How to identify the A and B ends of an optical cable

The A end of an optical cable is the transmitter (Tx) side, and the B end is the receiver (Rx) side, typically identified using A-B duplex patch cords, color coding, and connector orientation.

Understanding A-

B Polarity

In fiber optic networks, polarity ensures that the transmit (Tx) signal on one end aligns with the receive (Rx) signal on the other end. A standard A-B duplex patch cord provides a straight-through connection where the A end (Tx) connects to the B end (Rx) on the opposite side, enabling bidirectional data flow without errors (Polarity Basics - Optical Fiber Networks).

Identifying the A and B Ends

Duplex Patch Cords: A-B cords are designed so that when facing the connector in the "key-up" position, A is on the right and B is on the left. The A end is typically connected to the transmitter port, and the B end to the receiver port (Fiber Optic Polarity 101: A-B Polarity). Color Coding: Many patch cords use color codes to indicate Tx and Rx.

For example, blue for Tx (A) and white for Rx (B). Following the TIA-598-C standard ensures consistent identification across multiple segments (Polarity Basics - Optical Fiber Networks). Connector Orientation: Duplex connectors (LC, SC) have a key that prevents rotation.

Maintaining the key orientation ensures the correct Tx/Rx alignment. For MPO/MTP multi-fiber cables, Type A, B, or C methods define fiber mapping and key orientation to maintain polarity across high-density connections (MPO Polarity Explained: Type A, B, and C).

Practical Tips

Check the patch cord type: A-B cords are standard; A-A cords are less common and used only in special setups. Follow the key-up/key-down orientation for MPO/MTP connectors to ensure proper fiber mapping. Use labeling or numbering on patch panels to track A and B ends, especially in multi-segment or high-density installations.

Test connectivity with a fiber tester to confirm that Tx and Rx are correctly aligned before final deployment. By following these methods—using A-B patch cords, observing color codes, and maintaining connector orientation—you can reliably determine the A and B ends of an optical cable and ensure proper network operation.