Why Fiber Polarity Matters ( Part 1)

Again, polarity is the term used in the TIA-568 standard to explain how to fiber (wire) to make sure each transmitter is connected to a receiver on the other end of a multi-fiber cable.

Many data center managers are opting to use pre-terminated fiber assemblies due to their higher-quality factory termination, ease of use and quick installation. And many are using 12-fiber MPO backbone cables with cassettes and patch cords to transition to active equipment. When doing this, they choose a polarity method that makes sense for their operation.

Polarity Method A: This is the most straight-forward method. It uses straight-through patch cords (A-to-B) on one end that connect through a cassette (LC-to-MPO or SC-to-MPO depending on what the equipment connector is), a straight-through MPO-key-up-to-MPO-key-down backbone cable and a “cross-over” patch cord (A-to-A) at the other end.

Polarity Method B: The “cross-over” occurs in the cassette. The keys on the MPO cable connectors are in an up position at both ends, but the fiber that is at connector position 1 in one end is in position 12 at the opposite end, and the fiber that is in position 12 at the originating end is in position 1 at the opposing end. Only one type of patch cord is needed – A-to-B.

Polarity Method C: This is the most complicated. There is pair-wise “cross-over” in the backbone cable in this method. A-to-B patch cords are used on both ends, the cassette uses MPO-key-up-to-key-down and the backbone cable is pair-wise flipped so 1,2 connects to 2,1; 3,4 connects to 4,3; etc.

There is a fourth propietary method that I won't go over here since it's proprietary and not standardized.

If the end user does not get this correct and use all of the proper pieces together, their systems will not work. If you don’t understand what I’ve just explained above, you're not alone. There are diagrams in the TIA-568 standard as well as many white papers from leading structured cabling companies explaining fiber polarity in arrayed cabling systems. Here’s a link to Panduit’s white paper that may help. In the next post, I’ll explain how to upgrade to 40/100G and reuse your pre-terminated backbone.