Why is Bandwidth Finite?

When fiber was introduced,its enormous bandwidth and extremely low attenuation made it the No. 1 choice to replace copper in long-distance telephone networks. Once the telcos switched from multimode to single-mode fiber in 1984, they found almost boundless bandwidth.

Using the same fiber today, telco systems have gone from 45 Mbps on one wavelength to 10 Gbps over as many as 32 wavelengths each and still use only a fraction of the fiber’s capacity. It would be erroneous to assume that fiber has infinite bandwidth for premises applications. While the telcos switched to single-mode fiber and fast laser sources, local area networks (LANs), closed-circuit television (CCTV) and other premises applications continued to use multimode fiber with light-emitting diode (LED) sources. While single-mode fiber and lasers have more bandwidth, the combination of LEDs and multimode fiber is limited in bandwidth by several factors.

The LED sources are only capable of generating signals of 250 to 300 Mbps because of the LEDs’ light-generating mechanisms’ limitations. The fiber bandwidth is limited by two factors: modal dispersion and chromatic dispersion.

Multimode fiber carries light in many rays or modes. Each mode takes a different path through the core of the fiber and takes a slightly different time to get from transmitter to receiver. The signal pulse is broadened by the variation of times for each mode: modal dispersion.

In addition, light of different colors travels in the glass core of the fiber at different rates and angles: that’s why a prism disperses colors. This causes signal broadening too: chromatic (for color) dispersion.

Laser light is almost all one color, so chromatic dispersion is not a problem. LEDs, however, have many colors in their outputs. In multimode fiber with an LED source, chromatic and modal dispersion effects are about equal. The fiber is optimized for one wavelength (1,300 nm rather than 850 nm) to take advantage of the fiber’s lower attenuation at the longer wavelength.

Distance affects all forms of dispersion. Longer fibers have more dispersion. Eventually, it becomes a problem: two pulses will be widened enough that they cannot be distinguished and the fiber optic link will no longer operate.

All of this is academic for low-speed systems like 10 megabit Ethernet. The fiber bandwidth is high enough so dispersion is not a problem. At 100 Mbps or higher, as used by Fast Ethernet, fiber distibuted data interface (FDDI), ESCON and Fibre Channel, the LED/multimode combination limits link distances to 2 to 3 km (1.2 to 2 miles), which is still adequate for premises and campus installations.

In the gigabit range, LEDs are too slow. Coincident with the development of gigabit links, vertical cavity surface-emitting lasers (VCSELs) became available. A VCSEL is an inexpensive laser, currently available at 850 nm but soon to be available at 1,300 nm, that is ideal for gigabit links.

For GbE, the 850 nm VCSEL and multimode fiber was an ideal marriage. It allows users to upgrade current fiber optic backbones to GbE without recabling. However, the bandwidth of most installed multimode fiber limits GbE to about 250 meters (800 feet).

Should the user recable with higher bandwidth multimode fiber or go to single-mode? Most fiber manufacturers offer multimode fiber optimized for 850 VCSELs, but it has a 50-micron core diameter that is incompatible with today’s installed base of 62.5-micron core fiber. This means cables, links and patchcords––even test hardware––must be marked and segregated. New fiber users don’t have a problem; they can use the new fiber and enjoy the benefits of much higher bandwidth, knowing they are ready for the next generation of 10 GbE.

Converting to single-mode may not mean recabling. We advise users to install hybrid cables with both multimode and single-mode fibers, and many are simply switching over to single-mode transmitters and using the fibers already available. But single-mode transceivers are considerably more expensive than multimode ones.

One fiber manufacturer, Pirelli, suggests running 850 nm transceivers on single-mode fiber. It needs more field trials and VCSEL manufacturers may need to tweak their devices to optimize the power coupling, but their idea has merit. Pirelli’s “FineLight” fiber is designed for this application and offers better performance at 850 nm than normal single-mode fiber; however, there’s no apparent reason that other single-mode fibers would not also be useable in this way.

Despite the need for more research and development, the solution makes installing hybrid backbone cabling sensible. Whether the multimode fiber is regular 62.5/125 or 50/125 laser grade and single-mode is regular or FineLight, style is another, more complex, issue. EC

HAYES is a VDV writer and trainer and the 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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