Topic: Fiber Optic Link Polarity Table of Contents: The FOA Reference Guide To Fiber Optics

Fiber Optic Link Polarity

fiber optic data 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.

installed fiber optic link polarity

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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