4 Basic Rules of Fiber Polarity

In this blog, we cover fiber polarity systems using pre-terminated fiber optic cabling, regardless of end-user requirements (i.e., simplex/duplex/parallel optics).

This can be challenging as polarity isn't something that has received much attention. If something didn’t work, we 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 (8 fibers), with simplex, duplex and parallel optics, that is not a viable or good way to address the issue.

What is Fiber Polarity?

Polarity is about making sure that transmit talks to receive (a simple analogy—the mouth always talks to the ear). It sounds simplistic however, failing 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 detrimental to successful system installs.

Today’s world is more complex—solutions (or methods in TIA standards terminology) are comprised of varied components with similar labels (e.g, 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 complex has to do with how these components collectively interact. There are issues of general fiber polarity (straight/flipped/crossed/other), gender (male/female), orientation (KeyUp/KeyUp or KeyUp/KeyDown), end-face alignment (straight vs angled) and pinning (for MPOs, identical other than how alignment pins are set). All of these factors impact polarity. For now, we concentrate on how to achieve the correct fiber polarity in a simple, repeatable and flexible manner.

One Method for Achieving Fiber Polarity

In one method established by Belden (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—and addresses nearly all applications. If you have a situation that doesn’t fit the scenarios listed here, Belden experts are here to help.

Basics for Implementing Systems with Polarity

This approach is written for Base-12 with Base-8 listed as the exception.

1. All patch cords & trunk cables should be Type B.
   • If the patch cord has an MPO connector, it must be female-female
   • All trunks are male-male
   • If extending a trunk, use a male-female Type A trunk extender
     
     
2. When converting MPO trunks into a duplex environment, use Type A cassettes/hydras at each end.
   • Ensure port mapping is correct by using Type A cassette at one end & Type A alternate (to map ports correctly) at the other
   • If using a hydra to pre-wire the switch in a traditional harness-type orientation, use a Type A hydra assembly to the switch
   • When installing Type A & Type A alternate components, always leave the Type A highest in the network hierarchy to harness switches (as noted above)
     
     
3. When using OS2 connectivity, ensure any duplex assemblies align from an end-face geometry (i.e., straight or angled polished) to avoid damage or system inoperability. 
   • This applies to duplex connectors—all OS2 MPO connectors are APC, eliminating potential for mismatching
     
     
4. If Base-8, (also referred to as SR4) use SR4 components at each end requiring it vs Type A.
   • If desired, Base-8 trunks can also be used to connect Base-8 components vs a Base-12 trunk
   • Using Base-12 trunks (even for SR4-type applications) should be considered to reduce operational risk and the variance in component number 

With these four rules, you'll avoid having to stock both straight (Type A) and flipped (Type B) duplex patch cords and can be certain each and every time you plug something in that transmits will be talking to receive.

A Look at Fiber Polarity in Pictures

We often hear clients ask about gender-changing MPO connectors, polarity reversing or orientation flipping—thinking they’re a “magic bullet” and required. Nothing could be further from the truth. With these four rules, you'll never have a need for them. Furthermore, they pose a significant operational risk as each of those issues at each end must be verified each time. Failure to do so will risk (at best) a network that doesn’t work and (at worst) damage costly infrastructure or equipment. Why risk it when you don’t need it?

Now that we've covered the basics, let's address the topic in pictures. In the following examples, we've included some of the most basic types of simple links that can be added together to provide any type of concatenated channel you may require.

Example 1 (Duplex Optics)

Example 2 (Duplex Optics with Hub Harness)

Example 3 (SR4 Link Aggregation)

Example 4 (Parallel Optics Example)

The principles and rules shared above accommodate fiber-configuration requirements for the majority of applications. As new applications emerge (e.g., Base-16, multi-row MPO), we will address requirements to ensure successful system installations. 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!