Patchcords in Fiber Optic Networks
Patchcords are used to cross-connect installed cables and
connect communications equipment to the cable plant. The choice of
patchcords is very important to the performance of the network. Choose
patchcords with these criteria:
- Fiber type must match the type of fiber in the cable plant. For multimode cable, use only 50/125 patchcords with 50/125 fibers in cables and 62.5/125 patchcords with 62.5/125
fibers in cables. Although there are several types of each size fibers,
matching the fiber type exactly is generally not required, e.g you can
use OM3 patchcords on OM2 or OM4 cable plants. Singlemode fiber is
mostly the same, but some long distance fibers have different mode
field diameters and should use patchcords with matching fibers. Some
patchcords are made from special bend-insensitive fibers.
Although there has been some controversy about the compatibility of BI
fibers to normal fibers, manufacturers seemed to have adapted their
designs to ensure compatibility.
- Choose patchcords with proper cable jacket color codes. This can help prevent mismatching fibers, especially multimode fibers.
- Choose patchcords with the correct matching connectors.
Choose connector types that match the connectors on patch panels and
equipment. Since many cable plants use one type of connector and the
equipment another, e.g. STs on a multimode
cable plant and LCs on the equipment, you may need to stock hybrid
patch cables with different connectors on each end to make the
connection. For singlemode networks, the connectors must also have the
proper finished ferrule ends, either PC or ultra PC polish for most
systems or angled PC (APC) connections for higher speed systems or
those carrying AM video. Do not mix PC and APC connectors as that may
damage them! The TIA 568 color code for connector bodies and/or boots is
Beige for multimode fiber except aqua for laser-optimized fiber, Blue
for singlemode fiber, and Green for APC (angled PC) connectors.
The performance of a fiber optic network (and copper
networks too) can be undermined the performance of the patchcords used
for interconnects and connecting communications equipment. But it's
easy to take patchcords for granted. They are inexpensive, often not
tested properly if at all, and sometimes treated badly.
Most
users buy patchcords in bulk and store them in boxes until they are
needed, when they are unpackaged and plugged in. Some users hang them
on the sides of the equipment racks. That's not how they should be
handled.
When
it comes to patchcords, there are three issues: quality, cleanliness
and condition. You might assume that the patchcords you buy are good
quality. They come in plastic bags usually with a label that identifies
the fiber and connector type, length and test results. We’ve seen cheap
imported patchcords with connectors with locking mechanisms that would
not connect because they were improperly made and connectors with
losses that tested at 1 dB although the label said 0.2 dB. Under closer
examination, the label appeared to be from a copy machine not a
printer, so I doubt the cable was even tested. While it's inconvenient
and expensive, testing every patchcord is highly recommended per the standard test, FOTP-171. If you do not test them and have network problems, then look at them first when troubleshooting the system.
Testing Patchcords
For testing the loss of a
patchcord, you only need an 850 nm LED light source for
multimode cable or 1310 laser for singlemode, a fiber optic power meter
and some reference patchcords. Just remember that the patchcords used
for references in testing must be good for tests to be valid, so you
test them as you would other patchcords, just more often.
Testing
patchcords is similar to testing any fiber optic cable. Use one
reference patchcord to set a 0 dB reference. Connect a patchcord to
test to the reference patchcord with a mating adapter. Connect the
power meter to the other end of the patchcord and measure the loss.
Since the length of the fiber is short, the loss contribution of the
fiber is ignorable. And since one end of the cable is attached to the
power meter, not another cable, you only measure the loss of the one
connection between the reference cable and the cable under test, so you
can test each connector individually.
To
complete the testing of the patchcord, reverse the cable you are
testing to check the connector on the other end. Sometimes you will
find one bad connector and can replace it to make the patchcord useful
again. But often the cost of replacing the connector may be higher than
replacing the patchcord itself.
If
your test equipment has different connectors than the patchcords you
are testing, you will need hybrid reference cables with connectors
compatible with the equipment on one end and the patchcord connectors
on the other end. You will also need the correct connector adapters for
your power meter.
Obviously,
all reference cables used for testing must have high quality connectors
to get reliable test results. Use this same method to test your
reference cables against each other and discard any with high losses,
usually those with losses over 0.5 dB.
Cleaning Patchcords
Cleanliness
is an issue with brand new cables as well as used ones. All connectors
should always have the polished ferrule covered by a “dust cap” to
protect the end of the connector ferrule from damage and dirt. However,
a user recently told me that after examining many connectors with a
microscope, he understood why they were called dust caps, they were all
full of dust. The dust cap should be used to protect the connector
ferrule, but do not assume it keeps it clean. You must clean connectors
before inserting them in mating adapters or active devices to ensure
they are really clean. Here are directions for cleaning.
Cleaning Removes Dirt That Causes Loss
Those
cables connecting equipment to the cable plant or hanging on the sides
of racks are often mistreated, leaving kinks in the cable or scratches
and scuffing on the connector ferrules. With the reference cables used
for testing, the continual mating to other connectors, even when
carefully cleaned before use, will cause scuffing that increases loss
over time.
What this means is patchcords need continuous inspection
and testing. It only takes a minute but can save you lots of
troubleshooting time. Start by checking the connector ferrules with a fiber optic inspection microscope, preferably around 100 power, then test according to the procedure above.
Handling Patchcords
Patchcords are often subject to poor treatment. They are
often hung off communications equipment or patch panels stressing the
fiber at the back of the connector. When they are too long, they are
bundled and hung in large piles on the side of equipment racks. Kinking
is always a problem. Ideally, patchcords should be the right length,
supported below the connection and carefully placed to prevent stress.
Unfortunately, that is often not the case and network performance may
suffer. Don't let your patchcords look like this:
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