors under a single sheath.¹.

This was further refined by deliberately placing the buffer tube in the center of the cross section, and distributing the copper uniformly around it.² This further improved the flexibility of the cable.

Both of the above designs were envisioned more or less as drop cables, designed to serve a single ONU. Both originated from splice enclosures that were designed to accommodate separate fiber and copper distribution cables.

Note that individual cables are deployed to each of the ONUs. The cables in this strategy are pre-stubbed to save splicing time. Note also that the splice enclosure incorporates separate distribution copper and fiber distribution cables. One can see a clear architectural progression which saves labor and material costs for the user.

New Cable Designs and Closures
Pirelli CurbLink incorporates two to twelve fibers in a central buffer tube. The tube is gel-filled for water blocking within the tube. All other water blocking is achieved with water-swellable tapes and yarns. This construction simplifies access by eliminating the time needed to clean wax off of the conductors and buffer tubes.

CurbLink is available with 5, 13 or 24 pairs of 19 AWG copper conductors. Conductor counts greater than 5 pairs are stranded in distinct layers. This ensures uniform arrangement of the conductors, and simplifies the location of specific conductors, since each color combination can be found in a specific location.

The conductors are also S-Z stranded with a short pitch. This greatly facilitates taut-sheath, mid-span access of the cable. When wound off the buffer tube, the conductors will be longer than the tube. This makes them easier to organize out of the way of the fibers.

Interestingly, other recent designs continue to incorporate twisted copper pairs. Yet, the twisting is unnecessary if the copper is being used solely for power. Twisted pairs take up more cross sectional area than individual conductors do. Twisted pairs have been replaced in the present design, thus saving space and improving cable-handling properties.

The jacket is available in an armored or unarmored version. The unarmored version affords easier access, while the armor provides greater mechanical protection. The cable can also be supplied in a pre-stubbed form.

Although the S-Z stranding facilitates mid-span access, appropriate enclosures were needed, as well. Keptel worked directly with Pirelli to develop splice enclosures for this application. Specific attention was focused on the need for separate fiber and copper management. Closures were developed for both pedestal and aerial applications.

The interior of the case is organized to allow the larger cable to pass straight through the enclosure. Smaller cables can be spliced in and routed out of either side of the enclosure. A slitter accesses the central tube, and any number of fibers can be pulled out for splicing. Fibers that are passing through can be left in the tube that is protected by a channel in the sheet metal enclosure. Fiber and copper are separately organized to simplify handling and protect the fiber splices. The lid of the enclosure is omitted from the drawing for clarity.

The enclosure also incorporates easy-to-use hardware that positively couples the various cable elements to ensure long-term reliability of the system after extensive temperature cycling. The rubber grommets are designed to seal a wide range of cable diameters with minimal effort and no special tooling.

Deployment Topology

Single ONUs

The present design is available pre-stubbed with two fibers and five pairs of conductors, as mentioned above. Thus, where appropriate, each ONU can have a dedicated cable as previously described.

Multiple ONUs

In cases where it is undesirable to have a separate cable for each ONU, the present system is easier to deploy with multiple ONUs. Obviously, there will be more fiber splicing involved. However, the preparatory work for the splices is reduced, and the splicing process is more easily managed.

Such a deployment might be preferable where it is difficult to pull multiple cables. Some examples would be ducts and congested aerial environments. It would also be advantageous if service expansion is likely to be needed after the initial installation. This is especially true in aerial deployments when it is unclear where future ONUs will be located. Existing service can be installed with fibers to spare. Extra cable (e.g. slack loops) doesn't have to be distributed throughout the line to facilitate expansion. Future service can be established anywhere along the length of the cable by deploying the taut-sheath enclosure.

Local Distribution

The benefits of composite cable can be also pushed further back into the network. Note that the other designs in previous examples used separate copper and fiber distribution cables for all but the final link. The present design can be used as a local distribution cable, as well. This can be especially valuable in aerial installations. The time savings associated with pulling only a single cable can be realized in more of the network. If copper and fiber cables are separately mounted on the pole, it can save pole space. There are fewer sheaths to be removed, and there are fewer cable entries into the splice closures.

Easy Expansion

Extra fiber can be deployed at minimal cost, since it can be located within the same cable. The initial service requirements can be met during the initial installation. As new demand emerges, service can be expanded in two ways. New ONUs can be added along the length of existing cable runs or smaller cables can be branched off the initial installation.

