Design and installation practices for ultra-high-density fiber-optic cabling systems

Dec. 23, 2020

By Patrick McLaughlin

The need to connect data center facilities to one another, frequently referred to as data center interconnect (DCI), has been a primary driver for optical-fiber and fiber-optic cable manufacturers to develop products containing thousands of fibers. We refer to cables with 1728 or more fibers as ultra-high-density cables, and this article examines those products.

We gathered information from several providers of ultra-high-density fiber-optic cables about the design of these cables, as well as their specification and installation. This article presents the information we gathered from them in question-and-answer format. The following professionals provided their expertise for this article.

  • Patrick Dobbins, director, solutions engineering, AFL
  • James Young, director, enterprise data center division, CommScope
  • Arelis Soto, enterprise data center marketing manager, Corning Optical Communications
  • Mark Boxer, manager, applications engineering, OFS
  • Jon Fitz, director, product management, Prysmian Group

Q: What applications or user environments create demand for cables with a minimum of 1728 fibers?

Dobbins: Several different applications drive demand for extreme high-density cables. Currently, AFL manufactures extreme high-density cables with fiber counts of 1728F, 3456F, and 6912F. Current users of extreme high-density cable include hyperscale data centers that utilize mesh architecture for computing, which requires a high number of connections and parallel processing of information that drives up the number of fibers and interconnection to storage devices. Due to the size of these data centers, many are multi-building and multi-floors. Other users of extreme high-density fiber-optic cables are distributive access networks. Dense urban access networks that interface with wireless low-power WAN systems are another. A number of these networks are running in major urban areas. Competitive access providers that are building business bypass networks in urban areas are another user group of extreme high-density cables. As the cost of rights-of-way are very high in urban underground networks, many of these competitive access networks are attempting to maximize the use of underground conduit systems.

Young: Technologies like rollable ribbon and 200-micron fiber are generally deployed by hyperscale data centers to support the increased demand for inter-data-center connectivity. Another use is for intra-data-center trunking to support the scale of server farm requirements.

Soto: Demand for high- and extreme-high-density cables can be found in both carrier and data center environments. For data center operators, both hyperscale data centers and data center interconnects benefit from the use of extreme-density cable. In carrier applications, these cables can be utilized in high-density converged networks, as well as central office/headend-to-headend applications. Given the nature of the cable, any time you have congestion inside a given duct space, high-density cable delivers value. Many carriers are looking to maximize the capacity of their network, and extreme-density cable is designed with that in mind.

Boxer: Data center interconnect applications currently represent the bulk of extreme-high-density cable demand to this point. However, we’ve also seen these fiber counts used in converged metro network applications, where traffic for 5G or other wireless, FTTH, and FTTB networks are aggregated together and customers are looking to pack large amounts of fiber into precious duct space.

Fitz: Demand for extreme-high-density cable is being driven by applications that demand more fiber than would otherwise fit in a standard duct. The two most common applications are 5G and hyperscale data centers. The most common duct size in a telco underground network is 1.25 inches. For many years, 864 was the highest-count fiber-optic cable that would fit that duct, and 864 was a very popular fiber count. 5G networks take fiber demand to a new level. But in urban areas, new construction is prohibitively expensive, so there was a powerful incentive to make use of existing duct by increasing fiber density. Prysmian responded by developing a FlexRibbon cable with twice as many fibers—1728. This design is slightly smaller than the original 864 count. As impressive as a 1728 count may seem, it’s modest for some hyperscale data center operators. Some had been using conventional, flat-ribbon 1728 counts for a few years. These designs required larger ducts, which meant there was room for even higher counts of flexible ribbon fiber. This market is the primary user of 3456- and 6912-count cables. As the market becomes more comfortable with these cables, they are also being used as trunk cables in larger networks. Thus they’re the most recent continuation of a trend that has been underway since fiber was first deployed.

Q: Please describe the technology your company uses to achieve such high fiber counts within a small-diameter cable.

Young: CommScope uses rollable ribbon versus standard ribbon, which provides approximately 35% size reduction. Additionally, we use 200-micron fibers, which when combined with rollable ribbon, provides approximately 50% size reduction. We also have a factory-termination process that can lower the installation cost including equipment, labor and overall time.

