Do I Need G.657 Fiber in the Outside Plant?

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By: Terry E. Power

There seems to be a good bit of confusion (and controversy) right now about the use of G.657 (bend insensitive) fiber types in the outside plant.  There are a lot of people on both sides of the discussion.  Some say it is only for the last connection into a building.  Others contend there is a strong case for end-to-end use.

In truth, there are two main categories of bend insensitive fiber (G.657.A and G.657.B) with different characteristics.  Depending on which type you are discussing, either side of the argument could be correct.

Structure of ITU-T G.657 (2012)

Relevant specified bending radii for ITU-T G.652 and ITU-T G.657

In this article, we will look at the traditional uses for G.657 fibers, and then look at the case for end-to-end usage.  We will look at the two categories of bend insensitive fibers and address how each has a place in a total, end-to-end, solution for modern optical fiber networks.

Traditional Case
Bend insensitive fiber (Category B) has traditionally been used for the final drop and in-building fiber connections.  The number of corners and sharp turns faced in providing a visually pleasant installation of fiber inside residences created that need.

These fiber runs are typically short (less than 30 meters) and are accomplished with tight-buffered fibers specifically created for the purpose.  With G.657.B fibers, very tight bends are possible, with bends as small as 5 mm adding no additional losses from bending.  The compatibility differences in the transmission principles (particularly with CD and PMD) of the G.657.B fiber vs. traditional G.652.D fibers have minimal impact in these short distances.

The Case in the OSP
Category A types of G.657 fibers are fully compliant with the transmission principles of G.652.D fibers for loss and dispersion characteristics.  This allows G.657.A fibers to be deployed in the transport and access networks.  

Here are five points that need to be considered:

1. Smaller diameter micro-cables
   New advancements in micro-cables (especially using 200 um fibers) need to utilize G.657 fibers to pack the fibers in more tightly to achieve the smaller outer cable diameters.  The internal stresses created in the cabling process of micro-cables also create bending losses at the higher wavelengths.  Numerous manufacturers, around the globe, are now using G.657 fibers to reduce these losses and allow for the development of the smallest cable diameters we have ever seen in the industry.

2. Smaller Manhole and smaller enclosures
   Fiber densification has forced operators to use smaller manholes, cabinets, pedestals, and enclosures while demanding larger and larger fiber counts.  The use of G.657 fiber helps alleviate the potential for additional losses from macrobending in these small spaces.   

3. Lower Total Cost of Ownership
   According to the International Telecommunications Union (ITU) document, “ITU-T G.657 Fibers: Bend-insensitive single-mode fibers for access networks and customer premises”:

“ITU-T G.657 optical fiber cable reduces the roll-out cost for operators and the total cost of ownership (TCO) of an FTTH network.” 

Repair margins are greatly reduced when operators have less worry about bending loss at repair locations.  The minimally higher cost of the bend insensitive fiber over that of G.652.D is very quickly offset by years of additional life-span and reduced costs for repairs, adds, drops, and moves.

4. Engineer-Friendly Installation

The same ITU document mentioned above also refers to G.657 fibers being “more engineer-friendly.”  This is certainly a reference to the ease of installation in small spaces without fear of macrobending losses. 

As more fiber is deployed around the world, the demand for personnel will increase as well.  Bend insensitive fibers will reduce the impact of less experienced technicians without risking the addition of tight bends caused by their inexperience. 

5. Higher Bandwidth Made Easier

Higher bandwidth transmissions need to use the higher wavelengths where bending losses are highest.   The L-Band (1566 to 1625 nm) is particularly sensitive to bending and there is some discussion of opening the currently unused U-Band (1625 to 1675 nm), which is even more susceptible, in the near future.

Mitigating bending losses will be critical to operators attempting to utilize the L-Band, and possibly the U-Band, for high bandwidth DWDM systems.  G.657 fibers seem the best hope for the future of these high wavelength systems.

Conclusions

As we bring this look at G.657 type fibers to a close, there is one additional quote from the above-mentions ITU document that needs to be shared:

“Additionally, another favored application is in central offices where ITU-T G.657 fibers mitigate the risk of communication failure and/or high power damage under inadvertent bending.  Care needs to be taken to not impact long-term reliability.  Examples of the relationship between minimum bend radius and maximum power can be found in IEC TR62547.”

The questions we are asking about the uses for bend insensitive fiber are all adding depth to the conversation.  The future of optical fiber communications and networking will see great changes in technology and densification.  The discussions we are having today will guide us to the design decisions of tomorrow.

As we can see, there is a compelling case using of the full spectrum of bend insensitive fibers to provide a complete end-to-end solution from the central office to the receiver inside customer premises.  It will not be needed in every case; but adding it to our toolboxes will certainly help us better prepare for what the future brings.