Understanding Polarity in MPO Systems

MPO/MTP technology is one of the leading contributors in the migration to 10/100GbE. The technology is considered to be high density, flexible, and reliable with properties that are scalable and upgradeable. However, even though they have their advantages, networkers are faced with the task of ensuring the connections are made in a manner that ensures there is proper polarity on both ends of the system. For this to be done, it is important to understand the polarity of the MPO systems that will be explained in the article.

MPO Connector

The first thing you should learn about is the MPO connector. The MPO connector has a different number of fibers. It can have 12 fibers, 24 fibers, 36 fibers and even 72 fibers. On one of the flat sides added to the connector, there is a key. Key down is used to describe when the key sits at the bottom. Key up is when the key is on top. The fiber holes in the connector are marked from left to right, and there is a white dot on one side of the connector that indicates where the first fiber is. The numbers are referred to as P1, P2 and so on or fiber position. The polarity of the cable is determined by the key orientation.

Three polarization methods by three cables

TIA defines the three methods for proper polarity as method A, B, and C. there are different cables used for each method. These cables, A, B, and C are used for the three different connectivity methods. Here is a description of the different cables before moving on to the connectivity methods.

  • MPO Trunk Cable Type A - The cable is also known as straight cable. It is a straight cable that has a key up MPO connector located on one end, and the key down connector is located on the other end. Due to this, the fibers on each end have the same fiber position.
  • MPO Trunk Cable Type B - The cable can also be identified as reversed cable. It has key up connectors on both sided of the cable. This arrangement results in an inversion, and the fiber positions are reversed at each end of the cable. The fiber at P1 is mated with the P12 at the opposite end.
  • MPO Trunk Cable Type C - Cable type C is also referred to as Pairs Flipped Cable. The reason is that the adjacent pair of fibers on one end of the cable is completely flipped on the other end. The P1 fiber is moved to P2 on the other end of the cable.

The three connectivity methods

Different polarity methods will use different types of Trunk Cables. However, for every method, a duplex patch cable must be used for a fiber circuit to be achieved through the connection. There are two types of duplex patch cables that the TIA standard defines. The duplex patch cables must be terminated with LC or SC connectors for there to be a duplex fiber connection that is complete. Here is an explanation of each of the three connectivity methods that can be used to achieve proper polarity.

  • Method A - In this method of connectivity, an MPO module is connected on each side of the link using a Type A cable. To correct the polarity, two types of patch cords are used where on the right, a duplex A- to- A patch cable is used and on the left, the patch cable is a standard duplex A-to- B type.
  • Method B - In this method of connectivity, the two modules are connected by a Type B cable on each side of the link. Since the fiber positions are reversed at the ends, standard A-to- B duplex patch cables are used on both sides.
  • Method C - In method C, the Type C cable is used to connect the modules located on each side of the link. The patch cords used in method C are standard duplex A-to-B duplex patch cables.


The use of MPO/MPT components is meant to satisfy the growing need for high transmission speed. Polarity is a major problem that can be easily solved once the right types of MPO cables, connectors, MPO cassette and patch cables are used. The three polarization methods in placed can be used in different situations where they can be satisfactory.