Soto: RocketRibbon cables us a proprietary manufacturing process to create as many as 12 individually protected sub-units, each with 288 closely packed fibers. An extruded color-coded subunit protects and identifies each bundle and allows for direct and easy routing to splice trays, without the need for additional materials, ribboning processes, or furcation kits. RocketRibbon cables leverage next-generation cable production technology, while maintaining the benefits of a backward-compatible and proven industry-standard ribbon design.

Boxer: The most common technologies deployed today are rollable ribbons and/or 200-µm fibers. Each technology can be deployed on its own, or together, as in a 200-µm rollable ribbon. However, the demand for bandwidth and case for more fiber is getting stronger. That’s leading to additional innovation, which will play out over time, with potential smaller coating layers and/or glass fiber sizes. On top of that, multicore fibers (literally fibers with more than one core) have been in development for many years, and extreme fiber demand is leading to additional development in that space. It’ll be a while before we see these additional technologies deployed as commonly as we’re now seeing rollable ribbons and 200-µm fibers, but it highlights that the new technology pipeline in the fiber world is very robust, and there’s additional room for innovation.

Rollable ribbon (RR) is enabling technology allowing for placement of 1728F cables into typical 1.25-inch duct systems. Effectively, this doubles the fiber density in available duct systems. Rollable ribbons are ribbons that are intermittently bonded, enabling them to be rolled into very high-density circular packages. This enables many benefits for a network operator versus flat ribbons, including smaller and lighter-weight cables, ease of breakout of individual fibers, and potential use of smaller trays and easier routing. In addition, cable designs can be optimized to deliver cables with unique features addressing specific customer pain points. Examples of this include cables that are optimized for blowing, tight coiling, or frequent access.

Fitz: Flexible ribbons are an essential technology for very-high-fiber-count cables. Their flexibility is essential for minimizing cross section, by allowing them to be packed more efficiently than flat ribbons in round tubes. Of course, loose fibers are flexible, too. But it would take weeks to individual splice 6912 fibers, and the splice closure would be cavernous. Aggressive diameter reduction had shrunk microduct so much that the fibers alone occupied a major portion of the cable cross section. Prysmian responded by commercializing the first 200-micron fiber. Fifty microns might not seem like much, but it’s a 36% reduction in cross-sectional area. In a cable with thousands of fibers, it really adds up.

Dobbins: AFL uses a collapsible ribbon technology known as SpiderWeb Ribbon (SWR) developed by our parent company, Fujikura of Japan. Fujikura was one of the original inventors of the collapsible ribbon technology with NTT in Japan. The SWR is available in 250-µm and 200-µm fiber. This ribbon technology allows for the ribbon to fully collapse and creates a high packaging factor of ribbon into a cable that has been optimized to reduce the diameter. The individual 12-fiber ribbons have fibers that are intermittently bonded to the adjacent fiber similarly like a spider web. This allows for the fiber to align into a flat ribbon that can be mass fusion spliced, but still be collapsed to permit a dense packaging factor. Each of these ribbons is individually marked with band marking for identification down to an individual fiber color. The collapsible ribbon is arranged by binder grouping of 12-ribbon or 24-ribbon groupings known as binder groups. The binder groups follow the industry-standard color code for identification. All the binders are grouped in the core of the cable with water-blocking yarns to provide resistance to moisture migration down the core of the cable. This core of ribbon binder groups and water-blocking yarns are covered with a water-blocking tape and an outer sheath with strength members embedded in the jacket. It has two ripcords and a rib indicator on the jacket to identify the location of the ripcords to facilitate mid-cable access. The cable is called Wrapping Tube Cable (WTC) with SpiderWeb Ribbon (SWR) and typically is a 35% reduction in diameter and 50% reduction in cable weight from traditional ribbon cables.

Q: 200-micron fibers have become increasingly common in extreme high-density cable. Can you explain if or how a 200-micron fiber can be fusion spliced to a 250-micron fiber?

