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How to determine the A and B ends when laying optical cables

ODN and passive infrastructure knowledge brief Published 16 Apr 2023

Prepared for preliminary research by the GESTIONVAL ODN knowledge desk. Confirm network topology, split ratio, box or cabinet capacity, joint closure rating and loss budget against current standards and product documentation.

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
  1. 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) .
  2. 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) .
  3. 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.

How to determine the A and B ends when laying optical cables - GESTIONVAL ODN INFRASTRUCTURE S.L. [PDF]

How to determine the A and B ends when laying optical cables

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Technical note

This reference is intended for preliminary ODN and passive infrastructure research. Topology, split ratio, box or cabinet capacity, closure rating, cable type, test limits and applicable standards must be verified for the specific project.

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