Ensuring Proper Polarity in MTPMPO Array Connections for Efficient Data Center Networks

MTP/MPO systems offer a solution to cable congestion problems in data centers or enterprise networks due to their flexibility, reliability, and scalability. However, some network designers face the challenge of ensuring proper polarity in these array connections when using end-to-end MTP/MPO multi-mode fiber optic components. Maintaining correct polarity in fiber optic networks ensures that transmission signals from any active device are accurately directed to the receiving ports of the second active device, and vice versa. The TIA 568 standard provides three methods to ensure that MTP/MPO systems operate with the correct polarity, which will be explained in this article.

MTP/MPO Connectors and MTP/MPO Polarity

The MTP/MPO connector is an innovative, high-performance fiber optic connector with enhanced optical and mechanical performance. The unique design of the MTP/MPO connector ensures accurate polarity in MTP/MPO network systems.

But what is polarity? A typical optical link requires two fibers to complete the entire transmission process. For example, an optical module has a receiver (Rx) and a transmitter (Tx). During operation, it is crucial to ensure that the receiver and transmitter are interconnected, and this matching between the transmitter and receiver ends of the optical link is known as polarity. In common cabling systems, connectors like LC and SC can easily match, so there is no polarity issue. However, for high-density pre-terminated MTP/MPO cabling systems, the polarity issue must be addressed.

Three Polarity Methods with Three Cable Types

The TIA 568 standard defines three methods for ensuring correct polarity, named Method A, Method B, and Method C. To comply with these standards, three different structures of MTP fiber optic cables are referred to as Type A, Type B, and Type C, respectively, and are used for the three different connection methods. This section will first introduce the three different fiber optic cable types, followed by the three connection methods.

MTP Trunk Cable Type A: Type A cables are straight-through fibers, with one end having a key-up MTP connector and the other end having a key-down MTP connector. This ensures that the fibers remain in the same position at both ends of the cable. For example, the fiber at position 1 (P1) on one connector will reach P1 on the other connector. The fiber sequence of a 12-fiber MTP Type A fiber jumper is shown in the following image:

MTP Trunk Cable Type B: Type B cables (reverse cables) use key-up connectors at both ends of the fiber optic cable. This type of array pairing results in fiber inversion, meaning that the fiber positions are reversed at each end. The fiber at P1 on one end is paired with the fiber at P12 on the other end. The following image shows the fiber sequence of a 12-fiber Type B cable.

MTP Trunk Cable Type C: Type C cables (pair-swapped cables) appear similar to Type A cables, with one key-up connector and one key-down connector on each side. However, in Type C, every pair of adjacent fibers at one end is flipped at the other end. For example, the fiber at position 1 on one end is moved to position 2 on the other end of the cable. The fiber at position 2 on one end is moved to position 1 on the other end, and so on. The fiber sequence of a Type C cable is shown in the following image.

Three Connection Methods

Different polarity methods use different types of MTP trunk cables, but regardless of the method, duplex jumpers should be used to establish the fiber optic link connection. The TIA standard also defines duplex fiber optic jumpers using LC/SC connectors: A-to-A type jumpers (crossover version) and A-to-B type jumpers (straight-through version).

The following sections introduce how to use the components together in an MTP system to maintain proper polarity connection, as defined by the TIA standard.

Method A: As shown in the image below, Type A trunk cables connect the MTP modules on each side of the link. In Method A, two types of jumpers are used to correct the polarity. The jumper on the left is a standard duplex A-to-B type, while the jumper on the right is a duplex A-to-A type.

Method B: In the Method B connection, Type B trunk cables connect the two modules on each side of the link. As mentioned earlier, the fiber positions in Type B cables are reversed at each end. Therefore, standard duplex A-to-B type jumpers are used on both sides.

Method C: In Method C, the connection uses pair-swapped reverse trunk cables, connecting one MTP module on each side of the link. The jumpers on both ends are standard duplex A-to-B type.

Maintaining MTP/MPO Polarity Rules During Connection Establishment

When fiber optic jumpers have different polarity schemes, IT personnel need to be very careful when replacing jumpers on-site. Those unfamiliar with polarity or eager to start and run equipment may incorrectly use jumpers, affecting optical signal transmission.

MTP/MPO Cable and Jumper Connection Rules

In situations where both A-to-A type jumpers and A-to-B type jumpers are present, there are three general types of array (multi-fiber) fiber optic cable assemblies. Note that the key position on MTP/MPO connectors is crucial for maintaining proper polarity. Therefore, it is important to ensure the correct key position before using jumpers to connect MTP/MPO fibers.

The A-to-B type LC/SC duplex jumper is a standard crossover cable that maps the Tx port to the Rx port by flipping to ensure correct polarity for A-to-B type jumpers. The Type B MTP trunk cable reverses the fiber positions at each end (1 to 12 and 12 to 1), and the connectors have keys facing up. It is recommended to use this cable to maintain proper MTP/MPO The selection of MTP/MPO patch panels determines the choice of MTP/MPO cable. It is best to choose fiber optic patch panels with suitable keys to ensure a perfect match between the MTP/MPO patch panel and the MTP/MPO connectors on both ends of the MTP/MPO cable. Additionally, the adapters mounted on the back of the patch panel will define it as Method A or Method B, in compliance with the TIA standard.

Conclusion

Network designers use MTP/MPO components to meet the increasing demand for higher transmission speeds, and one of the biggest challenges – polarity – can be addressed by selecting the correct MTP cables, MTP connectors, MTP patch panels, and fiber optic jumpers. The three different polarity methods can be applied to different situations to meet various application requirements.