Cable Testing 101: Considerations for Mixing Multimode Fiber Types

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Cable Testing 101 Considerations for Mixing
Multimode Fiber Types-Mark Mullins
www.flukenetworks.com 2006-2017 Fluke
Corporation
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Cable Testing 101 Considerations for Mixing
Multimode Fiber Types
  In our last Cable Testing 101 series, we
covered the difference between OM3 and OM4 50µm
multimode fiber -- primarily the fact that the
fiber core of OM4 has been constructed to provide
better attenuation and higher bandwidth, thereby
allowing for longer link lengths. With OM4 at a
premium over OM3, many data centers and LANs not
requiring the extra distance afforded by OM4
continue to deploy OM3 multimode fiber cabling,
and it remains more widely deployed for that
reason. And while the two fiber types can be
mixed due to the same core size, there are some
considerations in general when it comes to mixing
multimode fiber types.
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Cable Testing 101 Considerations for Mixing
Multimode Fiber Types
Staying in Your Budget
It's important for cabling infrastructure
designers to know which type of fiber is being
used, and it's recommended to use one type of
fiber throughout an entire channel to avoid
potential performance issues. Due to its lower
attenuation and greater modal bandwidth, OM4
supports 40 and 100 Gig to a 150-meter distance
while OM3 fiber only supports these applications
to a 100-meter distance. If you're
troubleshooting a fiber channel greater than 100
meters that your customer claims is OM4, check
the cable legend. It could be failing insertion
loss because it was actually deployed with OM3.
Depending on their length, using OM3 fiber
jumpers on an OM4 link can also have an impact on
loss-- especially if the link is pushing the
upper limits of the budget to begin with. What's
more of a concern however is the mixing of OM3 or
OM4 components with legacy OM1 62.5/125 fiber
that has a larger core size of 62.5µm. Mixing
different core sizes is akin to connecting two
different size water pipes--when water flows from
the larger pipe into the smaller pipe, it's
evitable that you'll lose some. The same holds
true for light. Mixing OM1 with OM3 or OM4 will
cause high loss when transmitting from the 62.5µm
core into the 50µm core.
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Cable Testing 101 Considerations for Mixing
Multimode Fiber Types
Keep an Eye on the Color
Thankfully OM1 cable is orange in color, while
OM3 and OM4 are typically aqua. That makes it
easy to distinguish the two. But determining the
difference between aqua OM3 and aqua OM4 cabling
isn't as simple unless you can access and easily
read the legend on the cable. In most of Europe,
and starting to gain ground here in North
America, is the use of Erika violet for OM4
components. So if you see a "pretty in pink"
cabling plant, you'll know right away that you're
dealing with OM4.
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Cable Testing 101 Considerations for Mixing
Multimode Fiber Types
What About My TRCs?
If you're testing an OM4 cable plant, you don't
need to worry about the fact that the Test
Reference Cords (TRCs) that come with Fluke
Networks CertiFiber Pro are made with OM3 fiber.
The fiber used in these TRCs features tighter
core concentricity (i.e., common center) required
to maintain Encircled Flux  compliance, and you
can use them to test any 50µm multimode fiber
since we are measuring optical loss of the link
not modal bandwidth. But if you're testing legacy
OM1 62.5µm OM1 fiber, you'll need to make sure
you have a matching core size in your TRC. Not a
problem -- 62.5µm TRCs are available from Fluke
Networks as an optional accessory.
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