On duplex LC fiber, what does polarity refer to?

• A. Ensuring TX on A maps to RX on B; flipping pairs can restore light on straight-through runs.
• B. A method to physically twist fiber to adjust bend radius.
• C. A measure of optical loss in dB per kilometer.
• D. A standard for data link layer addressing.

Answer: (A)

Polarity in duplex fiber is about which fiber in the pair carries transmit versus receive signals and how those paths map between the two ends. For a working link, the transmitter on one side must connect to the receiver on the other; otherwise, light won’t be properly received and the link won’t come up. If the mapping is wrong, you can flip the polarity by swapping the fiber pair so that TX from one side goes to RX on the other. That idea—ensuring TX on one end maps to RX on the other and using a polarity flip to restore proper light—directly captures what polarity is about.

The other options describe unrelated concepts: twisting fiber to change bend radius isn’t about signal direction, attenuation measures loss per kilometer rather than signal mapping, and data link layer addressing is a networking protocol concept, not a physical-layer polarity issue.