Fiber Optics Troubleshooting
By Anthony Sequeira
Fiber optics provides an advantage over copper cabling for Ethernet in that fiber
optic cables can go farther and have higher capacity for throughput. They are not
subject to radio frequency or electromagnetic interference because they don't use
copper for cabling but instead use light signals for transmitting and receiving data.
Most of the problems presented can be avoided or resolved by simply following the standards.
Attenuation/Decibel Loss - If a cable is longer than the specification or
technology for which it is being used, the natural attenuation decibel loss of
the signal can cause the network to have faults or be completely unusable. The
correction is to apply repeaters or other methods of regenerating signals so that the
signal transmission is performed within the specification of the technology being used.
SFP (Small Form-Factor Pluggable)/GBIC (Gigabit Interface Converter) -
when fiber optics are used, switches can have a generic port that supports a
module such as an SFP or GBIC module or adapter. The adapter chosen should
be the correct one for the type of fiber termination as well as the type of
fiber cable that is in use. If the incorrect SFP module for the incorrect
cable type or termination is used, it could result in a failure. The resolution
is to verify the type of module required, as well as the type of fiber cable
that should be in place - for example, multimode versus single mode. Also within
these categories are additional specifications that should be checked and verified.
Bad SFP/GBIC - The SFP or GBIC module acts as a transmitter and receiver that the
switch uses to communicate over the fiber or Ethernet cable. If this adapter is the
incorrect type, it would cause a failure for the fiber connection. Using the vendor's
show commands can reveal additional details regarding the module, including any
failures that the module might be experiencing.
Wavelength Mismatch - The SFP, connectors, and cabling all must support the type
of frequencies used over a fiber cable. If there is a mismatch among the module type,
the cabling, and the termination being used, that could cause a failure. You should
verify the vendor's specifications and then confirm that the correct cable type and
module are being used.
Fiber Type Mismatch - There are several types of fiber cabling, including
multimode and single mode, and many variants within each of those categories. Confirm
the use of the correct type of fiber cabling by comparing the standards along with
the vendor's requirements to make sure that the correct cabling is being used, as well
as ensuring compatibility with the SFP, cable termination, and port.
Dirty Connectors - A dirty connector at the termination of the fiber-optic cabling
could prevent the signals from being received or sent successfully over the fiber-optic
cable. The fiber-optic cable itself might be the problem. Verify this by using a
fiber-optic cable tester and have the termination cleaned if found to be dirty. The
SFP adapter could be swapped out as part of a troubleshooting process and then the
fiber cable could be reconnected to correct the problem.
Connector Mismatch - SFP modules are designed for specific fiber-optic termination,
as well as specific frequency types to be used over the fiber cabling. Certain
types of cabling are designed for different types of connectors. If not all of these
items are compatible, it could cause a failure of the fiber optics. The resolution is to
check all the specifications, and correct any components that are incompatible with
the type of cabling or the technology used.
Bend Radius Limitations - fiber-optic cabling can be bent, to the extent that
is planned for and documented as part of the specification (or recommendation) for
that cabling. If the cable is bent in excess of the bend radius standards for fiber-optic
cabling, the optical signal might not perform as well. There is also potential
for permanent damage of the fiber due to causing miniature cracks in the cable. To
help mitigate the potential of cables being harmed, cable management including the
use of cable trays should be used. They can help keep the cables organized and
protected against excessive bending.
Distance Limitations - Going beyond the distance of a standard, whether it be
wireless, copper-based wire, or fiber-optic cabling, puts the network at risk. failures
are more likely to occur. Cable testers, including those specific for fiber optics,
can be used to verify cabling plants including the distance of a fiber-optic cable.
If a cable is found to extend farther than is supported by the technology, it should
be modified or replaced within specs.
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