Fiber Optic Link Polarity
Since
most fiber optic links use two fibers transmitting in
opposite
directions to create a full duplex link, you need to
ensure that
transmitters are connected to receivers and vice
versa. One of the most
common faults when a newly-installed fiber network
does not work is the
fibers are not crossed and transmitters are connected
to transmitters
and receivers to receivers.
Traditionally,
fiber links are made where pairs of fibers are crossed
between patch
panels so fiber 1 at one patch panel will be connected
to fiber 2 at
the patch panel on the other end, fibers 3/4. 5/6.
etc. are handled
likewise. Thus, when connecting patchcords, fiber 1
(or the odd
numbered fibers) can always go to the transmitter and
fiber 2 (or all
even numbered fibers) goes to a receiver and proper
connectivity is
maintained, allowing the use of straight through
duplex patch cords.
(Some
users number fibers by pairs, so the fibers in Pair 1
are 1A and 1B,
and all A's go to transmitters and all B's go to
receivers. It doesn't
matter as long as you are consistent.)
If the fibers are not crossed
in the permanent cable plant, one duplex patch cord in
the link needs
to be crossed or simplex patch cords can be used and
the proper
connections made manually. However, stocking different
duplex
patchcords and keeping it straight which is straight
or crossed or using simplex
patchcords is
harder to manage, and the likelihood for improper
connections is much
higher, making the crossed fibers in the permanent
cable plant easier
to manage.
This is described in TIA-568 as keying or polarity,
but
the standard, developed by consensus, offers
practically every option
imaginable. If you use the configuration shown above
for all your cable
plant (the permanent cables have fibers crossed as
described) and you
use straight duplex patchcords, you should not have
problems.
Obviously,
nothing substitutes from proper markings and
documentation of the cable
plant, a task that is often given little consideration
until the
network equipment installers are forced to use a
visual fiber tracer to
find which fibers to connect.
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