4 basic 'how-to' rules of fiber polarity

By HENRY FRANC, Belden -- Understanding fiber polarity and how to connect a system with absolute certainty is key to a successful installation. The tricky part, however, is that there is no “right” way to approach fiber polarity. Each manufacturer usually provides its own fiber polarity solution.

In this blog, we explain how to implement fiber polarity systems using pre-terminated fiber optic cabling – regardless of end-user requirements (simplex/duplex/parallel optics).

This is a challenge for many, including myself, who grew up in a world that didn’t pay enough attention to fiber polarity. If it didn’t work, we just unplugged the cord at one end and flipped transmit and receive. With the advent of MPO connectivity in Base-12 (12 fibers) and Base-8 (eight fibers), with simplex, duplex and parallel optics, that is no longer a viable or good way to address the issue.

What is Fiber Polarity?

Polarity in its most basic form is making sure that transmit talks to receive (a simple analogy: the mouth always talks to the ear). It sounds simple, but failure to fully understand polarity has often led to the “apathy” method of just “flipping” a pair of simplex connectors around as needed.

As fiber becomes the media of choice for high-speed data transport, the number of fibers (and connections) is increasing, making apathy toward understanding polarity detrimental to the success of system installation.

Today’s world is more complex; solutions (or methods in TIA standards terminology) are made up of varied components with similar labels (such as Type A, Type B and Type C) that can be interpreted in different ways. By way of example, Method B (or Belden’s modified Method B) is made up of various components that may be considered Type A, B or C. Types are components that are put together in a method.

The reason it’s so complicated has to do with how all of these components interact. There are issues of general fiber polarity (straight/flipped/crossed/other), gender (male/female), orientation (KeyUp/KeyUp, or KeyUp/KeyDown) and end-face alignment (straight or angled), as well as pinning (in the case of MPOs, which are identical except for how the alignment pins are set). All of these factors impact polarity. For now, let’s just concentrate on how to achieve the correct fiber polarity in a simple, repeatable, flexible way.

One Method for Achieving Fiber Polarity

In the method we share here, established by Belden (a modified Method B), there are four basic rules that will allow us to construct simple and complex channels using a series of common components and a unified strategy. They will address nearly all of the applications you’ll run across. If you have a situation that doesn’t fit the scenarios listed here, let us know and we’ll be glad to help.

Here are the basic rules for implementing a system with polarity, as written for Base-12, which I come across most often (with Base-8 listed as the exception):

1. All patch cords and trunk cables should be Type B.
   1. If the patch cord has an MPO connector, it must be female-female.
   2. All trunks are male-male.
   3. If you have to extend a trunk, use a male-female Type A trunk extender.
2. If you are converting MPO trunks into a duplex environment, you must use Type A cassettes/hydras at each end.
   1. To ensure that port mapping is correct, a Type A cassette shall be used at one end, and a Type A alternate (to map ports correctly) shall be used at the other.
   2. If you want to use a hydra to pre-wire the switch in a traditional harness-type orientation, use a Type A hydra assembly to the switch.
   3. When installing Type A and Type A alternate components, always leave the Type A highest in the network hierarchy (this will let you harness the switches as referenced above in 2b).
3. Make sure that, when using OS2 connectivity, any duplex assemblies align from an end-face geometry (i.e. straight or angled polished) or damage will occur, and your system will not work.
   1. This only applies to duplex connectors; all OS2 MPO connectors are APC, so there is no mismatch possible.
4. If Base-8, also sometimes referred to as SR4 (because this is the application it was originally written for), just use SR4 components at each end that requires it (instead of Type A).
   1. If desired, Base-8 trunks can also be used to connect Base-8 components instead of a Base-12 trunk.
   2. The use of Base-12 trunks (even for SR4-type applications) should be considered; this reduces operational risk to the facility and lowers the variance in the number of components.

That’s it – the four basic rules. No special tricks or strange/proprietary components required. You will never have to stock both straight (Type A) and flipped (Type B) duplex patch cords again. More importantly, you will be certain each and every time you plug something in that transmit will be talking to receive.

A Look at Fiber Polarity in Pictures

I often hear clients ask about MPO connectors that are gender changing, polarity reversing or orientation flipping, thinking that they’re a “magic bullet” and required. But nothing could be further from the truth. If you follow the four basic rules, you will never need them.

Furthermore, they pose a significant operational risk because you have to verify each of those issues at each end each time you are about to use them. Failure to do so will risk (at best) a network that doesn’t work, or (at worst) damage expensive infrastructure or equipment. Why risk it when you don’t need it?

Now that you know the rules, perhaps the easiest way to explain it is in pictures.

Take a look at the examples below. We’ve included some of the most basic types of simple links that can be added together as needed to provide any type of concatenated channel you may require.

By utilizing the principles and rules listed above, you can address the fiber-configuration requirements of the overwhelming majority of applications. As new applications emerge, such as Base-16 or multi-row MPO, we will provide you with content for absolute assurance that your systems will work.

If you’d like to learn more about fiber polarity, watch for future blogs on the topic – including a blog about what TIA has to say about polarity, as well as how the various types of components interact with each other. Subscribe so you don’t miss it!

With an emphasis on data center design, planning and building, Belden's Henry Franc acts as a trusted advisor for large or complex projects across all verticals, assessing clients’ business needs and finding the best technology options to meet them. He was also elected by industry colleagues to serve as vice-chair of the TR42 Telecommunications Cabling Systems Engineering Committee.