Understanding Polarity in MTP/MPO Fiber Cabling Systems
Polarity defines direction of flow, such as the direction of a magnetic field or an electrical current. In fiber optics, polarity is directional; light signals travel through a fiber optic cable from one end to the other. A fiber optic link’s transmit signal (Tx) at end of the cable must match the corresponding receiver (Rx) at the other end.
Unlike traditional 10GbE transmission, which utilizes a 2-fiber configuration, the 40GbE and 100GbE are implemented over multi fiber array MPO connectors. It is critical that proper connector orientations are established. TIA-568 standard has approved 3 methods for configuring system polarity in multi-fiber arrays – Methods A, B and C.
Each MPO connector has a key on one side of the connector body. The “Key Up” position refers to the orientation where the key is located at the top position of the connector. When looking at the end face of the connector, position 1 is on the far left while position 12 is on the far right. For a 24 fiber MPO connector, the same orientation applies with the top row being positions 1 to 12 and the bottom row being positions 13 to 24. Depending on the adopted connectivity orientation, the MPO adapter needs to be suitable for its application, which is either “Key Up to Key Down” or “Key Up to Key Up”. In addition, MPO connectors are differentiated to a Male and Female connector. A Male MPO connector has two alignment pins while a Female MPO connector has two alignment holes where the pins are to be inserted when a termination is performed. An MPO connection can only be performed between a Male and Female connector to ensure proper alignment which is required to maintain a low loss connection.
Method A:
Patch Cord Polarity Flip
The Patch Cord Polarity Flip, also known as the Transmit-Receive Flip, has fiber 1 in position 1 in the MPO connector at both ends of the patch cord and is maintained throughout the network. This is achieved by a 180 degree twist in the fiber orientation within the patch cord. Although the MPO connector orientation is maintained throughout the network, two different types of duplex patch cords need to be installed, which are a straight patch and a cross patch, at the Transceiver and Receiver end. This method is the simplest design to install and maintain, as the orientation of the connector remains consistent. Although two types of duplex patch cords are required, this can be easily maintained by allocating one type of duplex patch cord to a fixed terminal to avoid a wrong installation.
Method B:
Adapter Polarity Flip
As stated in the name Adapter Polarity Flip, the polarity at one end of the connector is flipped. At one end of the connector fiber 1 is located at position 12 while fiber 12 is located at position 1. At the Transceiver and Receiver end only a straight duplex patch cord is required at both positions. This method requires the installation of the patch cord to be exactly in accordance to the required orientation. The connector orientation at the Transceiver and Receiver end are the same. In addition, although the connector orientation at both ends of the patch cord is different, the physical appearance of the connector is the same. The installer must have deep knowledge of the equipment and their positions to perform a correct connection.
Method C:
Pair Polarity Flip
This method is the most complicated method. At one of the patch cord, two adjacent fibers are flipped in the MPO connector. Fiber 1 in position 2 while fiber 2 in position 1. This repeats throughout the whole connector. At the Transceiver and Receiver end only a straight duplex patch cord is required at both positions. This method is not suitable for a 100GbE transmission where Position 1 and Position 12 are not allocated. Since Positions 1 and 2 are paired as well as Positions 11 and 12, these four ports will not be usable. This reduces the usable positions to 8, which is insufficient for a 100GbE transmission.
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