Type A/B LC Duplex and Type B Female-Female MPO Patch Cables Are Common
Two types of duplex fiber patch cords are defined in the TIA standard: A-to-A (cross-over) type and A-to-B (straight-through) type.
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.
The graph below shows 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 the polarity is not maintained, such as connecting a transmitter to a transmitter (B to B), data will simply not flow.
Source: Fluke Networks
To help the industry select and install the right components to maintain proper polarity, TIA-568-C standards recommend 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 maintain the correct Tx and Rx position.