Understanding MPO/MTP polarity

MPO/MTP technology, which is high density, flexible, and reliable and has scalable and upgradable properties, is one of the main sources of migration to 40/100GbE. However, network designers face another challenge in determining how to ensure the proper polarity of these connection deployments using end-to-end multi-fiber MPO/MTP components. Maintaining the correct polarity over a fiber network ensures that the transmit signal from any type of operating equipment is directed to the receiving port of a second operating equipment, and vice versa. In order to ensure that MPO/MTP systems operate with the correct polarity. The TIA 568 standard establishes three methods that we will explain in this article.

MPO/MTP Connector and Polarity

MPO/MTP connectors are high-performance, multifunctional fiber optic connectors that have improved the optical and mechanical performance of fiber optics. The special design (as shown in the illustration below) of the MTP/MPO connector ensures polarity accuracy in the MTP/MPO network system.

What is polarity? A common optical link requires two optical fibers to complete the entire transmission process. The optical module has a receiving end (RX) and a transmitting end (TX). When in use, it is necessary to ensure that the receiving end and the transmitting end are in an interconnected state. The concordance between the transmitting and receiving end at both ends of the optical link is known as polarity. In common wiring systems, LC and SC connectors can be easily combined, as there is no polarity problem. However, for pre-terminated high-density MTP/MPO cabling systems, polarity issues need to be addressed.

Three wires with three polarization methods

The three correct polarity methods are established according to TIA 568 and are called method A, method B and method C. In order to comply with these standards, three types of MPO fibers with different structures are used, which are designated as Type A, Type B, and Type C for the three different connectivity methods, respectively. We will first explain the types of cable and then the three connectivity methods.

MPO Type A Trunk Cable: Type A cable, also known as straight cable, is a direct-pass cable with an MPO connector with top coupler on one end and an MPO connector with bottom coupler on the opposite end. This causes the fibers at each end of the cable to have the same position. For example, the fiber located at position 1 (P1) of the connector on one side will reach P1 on the other connector. The fiber sequence of a 12-fiber MPO Type-A cable is shown below.

MPO Type B Trunk Cable: Type B cable (reverse cable) uses a coupler connector on both ends of the cable. This type of deployment coupling results in an inversion, which means that the positions of the fibers are reversed at each end. The P1 fiber at one end is coupled with the P12 fiber at the opposite end. The image below shows the fiber sequences of a 12-fiber Type B cable.

MPO Type-C Trunk Cable: The Type-C cable (reversed wire pairs) resembles the Type-A cable with a top coupler connector and a bottom coupler connector on each side. However, in type C, each pair of adjacent fibers at one end are flipped at the other end. For example, the fiber in position 1 at one end is switched to position 2 at the other end of the cable. The fiber in position 2 shifts to position 1 at the opposite end, etc. The sequence of the Type-C cable fibers is shown in the illustration below.

Three Connectivity Methods

Different polarity methods require different types of MTP trunk cables. However, all methods must employ duplex patch cords to obtain the fiber circuit. The TIA standard also defines two types of duplex fiber patch cords that terminate in LC or SC connectors to complete an end-to-end fiber duplex connection. Type A to Type A patch cable — a crossover version and Type A to Type B patch cable — direct connect.

The following describes how MPO system components are used to maintain proper polarization connectivity, which is determined by TIA standards.

Method A: The connectivity of Method A is shown in the following image. A Type A trunk cable connects an MPO module to each side of the link. In method A, two types of patch cables are used to correct polarity. The patch cable on the left is standard duplex Type A to Type B, while on the right a duplex Type A to Type A patch cable is used.

Method B: In connectivity method B, a Type B trunk cable is used to connect the two modules on either side of the link. As mentioned above, the positions of the type B cable fibers are reversed at each end. Therefore, standard duplex patch cables from Type A to Type B are used on both sides.

Method C: The reversed pair trunk cable is used in Method C connectivity to connect the MPO modules on either side of the link. The patch cords at both ends are type A to type B standard duplex.