The A-B-C’s of Fiber Polarity

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The A-B-Cs of Fiber Polarity Author Mark
Mullins
www.flukenetworks.com 2006-2017 Fluke
Corporation
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The A-B-Cs of Fiber Polarity
Polarity defines direction of flow, such as the
direction of a magnetic field or an electrical
current. In fiber optics, it defines the
direction that light signals travels through an
optical fiber. To properly send data via light
signals, a fiber optic links transmit signal
(Tx) at one end of the cable must match the
corresponding receiver (Rx) at the other
end. While this seems obvious, polarity is one
area that seems to cause the most confusion among
technicians. So lets break it down and start at
the beginning
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The A-B-Cs of Fiber Polarity
Easy to Understand Duplex In duplex fiber
applications, such as 10 Gig, data transmission
is bidirectional over two fibers where each fiber
connects the transmitter on one end and to the
receiver on the other end. The role of polarity
is to make sure that this connection is
maintained. If you look at the graphic below,
you can easily see that the Tx (B) should always
connect to the Rx (A), regardless of how many
patch panel adapters or cable segments are in the
channel. If polarity is not maintained, such as
connecting a transmitter to a transmitter (B to
B), data will simply not flow. Obvious, right?
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The A-B-Cs of Fiber Polarity
To help the industry select and install the
right components to maintain proper polarity,
TIA-568-C standards recommends the A-B polarity
scenario for duplex patch cords. The A-B
duplex patch cord is a straight-through
connection that maintains the A-B polarity in a
duplex channel. Its also important to note that
every fiber connector has a key that prevents the
fiber from rotating when the connectors are being
mated and maintains the correct Tx and Rx
position.
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The A-B-Cs of Fiber Polarity
More Complex Multiples While duplex fiber
polarity may seem straight forward, it all
becomes a bit more complex when dealing with
multi-fiber MPO type cables and connectors.
Industry standards call out three different
polarity methods for MPOsMethod A, Method B and
Method C. And each method uses different types of
MPO cables. Method A uses Type A
straight-through MPO trunk cables with a key up
connector on one end and a key down connector on
the other end so that the fiber located in
Position 1 (Tx) arrives at Position 1 (Tx) at the
other end.
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The A-B-Cs of Fiber Polarity
When using Method A for duplex applications,
making the transceiver-receiver flip from
Position 1 (Tx) to Position 2 (Rx) is required in
a patch cord at one end. This is achieved with an
A-A patch cord that shifts the fiber in Position
1 to Position 2 at the equipment interface.
Method B uses key up connectors on both ends to
achieve the transceiver-receiver flip so that the
fiber located in Position 1 (Tx) arrives at
Position 12 (Rx) at the opposite end, the fiber
located in Position 2 (Rx) arrives at Position 11
(Tx) at the opposite end and so on. For duplex
applications, Method B uses straight A-B patch
cords on both ends since there is no need for the
transceiver-receiver flip. With the same type of
patch cord on both ends, concern about which type
of patch cord to use to which end is eliminated.
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The A-B-Cs of Fiber Polarity
Method C uses a key up connector on one end and
a key down on the other end like Method A, but
the flip happens within the cable itself where
each pair of fibers is flipped so that the fiber
in Position 1 (Tx) arrivers at Position 2 (Rx) at
the opposite end and the fiber in Position 2 (Rx)
arrives at Position 1 (Tx). While this method
works well for duplex applications, it does not
support parallel 8-fiber 40 and 100 Gig
applications where Positions 1, 2, 3 and 4 of the
MPO interface are transmitting and Positions 9,
10, 11 and 12 are receiving and is therefore not
recommended.
With three different polarity methods and the
need to use the correct type of patch cords for
each, deployment mistakes can be common.
Thankfully, Fluke Networks MultiFiber
Pro allows users to test individual patch cords,
permanent links and channels for correct polarity.
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