Extending Fiber's Reach

For almost 15 years, one had only two choices if installing fiber. The de facto-standard multimode fiber had a core/cladding size of 62.5/125 microns and was rated for use with FDDI (Fiber Distributed Data Interface) or Fast Ethernet, both 100 Mbps networks that used inexpensive LED sources as transmitters. Longer distances or higher speeds called for single-mode fiber with a small 8-micron core that required expensive laser sources.

When Gigabit Ethernet (GbE) began to appear in corporate networks, the 62.5-micron multimode fiber had bandwidth limitations that restricted GbE to 220m (about 700 feet). Longer links needed single-mode fiber, which was much more expensive when the laser-based electronics were considered.

Of course, 220m was much longer than the 100m limit of UTP copper cabling, but fiber and copper are considered differently in structured cabling. Copper is used from the local telecommunications closet to the desktop, so it is engineered to a maximum length of 100m, no small feat when you consider the high-speed signals it now carries. Fiber is used for backbones that may stretch between buildings and interconnect the closets in a building, so maximizing the distance capability is important.

Fiber manufacturers had not put any real engineering effort into multimode fiber in 15 years because 62.5/125 fiber met the industry’s needs. But with the advent of GbE, calls for longer distances on multimode fiber sent them back to the labs. And what they came up with was a brand-new, 20-year-old fiber.

GbE could no longer use LED sources as transmitters. LEDs do not have the capability of modulation at gigabit speeds, so GbE adopted a new type of inexpensive laser called a VCSEL.

Lasers have higher bandwidth in fiber for two reasons. First, they have less chromatic dispersion than LEDs, due to laser’s purer color. And their output power is concentrated in a narrower beam, reducing modal dispersion. (See our column of August 2002 for a more detailed explanation of fiber bandwidth.)

Since GbE used an 850nm laser for a source, the fiber manufacturers were able to revive a fiber that dated back to the early days when newly-developed 850nm lasers were used with one that had a 50-micron core optimized for use with lasers. This fiber had been put aside when the industry moved to 1310nm lasers and single-mode fiber around 1983 to take advantage of the lower attenuation at 1300nm and the higher bandwidth of single-mode fiber.

The 50-micron fiber had a bandwidth of 500 MHz compared to 160 MHz for FDDI-grade 62.5 fiber, allowing distances about three times as far as 62.5 fiber when used with the same 850nm VCSEL.

As the fiber manufacturers began making this 50-micron type, they found they could improve it using new technology to allow bandwidths of 2,000 MHz, giving GbE the capability of going up to 2km over multimode fiber, the same as earlier 100 Mbps networks. The timing for this new “laser-rated” fiber couldn’t have been better, as 10 Gigabit Ethernet was being developed and it really needed this new fiber to allow low-cost links over multimode fiber.

Now for the big questions: Should all new fiber installations use this new fiber? And should older fiber backbones be replaced with it?

The answer to both questions depends on the plans for the fiber. If it’s unlikely that the user will ever need speeds faster than 100 Mbps, there is no reason to pay the higher cost for laser-rated 50/125 fiber or the cost of recabling. Likewise, if the user has 62.5/125 fiber and expects the fastest network on their cabling to not exceed GbE, they are in good shape. But if it’s a large corporate network, odds are they will eventually migrate to 10GbE and the faster laser-rated 50/125 fiber is a good investment.

A word of caution: you cannot mix 50/125 and 62.5/125 fiber. Although the cladding diameters are the same, the larger core of the 62.5 fiber makes for high loss when it is connected to 50-micron fiber.

The loss is about 2 dB per connection—about the loss margin tolerable in GbE. In the other direction, you can connect 50 to 62.5 with minimal losses, but mixing them means you will have both conditions on every link, since two fibers are used to transmit in opposite directions.

Managing 50-micron fiber in a cable network with 62.5 installed also requires caution. It’s best to color code at the patch panels to indicate which fiber is terminated there and buy jumper cables in the same color. Post signs noting the use of both fibers and the consequences of improper use.

Trust me, if you do mix them, you will have problems. But as long as you know the issues, you should be able to troubleshoot problems quickly. EC

HAYES is a VDV writer and trainer as well as president of the Fiber Optics Association. Find him at www.JimHayes.com.


About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor
Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com .

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