The myths and realities of reduced bend radius fiber

By Pat Thompson and Mike Hoppe, ADC -- Although reduced bend radius fibers represent a major breakthrough in attenuation prevention, service providers and network architects who assume they are indestructible can put network performance at risk.

Although reduced bend radius fibers represent a major breakthrough in attenuation prevention, service providers and network architects who assume they are indestructible can put network performance at risk.

By PAT THOMPSON and MIKE HOPPE
ADC

Reduced bend radius fiber is a huge innovation for optical networks because it allows more flexibility in bending fiber around tight corners without any discernible increase in attenuation. These fibers are often referred to as "bend insensitive," or "bend resistant" -- terms that can be somewhat misleading when it comes to the actual capabilities of the fiber. Service providers and network architects need to know the differences between the realities and myths surrounding the use of reduced bend radius fiber in their networks.

Believing that new reduced bend radius fiber is no longer subject to mishandling, temperature extremes, improper routing, or other external forces is a mistake technicians cannot afford to make. Bend radius protection is only one of four aspects of good fiber cable management techniques, and the other three -- connector/cable access, cable routing paths, and physical protection -- should never be overlooked. This article will address some of the benefits to deploying reduced bend radius fiber. It will also explain the need for continuing to incorporate good fiber cable management techniques in light of the many misconceptions regarding the characteristics of reduced bend radius fiber

The attributes

So what exactly is meant by "reduced bend-radius fiber"? It boils down to the fiber's ability to achieve tighter bends within fiber frames, panels, and routing pathways. Prior to these new fibers, the standard bend radius for singlemode fiber ranged from 30 to 38 mm. The basic rule of thumb has been a maximum bend radius of 10X the outside diameter of the cable or 1.5 inches, whichever is greater. The reduced bend radius fiber decreases this standard by about 50 percent, or to 15 mm, without changing the attenuation.

The optical performance of these fibers has improved dramatically when it comes to bending. However, equating this benefit with total cable reliability under extreme conditions or simply ignoring the need for proper fiber cable management is cause for concern. The optimized bend radius characteristics do not render unnecessary other aspects of cable management designed to further protect the integrity and performance of the optical fibers.

The considerations

When thinking about long-term fiber reliability in a network, there are more considerations than bend radius capabilities. Even with reduced bend radius fibers, there are still limitations to the fiber cable. Thus, providers and technicians should not confuse the attenuation bend radius with the fiber cable bend radius. The fiber cable bend radius must still account for the mechanical and structural characteristics of the entire cable.

There are other concerns for the fiber cable as well, such as environmental considerations. For instance, the cable jacket and other components will shrink in low temperatures and can still alter the fiber's performance. Even though the low temperatures may not cause attenuation, there is still a real potential for damage or even breaks as cables expand and contract with extreme temperature fluctuations. End users should always follow the manufacturer's specifications for recommended operating temperatures.

Another common misconception is that deploying cables with very small bend radius characteristics makes them more rugged and durable. In fact, the mechanical properties of the glass have not changed and neither have the cable materials in most cases. The reduced bend radius fibers are simply able to better maintain the light within the core when the cable is bent -- they typically do not add strength or durability to the glass inside the cable. In point of fact, attenuation in traditional fiber provides the benefit of an "early warning" when there is a critical bend.

It is worth noting that, by itself, reduced bend radius fiber offers no attenuation improvements. Rather, it is the ability to bend more tightly without causing additional attenuation that makes it more useful in networking applications. Comparing a straight run of reduced bend radius fiber and standard optical fiber reveals no difference in the performance of one over the other. A review of the four aspects of fiber cable management, in light of reduced bend radius fiber, will help point out the need to continue good cable management techniques.

It is critical that providers and technicians understand that all four aspects of fiber cable management are still absolutely necessary for long-term reliability and performance in all segments of the optical network.

Cable management practices

As stated earlier, reducing the bend radius restrictions on optical fiber and patch cords is a significant step in protection from macrobends when deploying fiber in cabinets, panels, and around sharp corners. Yet while the bend radius restrictions may be reduced, they are not eliminated altogether -- and the same can be said for the need for good cable management. Proper slack storage is still a critical step in alleviating potential problem areas on frames and along the cable pathways, as well as in outside plant portions of the network.

When it comes to cable routing paths, it would be easy for technicians to believe they no longer have to be neat since there's not significant concern about bend radius violations. But technicians still need to perform rapid circuit routing, cable tracing, and reconfiguration. Good cable routing enables clear and easy paths for more effective routing, tracing, and reconfiguration. A benefit of reduced bend radius fiber in this area will be the ability of technicians to actually put their hands into a fiber bundle to physically trace a fiber out. In fact, this easier cable tracing method will make good cable routing paths even more important.

In the area of connector and cable access, technicians must be able to identify and access the right connector for installing, testing, reconfiguring, and cleaning. When using reduced bend radius fiber this consideration becomes even more important. Technicians might mistakenly believe they no longer have to be concerned with bending the fiber. Network architects should also keep in mind that with connectors more densely packed together, the potential for accidentally disconnecting the wrong cable increases. Easy connector access is important no matter what fiber is being used.

Finally, the physical protection of the fiber, despite what many might have you believe, is not diminished by the use of reduced bend radius fiber. Optical cable is still glass -- and although it can now bend without increased attenuation, the reality is that damaged or even broken fibers are still possible. However, one of the real benefits achievable with reduced bend radius fiber is the ability to get more terminations into a smaller space. In turn, this will enable smaller packages into wall boxes or cabinet enclosures, as well as a greater number of terminations on a frame without sacrificing cable management.

Going beyond the central office, fiber cable management deserves the same considerations in the outside plant portions of the network. For example, drop cables located within a hand-hole should be free and unobstructed to enable easy access and physical protection from anything that could destroy the integrity of the cable. Good cable management is not limited to the central office.

Choosing wisely

The bottom line is that, in part because of the terminology used for describing reduced bend radius fiber, there are many misconceptions about its application and actual characteristics. The ability to bend the fiber more without causing additional attenuation is a great asset in optical fiber construction and will have a dramatic impact on the network. The further ability for much-handled patch cords and multiple fiber assemblies to be less susceptible to macrobends makes these fibers very attractive.

However, it's just as important for service providers, architects, engineers, and technicians to know what characteristics these fibers do not have. These cables are not more rugged and durable, nor are they less likely to suffer damage or breaks due to mishandling or misrouting.

The core is still glass -- and proper fiber cable management techniques are still required to protect it. Front and rear connectors must still be made accessible for service turn-up, reconfiguration, and troubleshooting without disturbing adjacent fibers. There are physical limits to density, and when connectors cannot be properly removed and reseated, service performance can be altered. Reduced bend radius fiber has the same coatings and jacket as standard fiber, making it just as susceptible to physical damage as standard fiber if handled improperly.

As reduced bend radius fiber finds applications using higher densities, easy connector access and efficient cable routing paths will become even more critical. The elements of fiber cable management, coupled with these new brands of fiber, represent a significant stride forward to better long-term performance and efficiency for tomorrow's optical networks.

Pat Thompsonis program manager for central office fiber products at ADC, headquartered in Eden Prairie, MN.Mike Hoppeis ADC's product manager for fiber cable assemblies and accessories.

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