Choosing the Proper Polarity Method for MTP System

Whether in local area network (LAN) campus or data center backbones, we are in the process of migrating to higher-density cabling in order to meet system bandwidth needs and provide the highest broadband network connectivity density. Many network designers are turning to MTP trunk cable for today’s duplex fiber transmission and to provide an easy migration path for future data rates that will use parallel optics such as 40/100G Ethernet. To ensure reliable MTP system performance as well as support ease of installation, maintenance and reconfiguration, choosing the proper polarity method is very important. In this post, we are going to introduce three MTP polarity method for your reference.

What Is Polarity?

Polarity is the term used in the TIA-568 standards to explain how fiber (wire) to make sure each transmitter is connected to a receiver on the other end of a multi-fiber cable. To be specific, as we all know, optical fiber links typically require two fibers to make a complete circuit. Optical transceivers have a transmit side and receive side, and typically deploy a duplex fiber connector as the interface. In any installation, it is important to ensure that the optical transmitter at one end is connected to the optical receiver at the other. This matching of the transmit signal (Tx) to the receive equipment (Rx) at both ends of the fiber optic link is referred to as polarity.

Structure of MTP Multi-fiber Connector

To better understand each polarity method, it is important to make it clear for the MTP connector structure.

Each MTP connector has a key on one side of the connector body. When the key sits on top, this is referred to as the key up position, on the contrary, when the key sits on bottom, we call it key down position. Each of the fiber holes in the connector is numbered in sequence from left to right, and we call these fiber holes as positions, or P1, P2, etc. Besides, there is a white dot as shown below on the connector body to designate the position 1 side of the connector when it is plugged in. Generally, MTP multi-fiber connector is pin and socket connector—requiring a male side and a female side (male side has pins, while female side has no pins) as shown below. Cassette and hydra cable assemblies are typically manufactured with a male connector, while trunk cable assemblies typically support a female connector.

Three Polarity Methods for MTP System

Defined by TIA/EIA-568-B.1-7, there are three polarity methods for MTP system—method A, method B and method C. These methods define installation and polarity management practices, and provide guidance in the deployment of these types of MTP fiber links. Once a method is chosen, these practices must be put into place to insure proper signaling throughout the installation.

Method A: In method, it requires two type A cassettes with key-up to key-down adapters, a straight-through key-up to key-down MTP trunk cables as well as two patch cables. This method, shown below maintains registration of Fiber 1 throughout the optical circuit. Fiber 1 in the near end cassette mates to Fiber 1 in the trunk cable assembly, which mates to Fiber 1 in the remote cassette. The fiber circuit is completed by utilizing one “A-to-A” patch cord at the beginning and “A-to-B” patch cord to insure proper transceiver orientation.

Pros: It provides the simplest deployment, works for single-mode and multimode channels, and easily supports network extensions.

Cons: Requires pre-configured “A-to-A” patch cables, or field configuration of same.

Method B: In type B polarity method, method B cassette requires key-up to key-up adapters to link reversed cable or MTP trunk cable type B. The fiber circuit is completed by utilizing straight “A-to-B” patch cords at the beginning and end of the link, and all of the array connectors are mated key-up to key-up. This type of array mating results in an inversion, meaning that Fiber one is mated with Fiber twelve, Fiber two is mated with Fiber eleven, etc. To ensure proper transceiver operation with this configuration, one of the cassette needs to be physically inverted internally, so Fiber twelve is mated with Fiber one at the end of the link.

Pros: It requires single source for components and “A-to-B” patch cords only. Besides, it is a standard which provides migration path to parallel optics.

Cons: This key-up to key-up method requires a more in-depth planning stage in order to properly manage the polarity of the links, and to identify where the actual inversions need to occur. Moreover, it only support multimode fiber.

Type C: The method C shown below, with the key-up to key-down adapter in the cassette, looks like the type A method. However, the difference between this method and method A is that the flip does not happen in the end patch cords, but in the array cable itself. In this case, the fiber at position 1 on one end of the cable is shifted to position 2 at the other end of the cable. The fiber at position 2 at one end if shifted to position 1 at the opposite end, etc.

Pros: This method requires one cassette type, easy to produce and purchase, and it can support both single-mode and multimode fiber.

Cons: An additional drawback to this method is that it does not support parallel optics and is less reliable than method A.

Conclusion

We have discussed three polarity methods for MTP system, and indicate the pros and cons of each one. For choosing the proper method for MTP system, you should weight both advantages and disadvantages.