The A-B-C’s of Fiber Polarity
Polarity defines direction of flow, such as the direction of a magnetic field or an electrical current. In fiber optics, it defines the direction that light signals travels through an optical fiber.
To properly send data via light signals, a fiber optic link’s transmit signal (Tx) at one end of the cable must match the corresponding receiver (Rx) at the other end.
While this seems obvious, polarity is one area that seems to cause the most confusion among technicians. So let’s break it down and start at the beginning.
Easy to Understand Duplex
In duplex fiber applications, such as 10 Gig, data transmission is bidirectional over two fibers where each fiber connects the transmitter on one end and to the receiver on the other end. The role of polarity is to make sure that this connection is maintained.
If you look at the graphic below, you can easily see that the Tx (B) should always connect to the Rx (A), regardless of how many patch panel adapters or cable segments are in the channel. If polarity is not maintained, such as connecting a transmitter to a transmitter (B to B), data will simply not flow. Obvious, right?
To help the industry select and install the right components to maintain proper polarity, TIA-568-C standards recommends the A-B polarity scenario for duplex patch cords. The A-B duplex patch cord is a straight-through connection that maintains the A-B polarity in a duplex channel. It’s also important to note that every fiber connector has a key that prevents the fiber from rotating when the connectors are being mated and maintains the correct Tx and Rx position.
More Complex Multiples
While duplex fiber polarity may seem straight forward, it all becomes a bit more complex when dealing with multi-fiber MPO type cables and connectors. Industry standards call out three different polarity methods for MPOs—Method A, Method B and Method C. And each method uses different types of MPO cables.
Method A uses Type A straight-through MPO trunk cables with a key up connector on one end and a key down connector on the other end so that the fiber located in Position 1 (Tx) arrives at Position 1 (Tx) at the other end.
When using Method A for duplex applications, making the transceiver-receiver flip from Position 1 (Tx) to Position 2 (Rx) is required in a patch cord at one end. This is achieved with an A-A patch cord that shifts the fiber in Position 1 to Position 2 at the equipment interface.
Method B uses key up connectors on both ends to achieve the transceiver-receiver flip so that the fiber located in Position 1 (Tx) arrives at Position 12 (Rx) at the opposite end, the fiber located in Position 2 (Rx) arrives at Position 11 (Tx) at the opposite end and so on. For duplex applications, Method B uses straight A-B patch cords on both ends since there is no need for the transceiver-receiver flip. With the same type of patch cord on both ends, concern about which type of patch cord to use to which end is eliminated.
Method C uses a key up connector on one end and a key down on the other end like Method A, but the flip happens within the cable itself where each pair of fibers is flipped so that the fiber in Position 1 (Tx) arrivers at Position 2 (Rx) at the opposite end and the fiber in Position 2 (Rx) arrives at Position 1 (Tx). While this method works well for duplex applications, it does not support parallel 8-fiber 40 and 100 Gig applications where Positions 1, 2, 3 and 4 of the MPO interface are transmitting and Positions 9, 10, 11 and 12 are receiving and is therefore not recommended.
With three different polarity methods and the need to use the correct type of patch cords for each, deployment mistakes can be common. Thankfully, Fluke Networks’ MultiFiber™ Pro allows users to test individual patch cords, permanent links and channels for correct polarity.