Comparing fiber-optic cables to cut costs

For large optical networks, the ODN typically accounts for more than 70% of the entire network cost. While active equipment can be upgraded with the rapid bandwidth increase of the electronics, the ODN, if designed with scalability, can and should last for 20-plus years.

To protect the network investment, the ODN design must be mechanically robust to survive any MACs, the harsh environment areas of the enterprise, even natural disasters, while also supporting several future generations of high-bandwidth, high-speed emerging technologies and electronic upgrades. These assertions further reinforce the necessity of deploying ruggedized, helically stranded tight-buffered cabling in the network.

To minimize the pitfalls that can lead to network failure and the monumental per-minute costs of downtime, troubleshooting, and network restoration, the cabling infrastructure must also deploy cables that are both ruggedized and simultaneously highly resilient to macro-bending. Macro-bending results in excessive attenuation that seriously degrades system performance and signal transmission and can force the network down. Macro-bending can also cause physical irregularities that can cause micro-bending and severe damage to the fiber.

As any installer can attest, macro-bends can occur during routine network installations, such as routing a jumper in a data center patch panel, routing around sharp corners in an office environment, frequent MACs for dynamic broadcasting applications, or pulling cable around tight bends or within manholes in the OSP backbone.

A recent NTT Advanced Technology study polled network owners and cable installers on the sources of network failures. Following the more controllable causes of network failure stemming from connector and splicing faults, bending the fiber was shown to be the next leading cause of failure—supporting the fact that a more “bendable” and more macro-bending resistant cable, such as helically stranded tight-buffered cables, are necessary to achieve a zero-downtime network.

Helically stranded tight-buffered high-count/high-density indoor-outdoor cable resulted in no change to the BER level when bent, unlike ribbon cable, reinforcing its macro-bending resilience properties.

By deploying cable that is both ruggedized and macro-bending resilient, data center operators and enterprise network managers can achieve time and cost savings with an overall low cost of network ownership through:

•                    Faster and easier installations, upgrades, and MACs due to easier cable bendability and handling

•                     Less likelihood of fiber damage and costly replacement

•                     Reduced network maintenance time and labor for troubleshooting

•                     Indoor-outdoor tight-buffered cables that reduce installation and maintenance costs by eliminating traditional connections or splicing for the transition from outdoor to indoor cables, thereby eliminating points of network failure for a continuous fiber run

•                     Less likelihood of network failure, saving significant per-minute downtime costs

•                     Increased cable quality for a robust, scalable infrastructure that is installed once, lasting 25 years or longer

The last bullet concerning cable quality raises an important consideration. Cable bendability, macro-bending resistance, and ruggedness are properties inherent in the tight-buffered cable design and manufacturing process. And the quality of the design and development of tight-buffered cables vary by the manufacturer, so it is recommended that data center and network managers choose their tight-buffered cables carefully.

Design Comparison

The helically stranded, tight-buffered, tight-bound technology incorporated by some manufacturers, including OCC, results in fiber cables with much stronger pulling strength and much less bending of the diameter against sharp edges when compared to other straight lay cables, such as loose-tube and ribbon. The bendability is achieved because all the cable elements in the helically stranded cable under pulling stress tend to pull toward the center of the cable.

Unlike some ribbon cables in which elements at one edge could bend at a much sharper diameter than elements at the other edge (causing macro-bending-induced high stress and micro-bending susceptibility), the helically stranded elements are bent equally with the stress averaged across the lay length, thereby eliminating macro-bending and micro-bending within the cable.

Helically stranded tight-buffer technology yields not only a smaller diameter fiber cable—ideal for the higher port densities of data centers and congested conduit capacity of enterprise networks—compared with loose- tube cables and some ribbon cables with the same fiber count, but also provides the highest pulling strength, significantly reducing the likelihood of damage.

The ruggedness of the helically stranded tight-buffered cables can best be exemplified by the cables’ crush resistance, the highest in the industry, that exceeds 10 times the 220 N/cm called for by the ICEA S-104-696 standard body.

Additionally, the tight-buffer cables do not rely on extra strengthening elements, thereby making them more flexible than ribbon and loose-tube cables. There are also no messy gels that need to be cleaned before fusion splicing, which improves both installation time and splicing efficiency.

Another important benefit of helically stranded tight- buffered cables is the water-blocked/water-tolerant attributes that provide the very best water protection system available by combining the inherent water-tolerant features of tight-buffered and Core Locked tight-bound cable that features super-absorbent polymer aramid yarn. The design provides superb water resistance while retaining optimal performance and the termination cost advantages associated with totally gel/powder–free tight-buffered cable.

In recent years, gel-free, dry loose-tube and ribbon cables have been introduced to overcome the messy installation and termination headaches associated with the gel. To replace the gel’s water-blocking capability, manufacturers added water-swellable powders and tapes in the development of the dry cable.

However, the swellable agents can attribute to further micro-bending in areas where the powder particles, for example, can get wedged between the inner surface of the tube and fibers—an occurrence that is detrimental to the cable and, consequently, the data transmission integrity of the network. This is one reason, among others, why dry loose-tube cables have not replaced or made gel-filled loose-tube cables obsolete.

