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Guidelines For Testing And Troubleshooting Fiber Optic Installations (2)

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4. Testing And Troubleshooting Communications Equipment

After the cable plant has been tested, the communications equipment should be properly connected using matching known-good patch cords. If the cable plant loss is within the loss budget of the equipment (including the loss of the patch cords), the communications link should work properly. If the link does not work, most likely potential problems are the following.
a. Improper connections
b. Cable plant problems
c. Malfunctions of communications equipment

4.1. Testing And Troubleshooting Steps For Communications Equipment
a. Improper connections. The system requires a transmitter be connected to a receiver, of course, so it is important to verify this connection for each link. Even if the cable plant is properly documented, fibers may have been crossed at intermediate connections, so using a visual tracer or visual fault locator will allow quick confirmation of the connection.

b. The functioning of the communications equipment:
i. If it is connected to the cable plant but not operating properly, begin by checking the power at the receiver on one end of the link.
ii. Disconnect the cable at the receiver input and measure power with an optical power meter. Make sure the equipment is trying to transmit a signal. Some equipment has a testing mode to force transmission of a test signal or the equipment may simply keep transmitting to try to complete a connection.
iii. If the receiver power is within specifications, the receiver or electronics beyond the link may be the problem. Use equipment diagnostics or consult the manufacturer for assistance.
iv. If the receiver power is too high, it may be overloading the receiver and an optical attenuate should be inserted at the receiver end to reduce the power to the proper level.
v. If the receiver power is lower than required by operating specifications, the cause is either low transmitter power or too much loss in the cable plant.
vi. To test transmitter power, disconnect the patch cord connecting the transmitter to the cable plant and measure the optical power. If the power is low, there is a problem with the transmitter or patch cord.
vii. To determine which is the problem, try testing the transmitter with a known good patch cord. If the power is then within spec, replace the bad patch cord and test the link again.
viii. If the transmitter power is low with a known good patch cord, the equipment may need maintenance (cleaning) of the output port or replacement.
ix. If the transmitter tests as good but receiver power is low, the problem is probably in the cable plant. First try to switch the communications link to spare fibers to see if that solves the problem. Next test the loss of the suspect fibers in the cable plant with an OLTS to determine if the cable plant loss is excessive.

c. Cable Plant Problems
i. High loss in the cable plant can be caused by damage after installation and testing. Use a visual tracer or visual fault locator to confirm continuity and an OLTS to test loss. See directions above on testing the loss of the cable plant.
ii. If the cable plant is long enough (>100m), it can be tested with an OTDR to pinpoint problems.
iii. If the cable plant loss is not the problem, there are other possible issues related to the bandwidth of the cable plant.
iv. Multimode cable plants operating at 1300 nm with LED sources may have bandwidth problems caused by the total dispersion due to both chromatic and modal dispersion.
v. Multimode cable plants operating at 850 nm with VCSEL sources on nonlaser-optimized fiber (usually 62.5/125 FDDI grade fiber) may suffer nonlinear modal dispersion that can produce distorted pulses that will cause data transfer problems.
vi. Multimode cable plants operating at 1300 nm with laser sources may have an improperly installed mode-conditioning patch cord (offset-launch) or none at all.
vii. Single mode links may suffer from problems caused by reflections at connectors or mechanical splices.
viii. Reflections in single mode terminations or splices near the source may cause nonlinearities in the laser transmitter which distort pulse shapes, causing high bit error rates (BER).
ix. Reflections near the receiver or at both ends can cause multiple reflections in the cable that create “optical noise” that causes BER.
x. Reflections can be tested, if the cable plant is long enough (>100m), with an OTDR to pinpoint problems.
xi. Reflections can be reduced by introducing an index-matching gel or fluid in the joint (Vaseline or mineral oil works, but is messy to clean up) to see if that solves the problem.
xii. Highly reflective connectors or splices should be replaced as soon as possible. Remember most single mode terminations are made by fusion splicing factory-terminated pigtails onto installed cabling.

5. Update Documentation
After completing tests, troubleshooting and repairs, update documentation to reflect the necessary procedures and any changes to the network. If the fix is to switch to spare fibers and suspect fibers are not fixed, not that on documentation to prevent future problems.

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