Roll With It, Baby!

Figure 1. Rollable ribbon enables higher density cables.

Where a flat ribbon is connected by matrix material between fibers down the entire length, the fibers in a rollable ribbon are connected only at regularly spaced points down the length of the fiber, which makes it more flexible and "rollable". (See Figure 1.) This design allows the fibers to be spliced using ribbon splicing techniques and equipment.

Rollable ribbon cables can double the fiber density for a typical duct compared to flat ribbons, enabling thousands of fibers to go into spaces that were previously too small, while enabling mass fusion splicing.

Service providers are facing unprecedented demand due to increased bandwidth requirements, both from the launch of 5G cellular service as well as the response to COVID-19.¹

In addition, duct space is at a premium, especially in dense metro areas, so enabling more fibers in less space is even more important in those scenarios.

Much more attention is now being paid to the rural/urban digital divide, and fiber density may even help in some rural and underserved areas. Many of these areas have older poles with potential sag and tension limitations. Although this case is not quite as clear as the metro case, there still can be value to having more fibers in less space.

3 Applications = One Size Doesn’t Fit All

Telecom providers face a myriad of challenges as they install fiber closer to the end user. Thus, fiber cables must be deployed across a wide variety of environments.

As with flat ribbon cables, rollable ribbons come in different designs for different applications and network types. Three of these different situations are described below.

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Situation 1. Datacenter Interconnect Networks That Require Very-High Fiber Counts and Frequent Access.
With the explosion of demand for a wide variety of cloud-based applications, hundreds of new
hyperscale datacenters have been built over the past few years, with more planned for the future.²

Many of these networks are in urban or metro areas, and/or in areas where access to duct space is at a premium, so fiber density poses a major challenge. More fibers in less space is very important for these networks. These networks often start with fiber counts as high as 864, and 1,728 or 3,456 fiber counts are not unusual. These often need to fit in existing 1 ¼" or 2" ducts.

Precisely because these networks are in crowded areas, another challenge is to provide proper protection for the fibers in these environments. Many of these cables are co-located in areas with many other cables from different service providers, and may be handled very roughly over their lifetimes.

Adding more fibers in a cable and materials for robustness typically make cables larger. However, rollable ribbons and bend insensitive fibers are tools that enable cables to be both ultra-high fiber count, and very robust.

The advantage for rollable ribbons over flat ribbons comes down simply to geometry. In a circular cable, circular rollable ribbons are more space-efficient. This means, for a given diameter, thicker buffer tubes and jackets can be used to better protect fibers in rollable ribbons versus cables with flat ribbons.

Another challenge is that although some of these networks may be point-to-point networks, other networks may require substantial numbers of fibers to be accessed and dropped off. Accessing fibers in a central tube configuration is possible, but accessing ribbons in a ribbon in loose tube design can be easier for networks with ultra-high fiber counts, 864 and above.

For these types of applications, a rollable-ribbon in loose tube (RILT) design can be an effective choice. (See Figure 2.) It can significantly improve density, versus a flat ribbon design, but also can enable easier access to fibers along the way.

Figure 2. Ribbon in loose tube (RILT) cable design.

Some other specifics about the RILT design:
• It’s inherently very strong, since the robust central member can enable 1,000 lbs. pulling tension.
• It facilitates good blowing performance, and offers crush resistance.
• The design enables easier handling, versus cable designs with outboard strength members.
• It has extra layers of fiber protection during mid-span access applications.

Situation 2. Longer Distance Network Builds
For more traditional network high fiber count builds, challenges don’t just disappear. These networks can be longer than metro networks, and reducing splice points can be a significant driver for both cost reduction and lower optical loss. These requirements can drive the need for longer length cables.

The installation method for these networks is often cable blowing. Cable blowing performance is typically helped by a stiffer cable. Cables with outboard strength members have been field-proven for decades to have excellent blowing performance.

For these networks, a central core LXE-type design featuring outboard strength rods is an industry staple. (See Figure 3.) While these networks may not require quite as many points where individual fibers or ribbons need to be accessed, a workhorse such as this is needed for providers looking for high-density and good handling capabilities.

Figure 3. LXE-type cable design with outboard strength rods.

Some other notes about this design:
• Strong blowing performance.
• High fiber density, especially for armored cables.
• As a central tube design, it delivers low latency and lower excess fiber length than other designs.
• The outboard strength rods means that the cable has a preferred bending plane.
• Often available in longer lengths than ribbon in loose tube designs.

Situation 3. Tight Bends
Finally, some high fiber count networks need to operate within the constraints of previously installed network hardware, including small handholes and or pedestals. These networks require a high fiber count cable that is both very small and can be coiled into these tight spaces. These networks may also frequent access.

For this type of network, a very flexible cable may be needed. These cables may have flat but very flexible strength elements surrounding a central tube, enabling very tight coiling and also the smallest diameter for smaller fiber counts. (See Figure 4.)

Figure 4. Highly flexible rollable ribbon cable.

Other features of this design include:
• Durable, highly crush-resistant central core with helically applied strength members for no preferred bending plane.
• Tighter cable coiling for simplified handling and installation compared to LXE designs.
• Added protection of central core tube for increased robustness.
• Easier mid-span access, especially with lower fiber counts. The core tube allows for a safe ring cut and longitudinal split, and strength elements can then be cut with scissors.

Rolling Into the Future

Although rollable ribbon technology is very promising, there are a few considerations that should be mentioned:

• At this point in time, rollable ribbon cables are sometimes more expensive than comparable options. That being said, the right design can result in savings when compared to either standard loose tube or flat ribbon cables.

• There are some subtle operational differences between splicing rollable ribbons and flat ribbons. This fact means that there is a small learning curve involved as splicers work with it for the first time. However, these differences are not significant.

• Finally, cable and ribbon designs are not as standardized as more traditional designs. There are significant differences between ribbon and cable designs from manufacturer to manufacturer.

Even though rollable ribbons are relatively new on the scene, there are different cable designs available that are purpose-built for various applications.

Also, there is a tremendous amount of innovation in this space, so it’s a good idea to keep up to date on new product developments as they occur.

Like most things in life, remember that one size truly does not fit all. Choosing the most appropriate design for the specific application can help reduce installation hassles, potentially save significant amounts of money, and help keep the network humming for decades to come.

Resources and Notes
1. https://www.fiberbroadband.org/p/do/sd/sid=3116

2. https://www.datacenterknowledge.com/cloud/analysts-there-are-now-more-500-hyperscale-data-centers-world