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Understanding Fiber Polarity at the Component Level, Part 2

Henry Franc

In last week's blog, we reviewed the different component Types associated with fiber polarity – and what they do. In this blog, we’ll review how these components need to interact to provide a functional fiber system – as well as some operational considerations that need to be made while reviewing polarity Methods.

From a purely technical perspective, you may use any combination of components to provide the correct polarity. ANSI/TIA-568.3-D provides numerous examples of simple channel solutions; however, these are only guidelines, and they’re informative in nature. The options are essentially limitless when it comes to polarity. Furthermore, there are likely hundreds of different use cases. The standard describes how to build basic channels rather than listing potential configurations.


Even though, from a technical standpoint, there is a vast number of ways to create a channel, there are common themes you’ll want to adhere to in order to address other considerations, like repeatability, scalability, solution assurance and operational security. (Which is why we gave you a simple-to-follow set of rules in the first blog in this series.)


Let’s talk about components based on how they interact. Even though multiple components may be packaged together (cassettes, for example), they can be made up of multiple components.


Cable assemblies come in two basic forms:

  1. Trunk assemblies (that don’t plug into equipment)
  2. Cord assemblies (that can plug into equipment or connect to trunks)

Trunk or cord assemblies make use of simplex, duplex or MPO connectivity.

  • Simplex and duplex connectors are always male; couplers are always female. You can plug any simplex or duplex connector into a like connector without fear of damage.

  • Because MPO connectors can be either gender, and couplers are genderless, it’s much more important to plan correctly from a gender perspective. MPO equipment ports are always male; therefore, they need a female cord to plug into them. It’s recommended that you use female-female cords (so they can be used in either direction); you should use male-to-male trunks. But there’s always an exception to the rule: trunk extensions. Since a trunk should be male-to-male, if, for any reason, you have to extend a trunk assembly, you should use female-to-male. That way, the connectors will mate correctly, and the combined assembly will work and function as male-to-male.

Couplers are easier. There are two basic Types: KU/KU and KU/KD, as we covered in last week’s blog. We, however, recommend that you follow two different conventions for them, depending on the Type:

  • Simplex and duplex couplers are typically KU/KU (Type B) because of APC (angled polish connector) connections. With simplex or duplex connectors, the angled polish is side-to-side, so they mate correctly when the keys are up. If you use KU/KD and have to use APC connectors, they won’t mate properly and may cause damage. It’s better to avoid the possibility and standardize on KU/KU. Even from a polarity standpoint, if you use KU/KU simplex and duplex couplers, you will always have an odd number of flips in your channel – regardless of how many links you connect – so that polarity will always work.

  • MPO couplers are typically KU/KD (Type A). Because of the wide face of an MPO connector, the angled polish is in the vertical plane. You may not use a lot of MPO APC connectors – but, if you do, you cannot use KU/KU or they won’t mate properly and may cause damage. If you remember to always use KU/KD MPO couplers, you will never have to change when using APC connectors.

What about cassettes? What Type are they? From a standards perspective, they aren’t a Type at all. Instead, they’re a grouping of components that each has a Type. For example, a typical cassette will have a duplex coupler on one side (Type B), with a small cord assembly (of varying Type based on need) and an MPO coupler on the other side (typically Type A).


Essentially, for any traditional system of two fibers or more, you need to ensure that you have an easy way of mapping it to ensure that you have an odd number of “flips” in the longitudinal direction and an even number of “flips” in the vertical plane. That way, your transmit talks to receive on the same row. Lane-assigned systems are slightly different; you have an even number of lane assignments with vertical plane adjustments (if any) to ensure that alignment occurs.


Now that you understand what the different Types of components are and how they interact, there are only a few special notes to keep in mind for special cases:

  • Simplex: These are typically the exception rather than the rule. Remember, however, that most of systems are set up to support duplex (or parallel) operations. Your port numbering will be accurate, but, when patching using a simplex system at the head end, you will likely go fiber position 1 of port X, for example, on the other end. If a system is set up for duplexing, you’ll plug into position 2 of port X.

  • Port Inversion: When using parallel optics that utilize Type B components, they’re doing a longitudinal flip (1-12, 2-11 and so on). This means that you’re transposing the fibers as you should, but it also means you’re transposing the ports as well. If you don’t invert the ports (as in our alternate cassettes), you plug port 1 into port 6 (of a Base-12 solution).

  • APC Connectors: In addition to what we covered above, remember KU/KU for duplex and KU/KD for MPO connectors.

  • Multi-Row Optics: These need to do a longitudinal flip, but you need to keep the vertical orientation the same. Again, this is a very unique use case and currently outside the norm.

Simplex systems are the easiest to implement, followed by duplex and single row parallel. The most complex is multi-row parallel optics.


In our next blog – and the last in this fiber polarity series – we will explore what TIA has to say about polarity, and provide you with easy-to-use and easy-to-understand tools to address complex channel design and implementation. Subscribe so you don’t miss it!