Understanding Fiber Polarity Method: Which to Choose?

40G and 100G are now universally deployed in data centers. As the preferred array-based fiber connector option, the MPO/MTP connector and its cable assemblies are widely used for 40/100G connectivity in high-density data center environments. However, in complex high density cabling, the advantages of MPO/MTP cabling will be lost if you don’t have a proper polarity method. Thus, the TIA 568 standard provides three methods—Method A, B and C, for configuring systems to ensure that proper connections are made. In this blog, these three methods would be described in details which may guide you to select the best method for ensuring polarity across your array-based fiber installation.

Understanding MPO/MTP Basic Structure

Before looking at each method in detail, it is necessary to understand the basic structure of an MPO/MTP connector. As the following picture shows, an MPO/MTP connector contains several parts such as boot, coupling/housing assembly, ferrule, guide pins, and so on. When the MPO/MTP connector is designed with pins, it is called male connector. On the contrary, it is called female connector.

MPO MTP structure

In addition, there is a “key” on one side of the connector body. When the key sits on top, we call that it is the key-up position. In this orientation, each of the fiber holes in the connector is numbered in sequence from left to right. We will refer to these connector holes as positions, or P1, P2, etc. Generally, there is a marker called “white dot” on the side of the connector body that is used to designate the position 1 side of the connector when it is plugged in.

Polarity Method Introduction

The TIA-568-C.0 standard illustrated three array system connectivity methods—Method A, Method B and Method C. This section will introduce them respectively.

Method A
As shown in the picture below, two Method A cassettes with key-up to key-down adapters, a straight-through key-up to key-down MPO trunk cable as well as two patch cables are required in Method A connectivity. The straight-through key-up to key-down MPO trunk cable means that the fiber 1 located in P1 of the connector on the left will arrive at P1 at the other connector. What’s more, it should be noted that the transmit‐receive flip must happen in the patch cables for Method A. In other words, an “A-to-A” patch cable at one end of the connection while an “A-to-B” patch cable at the other end.

Method A

Method B
In Method B, as shown in the following picture, Method B cassettes which employs key-up to key-up adapters are required to link straight-through key-up to key-up MPO trunk cable. With the key up on both ends, the key-up to key-up trunk cable has a different fiber array with Method A type cable. In this type of trunk cable, fiber 1 (Tx) is mated with fiber 12 (Rx), fiber 2 (Rx) is mated with fiber 11 (Tx), and so on. Two straight “A-to-B” patch cables are required at the beginning and end of the link, namely patch cables do not need to be flipped in Method B.

Method B

Method C
Method C uses the same cassettes as Method A, but to link a special key-up to key-down trunk cable. For Method C, each adjacent pair of fibers at one end are flipped at the other end. Notice the swapping of the color positions in the picture below. Fiber channel is completed by utilizing straight “A-to-B” patch cables at the beginning and end of the link. Method C is similar with Method A. The only difference between this method and Method A is that the pair-wise flip occurs in the array cable itself rather than at the patch cables, so that odd-numbered Tx fibers leaving the near-end cassette are in even-numbered Rx positions when they arrive at the remote cassette, e.g. fiber 1 (Tx) is mated with fiber 2 (Rx).

Method C

Which to Choose?

The above section shows us the details of these three methods. The following table summarizes the advantages and disadvantages of them which may guide you to choose a proper one for your network. But, it is very important to know that the method choice should be maintained consistently throughout the installation. Do not mix them throughout the installations.

Method Pros Cons
A One cassette type, easy to produce and purchase Requires pre-configured “A-to-A” patch cables, or field configuration of same
Compatible with many legacy systems
Multiple sources for components
Industry standard
Single-mode and multimode
Standard provides migration path to parallel optics
Ribbon cables can be linked (need male/female connector)
B Single source for components Remote cassette must be flipped and re-labeled
“A-to-B” patch cable only Identification and maintenance of cassettes are different on each end
Industry standard Multimode only
Standard provides migration path to parallel optics Not compatible with legacy systems
Ribbon cables can only be liked using less available (Key Up to Key Up) adapters (need male/female cable)
Fewest vendors
C One cassette type, easy to produce and purchase Less reliable than Method A
Singlemode and multimode Specialized ribbon cable assembly
Industry standard Does not support parallel optics
“A-to-B” patch cable only Not compatible with legacy systems
Less vendor support than Method A
Difficult to extend link

This post introduced three array system connectivity methods and listed their pros and cons that may guide you for polarity selection. In a word, the Method A is polarity flip in A‐to‐A patch cord. The Method B is polarity flip in cassette. And the Method C is flip by pairs. When choosing one of them for your network, the most critical consideration is to select one method and stick with it.

Polarity and MPO Technology in 40/100GbE Transmission (FS.COM)
ANSI/TIA-568-C.0 Standard
Best Practices for Ensuring Polarity of Array-Based Fiber Optic Channels (PANDUIT)

Understanding Fiber Polarity Method: Which to Choose?

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