Fiber Polarity Rules

Reel Time, February 2008 - Cabling Business Magazine

Question: In the design of our data center, it was an easy decision to choose a modular pre-terminated fiber system for our fiber plant and backbone cables. However, selecting a connectivity method and all the corresponding components is very confusing.

Answer: Good choice with selecting fiber cassettes, which are the heart of a modular fiber system and perfect for the data center environment. One of the main concerns in today’s data centers is space efficiency and especially as it relates to airflow, which is critical for the equipment. Therefore, selecting smaller diameter cables and high-density, pre-terminated fiber cassettes is preferred when designing your data center’s cabling infrastructure.

As you have probably already realized, modular pre-terminated fiber optic cabling systems increase optical port density, reduce congestion in cable pathways and spaces, save on installation time and offer superior optical performance and reliability. They are also supposed to offer system interoperability, but that’s where it is confusing – the connectivity method. How do you choose?

The key issue is maintaining polarity. The term “polarity” conjures up meaning of having two oppositely charged “poles,” one positive and one negative. In fiber optics, which is a duplex (two-way) or bidirectional transmission method using separate fibers in each direction, the cabling system is based on the same polarity principle as in electricity. The cabling system must provide proper signal “polarity” which means that the transmitter on one end of the channel will connect to the receiver on the other end. Without correct transmit-to-receive polarity from point to point, the channel simply won’t work.

Prior to 2002, there were proprietary connectivity methods to achieve proper polarity using a myriad of connectors, cables and patch cords. Basically, you had to select one manufacturer and stick with it. So, TIA stepped in to put some rhyme and reason” to the selection, so that end users and installers, like yourself, can follow these termination guidelines to properly select the method and interoperable components that works best for each scenario.

Specifically, the TIA/EIA-568-B.1-7-2006 (Commercial Building Telecommunications Cabling Standard) addresses the polarity issues associated with multi-terminated fibers (also known as “array”) connections. In doing so, they suggest three connectivity scenarios, or “methods” with the same goals – to maintain polarity. The confusion stems from the components within each method – three different cable types (A, B, & C) and two different patch cords (A-to-A and A-to-B) and connector and cassette keying. Let’s take a closer look at these components to figure out how to build a fiber link in each of the three methods.


Read the complete article, Fiber Polarity Rules: Reel Time Feb 2008 - Cabling Business Magazine, to learn more about this topic.