Understanding Polarity in MPO/MTP System
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Understanding Polarity in MPO System

Posted on by FS.COM

MPO/MTP technology, which is of high density, flexibility and reliability with scalable, upgradeable properties, is one of the contributors that lead the migration to 40/100GbE. However, the network designers face another challenge which is how to assure the proper polarity of these array connections using multi-fiber MPO/MTP components from end-to-end. Maintain the correct polarity across a fiber network ensures that a transmit signal from any type of active equipment will be directed to receive port of a second piece of active equipment – and vice versa. To ensure the MPO/MTP systems work with correct polarity, the TIA 568 standard provided three methods, which will be introduced in this article. You can also refer to our white paper Understanding Fiber Polarity for completed polarity guide of

our products.

MPO/MTP Connector And MPO/MTP Polarity

The MPO/MTP connector is a multi-innovative, high-performance fiber optic connector that has enhanced optical and mechanical performance. The special design (shown in the following figure) of MTP/MPO connector ensures the accuracy of the polarity in the MTP/MPO network system.

MPO/MTP connector

But, what is polarity? A general optical link requires two optical fibers to complete the entire transmission process. For example, 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, and such matching between the transmitting end and the receiving end at both ends of the optical link is known as polarity. In common cabling systems, connectors such as LC and SC can easily be matched, so there is no polarity issue. However, for pre-terminated, high-density MTP/MPO cabling systems, polarity issues must be addressed.

Three Cables for Three Polarization Methods

The three methods for proper polarity defined by TIA 568 standard are named as Method A, Method B and Method C. To match these standards, three type of MPO fibers with different structures named Type A, Type B and Type C are being used for the three different connectivity methods respectively. In this part, the three different cables will be introduced firstly and then the three connectivity methods.

MPO Trunk Cable Type A: Type A cable also known as straight cable, is a straight through cable with a key up MPO connector on one end and a key down MPO connector on the opposite end. This makes the fibers at each end of the cable have the same fiber position. For example, the fiber located at position 1 (P1) of the connector on one side will arrive at P1 at the other connector. The fiber sequence of a 12 fiber MPO Type A cable is showed as the following:

Type-A-cable

MPO Trunk Cable Type B: Type B cable (reversed cable) uses key up connector on both ends of the cable. This type of array mating results in an inversion, which means the fiber positions are reversed at each end. The fiber at P1 at one end is mated with fiber at P12 at the opposing end. The following picture shows the fiber sequences of a 12 fiber Type B cable.

Type-B-cable

MPO Trunk Cable Type C: Type C cable (pairs flipped cable) looks like Type A cable with one key up connector and one key down connector on each side. However, in Type C each adjacent pair of fibers at one end are flipped at the other end. For example, the fiber at position 1 on one end is shifted to position 2 at the other end of the cable. The fiber at position 2 at one end is shifted to position 1 at the opposite end etc. The fiber sequence of Type C cable is demonstrated in the following picture.

Type-C-cable

Three Connectivity Methods

Different polarity methods use different types of MTP trunk cables. However, all the methods should use duplex patch cable to achieve the fiber circuit. The TIA standard also defines two types of duplex fiber patch cables terminated with LC or SC connectors to complete an end-to-end fiber duplex connection: A-to-A type patch cable—a cross version and A-to-B type patch cable—a straight-through version.

duplex-patch-cable

The following part illustrates how the components in MPO system are used together to maintain the proper polarization connectivity, which are defined by TIA standards.

Method A: the connectivity Method A is shown in the following picture. A type-A trunk cable connects a MPO module on each side of the link. In Method A, two types of patch cords are used to correct the polarity. The patch cable on the left is standard duplex A-to-B type, while on the right a duplex A-to-A type patch cable is employed.

Method A

Method B: in Connectivity Method B, a Type B truck cable is used to connect the two modules on each side of the link. As mentioned, the fiber positions of Type B cable are reversed at each end. Therefore standard A-to-B type duplex patch cables are used on both sided.

Method B

Method C: the pair-reversed trunk cable is used in Method C connectivity to connect the MPO modules one each side of the link. Patch cords at both ends are the standard duplex A-to-B type.

Method C

24-Fiber MPO/MTP Polarity Solution

To keep up with the pace of development, migration to 40/100GbE has become the trend. To facilitate smooth migration to 100G network, 24-fiber MTP/MPO cable is recommended. However, the polarity maintenance of base-24 MTP/MPO cabling is complicated. Currently, there is no standard in the industry to define the types of 24-fiber MTP/MPO cables. FS.COM recommends the Type A (key-up to key-up) MTP/MPO trunk cable. With our Type A and Type AF cassette, no need for A-to-A patch cable any more. The connectivity method is illustrated in the following picture.

24-Fiber MPO MTP Polarity Solution

Keep MPO/MTP Polarity Rules When Building Connection

When fiber patch cords have different polarity schemes and gender, IT staff needs to be very careful when replacing patch cords in the field. Those who don’t understand polarity or are in a rush to get equipment up and running can potentially use the wrong patch cord and impact signal transmission.

Rules for MPO/MTP Cable And Patch Cable Connection

Where there are A-to-A type patch cable and A-to-B type patch cable, there are three general types of array (multi-fiber) cable assemblies. It is noted that the alignment pins on the MPO/MTP connectors are important for maintaining the correct polarity. Therefore, it is necessary to make sure of proper pin position before you connect MPO/MTP fiber with patch cable.

A-to-B type LC/SC duplex patch cable is the standard crossover cord that maps the Tx port to the Rx port. With the flip, A-to-B type patch cable maintains proper polarity. And MPO trunk cable Type B reverses the fiber positions at each end (1 to 12 and 12 to 1) and the connector keys are both oriented face up. This kind of cable connection is recommended to keep proper MPO/MTP polarity.

Rules for MPO/MTP Cable And Cassette Connection

The selection of the MPO/MTP cassette will also determine the choice of MPO/MTP cable. You’d better choose a cassette with proper alignment pins, so that MPO/MTP cassettes can nearperfect mate with the MPO/MTP connectors at ether end of MPO/MTP cables. In addition, the rear of the adapter mounted on the cassette defines it as either Method A or Method B to correspond with the TIA standard.

We always recommend using female MPO cassettes and male-to-male type B MPO trunk cables. Because female-to-female MPO patch cords are required to avoid transceiver damage, MPO-to-LC cassettes should be female. These in turn require the use of male-to-male type B MPO trunk cables.

Conclusion

Network designer using MPO/MTP components to satisfy the increasing requirement for higher transmission speed, during which one of the big problems—polarity, can be solved by selecting the right types of MPO cables, MPO connectors, MPO cassette and fiber optic cables. The three different polarization methods can be applied according to the satisfy requirements in different situations. For more information about polarity in MPO systems and 40/100GbE transmission polarity solutions, please read the tutorial at “Polarity and MPO Technology in 40/100GbE Transmission“.

Related article: Cleaning Recommendation for MPO/MTP Connectors

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