Soto: The fiber size reduction to 200 microns is achieved by making the coating layer smaller. The glass (core and cladding) dimension remains the same with the cladding diameter at 125 microns for both 200- and 250-micron fiber. There are no changes to performing a single-fiber splice on 200-micron fiber to a 250-micron fiber with a core or cladding alignment splicer due to the glass dimensions remaining the same. There are various 200-micron fibers on the market, and some have different mode-field diameters (MFD). If the MFD of the two fibers is different, this can lead to an exaggerated loss or a gainer showing up at the splice when testing the fibers. Bidirectional testing is recommended in this case. The challenge with mass fusion splicing 200-micron fiber ribbons to 250-micron ribbons lies in aligning the fibers in the v-grooves where the alignment of the fibers (typically 12 fibers in a ribbon) is at 250-micron spacing. The alignment of a 200-micron ribbon to 250-micron v-grooves can be achieved with the ribbon holder, which spreads the 200-micron ribbon into the 250 spacing. There are various options in the market to achieve this.

Boxer: This has been an issue in the past, which is now solved by a couple of innovative pieces of technology. First, the Fitel S124-M12 splicers has removable v-grooves, so it is easy to interchange 200 and 250-µm v-grooves in and out of the splicer. Second, it uses a very clever transition fiber holder, so 200-µm fiber goes in one side and comes out with 250-µm spacings. Because the glass diameters are the same, at that point, it’s just like splicing 250- to 250-µm fiber ribbons, which has been done for decades. However, it should be highlighted that the 200-250 transition takes longer than splicing either 200-200 or 250-250 µm ribbons. Overall, 250-µm fiber ribbons are more common with telecom applications, and data center interconnect applications often migrate to 200-µm rollable ribbons due to its inherent higher density.

Fitz: Various manufacturers of fusion splicers have developed tools and methods for splicing 200 to 250-micron fiber ribbons. The particulars vary, but the 200-micron fibers are spread out in a way that is compatible with the wider pitch of a 250-micron fiber ribbon.

Dobbins: In the 200-µm version of AFL’s SpiderWeb Ribbon, the fibers have a cladding of 125 µm ± 0.7 µm and a core that is approximately 8.6 µm ± 0.4 µm. This is a dual-listed fiber that is compliant with ITU-T G.652.D and G.657.A1. What is unique is that the ribbon “pitch,” or the spacing between the fibers in the ribbon, are based on 250 µm. This means when the coating is removed on the fibers of the SWR ribbon, the center of the cores is spaced the same as a 250-µm traditional flat matrix ribbon for mass fusion splicing. Additionally, the 200-µm SWR can be de-ribbonized and spliced using single fusion. In this case a core-alignment splicing machine would be used to splice to traditional 250-µm single fibers. A large part of the key to this technology is the evolution of mass fusion splicing equipment, mass fusion splice fiber holders and accessories like the thermal strippers and cleavers that are optimized to improve efficiency and splicing time of extreme high-density cables.

Young: There are a few ways the fibers can be spliced. The first, which is probably the most common, is to use a fusion-splice manufacturer’s fixturing to splice together the fibers. These manufacturers have been doing it for a while now and have the expertise to make it fast and easy. An alternative to splicing is using multifiber connectors, which can take away the pitch mismatch. Now, there is another option, which is fusion splicing technology that allows 200-micron and 250-micron to be spliced together.

Q: Do you have any requirements or recommendations for contractors who are installing your extreme high-density cables?

Boxer: The first step is to choose the right product for the application. We have different extreme high-density cable designs for different applications. We want our customers and their contractors to be educated as to which products perform the best in which applications. From there, we want contractors who deploy our products to be knowledgeable about them and will provide installation training to support our high-density products. Extreme high-density cables represent large investments for those companies deploying them, and we want to provide guidance to help their installations be successful.

In general, installation and cable access techniques are similar to standard fiber cables. The same bend radius guidelines used for standard fiber-optic cables also apply to high-density cables, e.g. minimum bend radius = 15x OD or 20x OD depending on the cable design and application. It is important to recognize that high-density cables have larger ODs, which lead to larger minimum bend radii requirements for the cable installation equipment. For example a 50-inch-diameter capstan may be required to pull the high-density cable and larger sized cable sheaves and fiber-optic quadrant blocks may be required to guide the cable through manholes and handholes. Larger sized handholes may also be required to accommodate larger slack storage coils and splice closures. Although the cables can be installed using conventional pulling techniques, cable blowing is the preferred installation method. FEC/OFS 6912-fiber cable has been successfully installed in a continuous 4000-foot length using commercially available cable-blowing equipment.

