Gail Ow
Product Manager, Ixia Solutions Group
Blog

Do you find MPO Polarity exasperating?

September 20, 2019 by Gail Ow

Harold Crick thought life was Stranger than Fiction.

So I thought of Harold as I was writing this explanation about why MPO Polarity can be so confusing.

If you’ve seen the trailer, I’m sure you’re wondering about the connection between Harold Crick, MPO Polarity, and the voices in Harold’s head.

I mean, the trailer doesn’t mention fiber optic Christmas decorations, much less fiber optic cabling. But If you’ve been working with MPO cabling, you’ll know that trying to understand MPO polarity is enough to make your head spin.

Go ahead. Do an internet search on “MPO Polarity”.

What you’ll find are a lot of words about MPO connectors, Key up/Key Down, fiber positions, and the importance of the White Dot. You’ll find diagrams, some useful, and some less useful-- like the diagram of the unlabeled front view of a connector that doesn’t tell you if it’s a socket or a plug. Really?!

Most curious of all, what you won’t find is an explanation for why your cables usually work when the datasheet says to ‘Build Type C cables’, and the exasperating other times that they simply do.not.work.

Wait. What?

But what about all those published standards that are supposed to prevent this type of confusion?

ANSI/TIA-568.3-C was published in 2008 and is where you’ll find the definition of array adapters (aka MPO-12) containing ONE row of 12 fibers (40GBASE-SR4).

ANSI/TIA-568.3-D was published in 2016 and is where you’ll find the addition of array adapters (aka MPO-24) containing TWO rows of 12 fibers (100GBASE-SR10) for use with CFP transceivers.

So for a period of time between 2010 and 2016, manufacturers were forced to create their own polarity method definitions for array adapters containing TWO rows of 12 fibers (100GBASE-SR10). Unfortunately, not all manufacturers chose the same definitions as the IEEE 802.3.

Because the internet still has a lot of information out there about ANSI/TIA-568.3-C polarity methods, you need to pay close attention. If you do, you’ll notice that what used to be a Type C cable in ANSI/TIA-568.3-C, is now a Type A cable in ANSI/TIA-568.3-D.

I thought Ben Franklin summed this problem up quite elegantly in December 1791 when he said, “What happens on the internet stays on the internet. Forever.”

Wait. What?*

_________________________________________________

* Ben Franklin wasn’t around in 1791. For that matter, neither was Al Gore…ahem…to invent the internet. In any case, if MPO polarity has been making you hear voices in your head, rest assured. They can stop now. Just pay attention to the referenced standard, in addition to the Cable Type A, B or C-- before you start building MPO cables.