Industry Insights

Connecting The Right Fiber Polarity Dot

Wednesday, May 25, 2016 | by Rudy Montgelas


What Exactly Is Polarity?

Polarity is a term often used to describe a direction of flow.  For instance, a battery’s positive and negative polarity terminals define the direction of its electrical current.  A magnet’s north and south poles indicate the direction of its magnetic field.  Transmit and receive (Tx/Rx) polarity of a fiber optic link defines the direction of light travelling through the optical fiber.

For Simplex fiber links, like those for remote security cameras, only a transmitter is required for the one-way communication from the camera to the video receiver at the other end unless PZT* is required.

*Pan, Zoom Tilt (PZT) is a camera control signal sent from the receive end.

 Data center switches typically employ full Duplex fiber optic links that connect pairs of small form factor pluggable (SFP) LC optoelectronic transceivers sending and receiving digital light pulses through duplex patch cords and multi-fiber trunk cable permanent links. One fiber in the pair is transmitting a signal, and the other fiber is dedicated to receiving the signal for a true two-way communication highway.

Transmit/receive polarity is defined in the TIA 568 Commercial Building Telecommunications Cabling Standard as A and B duplex connector plug orientations with respect to the transceiver’s duplex receptacles.  The LC or SC connector latches are oriented outward from the paper in the diagram below.

Polarity Is A Combination Of Type + Method: 

The concept of polarity describes different Methods you can use to ensure that a fiber communication link will operate properly.   With each of the 3 Methods of polarity, come unique Types of components.

For instance, Patch cords can typically be made with fibers passing straight through, or crossed over to reverse polarity.  Remember to always understand the definition and how it applies to either a Type A to A or a Type A to B polarity cord.    For instance, here are the Legrand duplex LC SpaceSaver™ cords. 

Did You Know?

The TIA specification also defines the 12-fiber based solutions using MPO parallel optical connections, which means we can increase our speeds from 10G to 40G by adding lanes using a fiber array. This happens when we use space division multiplexing (SDM) to multiply the effective channel bandwidth from a single duplex 10G channel to a 40GBASE-SR4 (QSFP - quad SFP) MPO fiber array channel.  In the MPO connector, only the four outside fiber positions are used for both transmit and receive.  With the increase in channel bandwidth comes an increase the connection complexity.  It’s further complicated with cross connects, cassettes & harnesses added to the link.      


Modules And Cross Connects:

Duplexing cassettes and harnesses, such as Legrand’s HDCA series cassettes, provide a transition from the 12-fiber MPO trunk connectors at the rear to LC and SC duplex connections at the front panels.  A cassette is a plastic or metal module that provides both front patching duplex adapter panels with an internal fiber MPO to LC breakout harness for converting and routing the connections. 


A Harness can be used In Lieu Of a Cassette with the option for a HILOC strain relief bullet.  A harness provides the advantage over a cassette of eliminating the insertion loss at the front panel duplex patch cord connection.

Transition cassettes and harnesses provide connections for a 100G core switch to a 10 or 40G edge switch.  The correct fiber polarity routing and MPO connector gender assures proper connection with the installed trunk cables.

Duplexing cassettes and adapter panels, as defined by the TIA, fall under two Types: Type A and Type B.  For polarity Methods A and B, the Types of cassettes and the Types of trunk cables should match.  
Note that For Method C polarity, the Type C trunk array cables should be connected to Type A cassettes
Migration from 10G to 40G or 100G requires removing transition cassettes and harnesses from the link and replacing them with the proper Type array patch cords and MPO adapter panels.

These Are The Methods To All Of This:

The objective of the three different Methods, proposed by the TIA, for managing and controlling the polarity in multi-fiber array links, is to cause a polarity flip (Tx to Rx fiber swap) to occur at some point in the link, for every fiber in the channel, so that a transmitter ends up being connected to the correct receiver at the other end.

Polarity Type A array trunks and interconnect cables map all of the MPO connector fibers the same at both ends of the link.  In order to flip the polarity, an A to B patch cord must be connected to one end and an A to A patch cord connected to the other end.  The MPO array cord connector keys are pointing down at one end of the cable and up at the other end.  Type A MPO adapters all have a key up on one side and the mating connector key down on the other side.

Note: MPO/MTP* connectors utilize a pin and socket feature to ensure proper alignment of the connectors when mating two connectors. Without the pin and socket alignment, it would be difficult to get the optical precision required and high loss would occur.  MPO connectors have an extrusion, called a “key”, on one side of the MPO/MTP connector to ensure proper polarity when connectors are mated. When discussing polarity, you will hear the terms “key up” and “key down”; these terms describe the orientation of the connector. “Key up” means that the extrusion on the connector is facing up. “Key down” means that the extrusion on the connector is facing down.

* MTP is a registered trademark of USCONEC. 

Type B Array Cords - All Keyed Up: 

Polarity Type B trunks require an array cord that is configured the same as Type A, except that at both ends the MPO connectors are oriented with keys facing up.  In addition, all MPO adapters in the Method B link are Type B key Up to Key Up adapters.

Polarity Type C array cords use reverse-pair positioning, through crossovers in the array cord, to swap the polarity of pairs of fibers. 

With this type of array cord, adjacent pairs of fibers are crossed from one end to the other.  The fiber at position 1 at one end of the cable is shifted to position 2 at the other end of the cable.  The fiber at position 2 is shifted to position 1 etc..  By crossing over each fiber pair in the array cable, the Tx/Rx polarity is swapped.  Note that for Method C polarity, Type A cassettes should be used with Type C array cords.

In Summary, Here Are The Types Of Components To Use With Each Method: 

What To Consider Next?

We have explored some of the considerations for assuring the proper polarity for 10G duplex 12-fiber based solutions for 40G parallel optical links.  Polarity can become even more complicated when using 24-fiber MPO connectors for 100GBASE switch applications.

Stay tuned for further CTDs to discuss 40G 8-fiber MPO connector array solutions to eliminate the four unused fibers that 12-fiber based solutions require.