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using 62.5 mm multimode fiber (MMF). The 50 mm MMF fared better, but it still
couldn't reach 300 m unless you used fiber with a high modal bandwidth (or were
lucky enough to have installed it).
For the TIA standards, the answer to the multimode problem is to more clearly
delineate the characteristics that make 10 Gb operation feasible. One way to do this
is to optimize multimode fiber for laser operation at 850 nm. As you recall from
Chapter 11, the VCSEL laser diodes work best with compatible fiber. This was first
described in TIA-568-B.1 Addendum 4, -B.3 Addendum 1, and in TIA-492, and is
now part of TIA-568-C. Use the right type of fiber and 10 Gbps is a cinch.
Another great solution to the multimode problem is to use MMF with the
IEEE's 10GbaseLX4 standard. For most applications, properly installed and termi-
nated 62.5 mm MMF should be able to work at 300 m with no problem—if you
use LX4. Likewise, 50 mm MMF will work at 240 to 300 m, depending on the
model bandwidth. As always, you are rewarded for spending a few percent more on
higher quality fiber. The only drawback to LX4 is the additional expense of the four-
way wave-division multiplexing (WDM) scheme. Splitting the signal into four wave-
lengths, and later recovering it, requires additional circuitry, optical filters, and
splitter/combiners.
Single-mode fiber (SMF), on the other hand, can operate very well at 10 Gbps,
supporting distances from 10 to 30 km—far above the needs for LAN cabling. The
biggest drawback to single mode is that the equipment costs are more, because the
cost of the transmit optics are more. Many cabling designers are putting in six to
twelve (or more) strands of single-mode fiber and the same number of multimode
strands. It generally takes two strands per link, with one for transmit and one for
receive.
This brings up a good topic: color coordination of the fiber termination con-
nectors. There has been a lot of confusion between MMF and SMF facilities, once
they are installed. Traditionally, the adapters into which the terminated fibers were
patched were a black color. Unless the installation was well documented, you could
not determine whether the installed link cable was single- or multimode. A mis-
match is disastrous. The loss from a mode mismatch can make a link fail.
Troubleshooting this type of problem is very difficult and time-consuming. The best
way is to mark the connections as to mode, and the TIA suggests doing this with
connector color. Here is the fiber connector color code:
50/125 mm
Black
62.5 mm
Beige
50/125 µm laser-optimized
Aqua
Single-mode
Blue
Angled single-mode
Green
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