The taut-sheath splice enclosure makes this option particularly user-friendly for aerial deployments. A new enclosure can be added at any point along the cable. The ONU can be pole mounted, or a pre-stubbed drop can be routed from the enclosure to the ONU.

The use of the composite design as a local distribution cable can also facilitate expansion in suburban, buried applications—particularly in areas of rapid growth.

The first area, shown at the top, is an established neighborhood. This area is shown mostly as a template for how the new area might expand. Although the schematic drawing is greatly simplified, the cable path will generally not be the shortest distance between two points. Rather, a route may be selected to avoid road crossings and other, more costly obstacles.

The second area is one of rapid growth. Most of the initial installation can be limited to the areas that are already occupied. As the construction fills out, additional cable can be installed. Thus, most of the installation cost is limited to areas that provide immediate revenue.

Cost Savings
Actual labor rates can vary considerably from one area to another. Thus, it is easier to define cost savings in terms of relative savings. System designers familiar with their own cost structure can easily convert these figures to hard figures. For comparison sake, the present system is compared with a typical system using separate cables throughout. This is the most general comparison, since most other scenarios will be a subset of this one. For example, an installer may already be using composite drop cables, but still using separate distribution cables. In this case, the incremental savings would be limited to the upgrade to local composite distribution, and any cost savings associated with the ease with which the new composite cables can be installed.

Right of Way Savings

Using a single cable can cut pole attachments by as much as 50 percent. This saves on hardware, as well as any costs associated with obtaining pole space. The compact cross section also minimizes ice and wind-loading problems.

The present cable can also make better use of duct space. It has a higher packing density than many smaller cables with the same number of elements. Even in an armored version, the largest fiber/conductor count is only 0.85 inches, which allows installation in one-inch ducts

Cable Installation Savings

It takes less time to pull a single drop cable to several ONUs. It is considerably less complicated to pull a single cable for aerial and duct installations. If a single drop contains extra fibers, future drops and cable pulls can be avoided. If the cable is installed incrementally in a growing area, the installation costs can be deferred until the additional service is needed. Slack loops are not needed for future access, so this step can be eliminated.

Cable Access Savings

Only one sheath opening is needed at each access point, as compared to two or even three. The conductor layers are S-Z stranded, which allows the conductors to be quickly separated and organized. Each conductor pair is located in a particular position to simplify the location of specific pairs.

The splice enclosure requires fewer openings, and there are fewer entries to prep and secure to the enclosure. Aerial enclosures can be added anywhere along the cable, allowing them to be put as close as possible to new ONUs and in the most convenient location for installation crews.

Once again, neighborhoods can be built out incrementally without penalty, thereby deferring costs until a new area will provide revenue. This approach does require some planning, but can significantly improve cash flow during construction.

Splicing Costs

The principal drawback of multiple ONUs on a single cable is that some of the ONUs will require fiber splices. This must be considered carefully for each installation to determine what makes the most sense overall. The cables are available in a pre-stubbed form. However, consideration must be given to the tradeoffs involved in this approach.

Pre-stubbed cables can be more difficult to install. This is particularly true when they are pulled from the ONU back towards the Central Office. The storage reel must be completely emptied in order to get at the inside end of the cable. Then, this end must be combined with others as the installation gets closer to the distribution splice.

In any case, cables are available in either configuration. The system designer must consider all factors when deciding which approach to use. Although the splicing costs are more obvious, they may not be as high as the expense of a more complex installation.

System Performance
The components of the system were tested in accordance with various industry standards. The results are summarized in Table 1 below.

Conclusion
A cable and enclosure system has been developed for the FTTC environment. All elements were tested according to applicable industry standards. The system can be shown to save installation costs by eliminating steps, simplifying procedures, and streamlining the deployment topology. Further benefits are achieved by allowing easy expansion of the system to service additional customers. Finally, the system can be applied to a wide variety of installation environments, including ducts, aerial lashing, and direct burial.

References
1. W.E. Beasley, S.E. Stokes, G. Karl, "Delivering Fiber To The Node-A New Composite Service Cable," National Fiber Optic Engineers Conference, pp 221-231, 1997.
2. K.L. Strause, "An Improved Network Deployment Strategy for FTTC," National Fiber Optic Engineers Conferences, pp 237-245, 1998.
3. Ibid.