Ethernet in Metro and Long Haul Networks
(continued)

Dark Fiber

Connecting directly to unused (“dark”) fiber is the most straightforward approach.  For example, Cisco Systems offers an optical interface called a Gigabit Interface Converter (GBIC). At the time of this writing, I am aware of at least three GBICs from Cisco. They are:

SMF = Single mode fiber
NDSF = Non Dispersion Shifted Fiber
NZ-DSF = Non Zero Dispersion Shifted Fiber

Depending on your application, you will plug the appropriate GBIC directly into your Ethernet switch or router port. Then you plug your optical cable into the GBIC. The GBIC type will depend on the type of cable that is deployed. This simple chart’s IEEE types column implies that there are just three types of glass fibers, which is not true.  For this discussion, we will focus on only these three.

The 1000BASE-SX is meant for multimode fiber, which severely limits distance. If you price out single mode fiber, you may be surprised to see that it is cheaper. Wavelength is one other issue regarding 10000BASE-SX. It turns out that glass fiber appears “more transparent” as the wavelength goes from 850 to 1550 nanometers. It does not go on forever though, and at 1550 nanometers, the fiber begins to become less transparent and at around 1610 nanometers, it is nearly opaque.  Does this mean we can use wavelengths between 850 nm and up to say 1600 nm? The short answer is yes, but the technical answer is that some wavelengths are much better than others and some are downright worthless.  Let’s look at the wavelengths that the industry commonly uses. They are 850, 1310, and a range from 1530 to 1570 nm. The 850 nm wavelength is great for short distances because we can make lasers dirt-cheap. At 1310 nm, an interesting thing happens. Physical properties of a NDSF fiber, which was and is the darling of the SONET industry, will hold the optical signal together and we experience nearly zero bit errors at bit rates much higher than one gigabit per second. Therefore, NDSF fiber, which is tuned for 1310 nm, can be considered an industrial strength fiber.  Since there is a lot of NDSF fiber in the ground, we have a Gigabit Ethernet standard (1000BASE-LX/LH) that knows how to use it to support distances up to 10 kilometers.

Of course you are not going to be satisfied with a limitation of 10 kilometers are you? If you need to support greater distances, then consider NZ-DSF fiber, such as Corning’s LEAF (Large Effective Area Fiber). This kind of fiber is tuned for the 1550 nm band. At this wavelength, fiber is outrageously “clear.”  Signal attenuation is at a microscopic level of .19 dB per kilometer and the NZ-DSF fiber easily holds your optical signal together over a 70-kilometer span and with the right fiber, like Corning’s LEAF, go ahead and push the limit to 100 kilometers.  (Just make sure your vendor will support you!)

Figure 6

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