Fitz: Respect the minimum bend radius. If your bread-and-butter installation is low-count cable, you don’t need big capstans, wheels or quadrant blocks. If you’ve installed thousands of miles of cable, it’s natural to get comfortable with the tools you’re using the install it. The simple fact is this: Bigger cables require a larger bend radius. Even though the fiber densities are high, a 1728 count is typically around an inch in diameter. All the applicable industry standards call for the dynamic bend radius to be 20x the cable diameter. Remember that diameter = 2x radius. So a 1-inch cable should be pulled over 40-inch wheels. Many contractors are surprised to hear this and object to the cost of appropriately sized capstans, wheels and quadrant blocks. This is understandable. However, bend radius violations are one of the most common causes of cable damage. Consider how expensive it will be to replace a damaged 6912-fiber cable.

Dobbins: The installation of these extreme high-density fiber-optic cables does not require special treatment for installation. Other than larger cable diameters and corresponding larger bend radius requirements, SWR utilizes the same installation methods that are currently used today for traditional fiber-optic cables. SWR can be pulled or jetted using commonly available equipment. SWR has been installed in all the traditional environments all over North America and the world. Techniques and figure 8’s and “racetrack” storage for cascading are the same. Using multiple jets or capstans that are synchronized to work together are the same methods as traditional fiber-optic cables. Cable entry for end preparation or for mid-cable access are very similar to traditional cables. Normal care and good practices are important as the dollar value of these extreme high-density cables is much higher. Good construction technique and prudent care when handling these cables will yield good results.

Young: The massive amount of fiber creates two big challenges for the data center. The first is, how do you deploy it in the fastest, most efficient way: how do you put it on the spool, how do you take it off of the spool, how do you run it between points and through pathways? Once it’s installed, the second challenge becomes, how do you break it out and manage it at the switches and server racks?

Cable terminations need to be of the highest quality. Fiber that is dirty, bent, poorly connected or has obvious physical defects will not meet the optical specifications detailed in the contract and will have a detrimental effect on the optical network, if not immediately then further down the road when critical network outages will occur.

One area that some may not consider is fiber characterization. Fiber characterization is the process of validating that a fiber path will support a given use before lighting it up. It’s a critical step, not only when expanding or upgrading the network but during the initial build-out as well. In fact, fiber characterization may be one of the most important steps to ensure a solid foundation for your network.

Soto: Corning recommends contractors follow the Power and Communication Contractors Association Critical Installation Steps. The steps are listed below and can be found at

  • Utilize tools that provide minimum cable installation diameter under load
  • Pull forces less than manufacturer’s cable installation load
  • Figure-8 cable slack during install to minimize cable twist
  • Prevent impact to cable to prevent crush

Corning also encourages contractors to read our RocketRibbon Extreme-Density Cable Installation Checklist, AEN166, for more information.

Q: Are there any best-practice recommendations for managing such a high number of fibers in an installed environment?

Fitz: Managing a high-count cable is kind of like storing a big load of paper files. You can save a lot of time by stuffing them all in a box. But, imagine trying to find a file later—except in this case, if you touch the wrong file, it could cause a service outage. A well-organized closure and proper tray labeling will improve network reliability and expedite new service provision.

We suggest putting wire loom (braided sleeving) on the fiber where it’s not in a tray, unless the fibers are cabled in flexible buffer tubes. This prevents fibers from being snagged during handling and subsequent re-entry. Even if they’re not flexible, buffer tubes will allow large quantities of fiber to be sorted, handled and protected more quickly and safely. Consider storing all the fibers in splice trays on day one—even the ones not being spliced. This costs more up-front. But identifying, separating, and traying fibers from storage is tricky. Finally, we suggest allocating fibers in multiples of 12 so that each splice is a complete, single ribbon. This keeps the closure neater and easier to manage in the future.

Dobbins: Best practice recommendations for managing extreme high-density fiber cable is to organize the fiber in a splice closure tray or optical entrance enclosure tray to keep the binder identification all the way to the splice tray. The color coding of the fiber in the ribbon identifies the fiber number, the band marking keeps the ribbon number identification, and binder color code keeps the binder number identified. This makes identifying a specific circuit easy to locate in the splice tray without having to trace back to the organizer basket or cable core.

