Polarity problems exist in both copper and fiber-optic networks. The feedback received over the years from many technicans servicing both types of networking is that fiber-optic networks experience polarity problems more often than copper networks. Polarity problems in a fiber optic network are not the result of a dirty connector, broken optical fiber, or a macrobend, they are the result of improper labeling and or human error.
A continutiy tester is the best tool to assist in polarity verfication because it emits visible eye-safe light. Depending on the complexity of the network, verifying the polarity of a channel may be time consuming. It is a good idea to bring a note book and record your observations as you work your way from one end of the chanel to the other.
ANSI/TIA-568-C defines two polarity schemes for duplex Fiber Optic Patch Cable: A-to-A and A-to-B. The most commonly deployed scheme is A-to-B, therefore this section focuses on that scheme. Because there are many different network configurations, we will only describe polarity verfication techniques for the configuration shown in Figure 17.28.
Each end of a duplex patch cord should identify position A and position B. This is typically accomplished with raised lettering on the latch that holds together the two connectors at each end of the patch cord. A raised letter latch holding together two SC connectors is shown in Figure 17.29. Because of the physical size on an SC connector, the raised lettering is easy to read. However, this is not the case with small form factor connectors such as the latched LC pair shown in Figure 17.30.
The first step in verifying the polarity the channels shown in Figure 17.28 is to de-energize the equipment at both ends of the network. Since latched duplex patch cords are used, unplug both connectors from both ends of the patch cords at each end of the network. Using the continuity tester, verify that connectors on each end of a patch cord are oriented so position A goes to position B. You can do so by inserting the ferrule of the A position connector into the continuity tester as shown in Figure 17.31.
With the connector inserted, energize the continuity tester and check to see if light is exiting the optical fiber in position B at the opposite end of the patch cord. If light is exiting the optical fiber in position B, the polarity is correct. If light is exiting the optical fiber in position A, the polarity is not correct. With the continuity tester still attached and energized, unlatch both connectors, swap locations, and relatch. Verify light is exiting from the optical fiber in position B. Repeat this test for the other patch cord and correct as necessary.
With both patch cords properly configured, the next step is verify the polarity of the horizontal cabling. To minimize access to other horizontal cabling, you should work from the equipment outlet to the patch panel. Remove the cover of the equipment outlet and plug both connectors at one end of the patch cord into the receptacles on the equipment outlet. Do not disturb the horizontal cabling connections.
Insert the ferrule of the A position connector at the end of the patch cord into the continuity tester as shown earlier in Figure 17.31. With the connector inserted, energize the continuity tester and check to see if light is exiting the horizontal cabling optical fiber in position B at the patch panel. If ligth is exiting the optical fiber in position B, the polarity is correct. If light is exiting the optical fiber in position A, the polarity is not correct. With the continuity tester still attached and energized, unlatch both horizontal cabling connectors at the back of the equipment outlet, swap locations, and relatch. Light should be exiting from the optical fiber in positon B. Reinstall the equipment outlet cover.
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