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Understanding Fiber Polarity at the Component Level (Part II)

Henry Franc

In Part I, we reviewed the different component types associated with fiber polarity and what they do. In this blog, we review how these components interact to provide a functional fiber system as well as operational considerations regarding polarity methods.


From a  technical perspective, any combination of components can be used for the correct polarity. ANSI/TIA-568.3-D provides numerous examples of simple channel solutions however, these are only guidelines. Essentially, options are 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.


From a technical standpoint, there are myriad ways to create a channel. It's important to recognize common themes to effectively address other considerations such as repeatability, scalability, solution assurance and operational security. (Refer to the simple-to-follow set of rules in the first blog in this series.)


Let’s review components based on how they interact. Why? Although multiple components may be packaged together (e.g., cassettes) they can be made up of multiple different components.


Cable assemblies come in two basic forms.

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

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

  • Simplex & 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.

  • Given MPO connectors can be either gender—& couplers have no gender—it’s important to plan correctly.
  • MPO equipment ports are always male & therefore require a female cord to plug. Female-female cords are recommended as they can be used in either direction. Male-to-male trunks are also recommended, however there’s always an exception to the rule—in this case, that's trunk extensions. A trunk should be male-to-male. If for any reason, you have to extend a trunk assembly, be sure to use female-to-male to ensure the connectors mate correctly & the combined assembly will operate as male-to-male.

Couplers are easier.

  • There are two basic type—KU/KU & KU/KD We recommend users follow two different conventions depending on type: 
  • Simplex & 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. NOTE: If using KU/KD with APC connectors, they won’t mate properly & may cause damage.  Avoid this by standardizing on KU/KU. Even from a polarity standpoint, using KU/KU simplex & duplex couplers will always mean an odd number of flips in your channel regardless of how many links you connect—meaning polarity will always work.

  • MPO couplers are typically KU/KD (Type A). Given the wide face of an MPO connector, the angled polish is in the vertical plane. For those who many not use many MPO APC connectors, it's important to note KU/KU won’t mate properly & may cause damage. Always using KU/KD MPO couplers means you'll never need to change when using APC connectors.

What about cassettes?

  • From a standards perspective, cassettes aren’t a type but a grouping of components with each having a type (e.g., a typical cassette will have a duplex coupler on one side (Type B) with a small cord assembly (of varying Type based on need) & an MPO coupler on the other side (typically Type A).


For any traditional system of two more more fibers, ensure you have an easy way of mapping to confirm there is an odd number of “flips” in the longitudinal direction and an even number of “flips” in the vertical plane. This guarantees your transmit talks receive on the same row. Lane-assigned systems are slightly different given there is an even number of lane assignments with vertical plane adjustments (if any) to ensure that alignment occurs.


Special Cases

  • SimplexTypically the exception rather than the rule however, most systems are set up to support duplex (or parallel) operations. The port numbering will be accurate however, 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 InversionWhen using parallel optics with Type B components, they’re doing a longitudinal flip (1-12, 2-11, 3-10 etc.) meaning fibers are being transposed correctly as well as the ports. If the ports are not inverted (as in alternate cassettes), port 1 will plug into port 6 (of a Base-12 solution).

  • APC ConnectorsRemember KU/KU for duplex & KU/KD for MPO connectors.
  • Multi-Row OpticsThis is a unique use case requiring a longitudinal flip while keeping the vertical orientation the same.

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