Young: Carefully control the cable bend radius. This will help avoid damage and minimize microbending and macrobending losses. The former can result in light attenuation, while the latter can cause light leakage. Long-term, if excessive stress is placed on the fibers, the fibers may fail, necessitating an expensive repair process.

With rollable ribbon fiber cable, the fibers are attached intermittently to form a loose web. This configuration makes the ribbon more flexible, allowing manufacturers to load as many as 3456 fibers into one 2-inch duct—twice the density of conventionally packed fibers. This construction reduces the bend radius, making these cables easier to work with inside the tighter confines of the data center. Inside the cable, the intermittently bonded fibers take on the physical characteristics of loose fibers, which easily flex and bend, making it easier to manage in tight spaces. In addition, rollable ribbon fiber cabling uses a completely gel-free design, which helps reduce the time required to prepare for splicing, therefore reducing labor costs. The intermittent bonding still maintains the fiber alignment required for typical mass fusion ribbon splicing.

A smaller fiber allows additional reductions in cable diameter. Fibers with 200-micron coatings are now being used in rollable ribbon fiber and microduct cable. Replacing a number of cables with high-count fiber cabling conserves valuable space in data center pathways while reducing the cable weight and management complexity. And suspending fiber raceways (pathways) from the ceiling creates a more-direct path from one end of the data center to the other. As well, this paradigm helps support the easier addition, removal or replacement of fiber cables.

Soto: Capacity and size/density are important factors when selecting hardware for these high-count applications. Space is often a premium, whether it is a wall- or rack-mount hardware indoors or in a manhole or vault outdoors. It is important to match your cable to the right hardware. For example, check the splice tray size and mass-fusion splice capacity to see if it aligns with your cable. Routing and furcation are also important tasks that should be considered. A roundtable subunit that can be installed directly to the splice tray can eliminate furcation, lowering cost and speeding up the installation process.

Boxer: Planning is always key. Discussing plans with suppliers for recommendations for a particular application is always helpful. An ecosystem of network components for high-density applications has been developed over the past few years, and suppliers are typically happy to help. In general, the cable types and markings are similar to that deployed for decades, but everything is scaled up to meet the fiber counts.

There are multiple choices of distribution cabinets available for building entrance and data center applications. Regardless of the cabinet choice, it is critical to organize and label the fiber bundles as required for the splice trays. Ribbon bundles in the outside plant and building cables may be organized differently, which may complicate ribbon bundling and routing in the cabinet. It is always best to plan ahead.

1728-fiber cables are common in telecom applications, and 3456- and 6912-fiber cables are commonly used in data center applications. The application may dictate the choice of cable and closure design. OFS recommends that end-users review the application with the cable and closure vendors to assure compatibility between the cable and closure. Again, organization of ribbon bundles in OSP cable may differ between vendors so it is important to properly identify, bundle, and label the ribbon bundles as needed for splicing.

OFS recommends that high-density closures not be used for routine fiber distribution and rearrangements. If frequent fiber access is required, we recommend that an “off ramp” closure be used for routine maintenance activity. Using the “off ramp” closure will greatly reduce or eliminate unintentional damage to the express fibers in the high-density cable.

Finally, a significant amount of documentation is available to help make installations successful, including procedures, computer-based training, and online videos. We encourage customers to contact us if we can help in any way.

(Editor’s note: In addition to the companies that provided information for this article, Sumitomo Electric Lightwave is a company that offers an ultra-high-density fiber-optic cable. Sumitomo Electric’s pliable ribbon, called Freeform Ribbon, was designed “with the intent to fill central tube cables with more fiber than ever before,” the company said. It added. “The Freeform Ribbon structured has no preferential bend, therefore allowing the fibers to collapse on top of one another while still attached in ribbon form. This feature allows the circular central tube of a cable to be completely occupied with fiber rather than having space left empty by using a rectangular or square stack of traditional flat ribbons. Moreover, Sumitomo has the ability to construct Freeform Ribbon with 200-µm-sized fiber to make design even more densely packed, space-saving cables for today’s ever-increasing demand for more bandwidth.”)

Patrick McLaughlin is our chief editor.        

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