These proprietary helically stranded tight-buffered cable design technologies advancing ruggedness, bendability, installation ease, and friendliness to save time and costs—while demonstrating outstanding transmission integrity—were first developed by OCC in the early 1980s, with ongoing advancements continuing ever since.

Initially, these rugged helically stranded tight-buffered cables were designed for the most reliable military tactical deployments required by the Department of Defense (DOD).

BER Testing High Data Rate Performance

Recently, Bit Error Rate (BER) testing of an OCC HC-Series – High-Density cable and a ribbon cable was performed to compare the system performance of these two different cables.

BER testing is a system-level evaluation of a physical layer optical network to meet the expected requirements for overall signal fidelity. It tests data transport accuracy through actual optical link conditions to ensure the physical layer signal path integrity of the designed optical network. The test results account for effects of all the parameters of the optical link, such as insertion loss, return loss, chromatic dispersion, and differential group delay for MMF. BER testing is critical, especially for high-bandwidth optical networks designed to meet current and future high–data rate operations.

The helically stranded HC-Series – High-Density Tight- buffered Indoor/Outdoor cable with splice-on blades is a popular solution for campus rings and other high-density applications. Another available choice on the market is ribbon cable with MPO-LC cassettes.

The system performances of these two configurations are compared via the BER test. More specifically, the HC-Series – High-Density cable used in the test was a 48-fiber single-mode (SM) cable with bend-insensitive fiber from OCC equipped with OCC’s splice-on Procyon Blades on both ends. Comparably, the ribbon cable was also a 48-fiber SM cable with bend-insensitive fiber supplied from a highly reliable, quality manufacturer; the ribbon cable employed MPO-LC cassettes on both ends.

The test results reveal that the design benefits of outstanding ruggedness, bendability, and water resistance of the HC-Series – High-Density helically stranded tight-buffered cable with spliced-on blades did not degrade optical performance at all. Rather, it marginally outperformed the ribbon cable link.

The 10km HC-Series – High-Density cable was error free when further tested under normal operating conditions at the 10Gbps data rate for two weeks and demonstrated the BER of better than 10^-15 at a 99% confidence level, further validating that the optical link of the helically stranded HC-Series – High-Density tight-buffered cable was technically error free.

The BER level was monitored for fluctuations while making the same degree of cable bending against sharp edges for the HC-Series – High-Density and ribbon cable. It is noticed that when a ribbon cable was bent sharply, the BER level under impact increased dramatically, indicating the anisotropic Macro-bending induced attenuation and micro-bending susceptibility that compromises signal integrity and transmission reliability.

Conversely, when an OCC HC-Series – High-Density cable is bent in the same angle (or any other helically stranded, tight-buffered OCC cable, since the mechanical design properties are the same), the BER level was not affected at all. This further proves that the helically stranded, tight-buffered cable is more macro-bending and micro-bending resistant.

Additional Considerations

Helically stranded, tight-buffered cable is much more macro-bending resistant than the ribbon cable because the fibers inside the tight-buffered cable are bent uniformly, while the fibers inside a ribbon cable are bending-direction dependent. When the macro-bending occurs along the ribbon surface, fibers at both sides of the ribbon can suffer enormous stress.

Many helically stranded, tight-buffered cables, including the HC-Series – High-Density cable, are also much smaller than the ribbon cable (7mm diameter versus 10mm diameter for a 48-fiber cable). A reel of 1km 48-fiber HC-Series – High-Density cable, for example, is approximately two-thirds the size of that with the same fiber count ribbon cable.

The helically stranded, tight-buffered cable is also mechanically stronger (more rugged) than the loose-tube-like ribbon cable. Because the ribbon cable relies on multiple strength elements for mechanical strength, it is not as bendable as the HC-Series – High-Density cable and all OCC helically stranded tight-buffered cables for ease of handling and installation, such as being easily pulled through several right-angled conduits.

However, ribbon cables do have the advantage of massive splicing of up to 12 fibers at a time. Helically stranded, tight-buffered cable can also be easily ribbonized with commercial-available kits before performing mass splicing.

Concluding Summary

The continuously growing demand for increased network bandwidth is one of the most critical issues facing data center operators and enterprise network managers, as more and more are deploying 10-100Gb Ethernet and beyond. Consequently, one of the most important decisions is which optical fiber cable is best to deploy for your OSP and inside plant network applications. This study has built the case that optical fiber cable needs to be both rugged and macro-bending resistant to achieve a long-life (25 years), scalable, future-forward network at a low cost of ownership.

Through BER testing, it is shown that helically stranded tight-buffered cables that feature these two crucial properties, specifically the HC-Series – High- Density compared to ribbon cable, demonstrate the outstanding transmission performance for zero-downtime fiber-optic networks.

The superior ruggedness and macro-bending resilience—along with other cable features—of helically stranded tight-buffered cables are necessary to achieve network longevity, zero downtime, and minimal maintenance and troubleshooting that ultimately results in optimum network performance and cost savings.

Joe Yue is Senior Engineer with the OCC Fiber Division.