MTP/MPO Fiber Optic Polarity Explained

With the increasing bandwidth and network connection needs of data centers, MTP/MPO fiber cables have become the main solution for 40G/100G/400G high-density environments. Individual cables come in 8 through 72 fiber strands (12 fibers being the most common) and have a variety of pinout options to suit different application requirements. In this article we will go over those MTP/MPO Fiber Pinout options.

Anatomy of an MTP/MPO Connector
The MTP/MPO connectors can contain from 8 through 72 fiber strands, 12 strands being the most common. The connectors come in a Male (with pins) or Female version (no pins), this helps keep the end face aligned at the fiber connections – The end-face alignment/polish usually come in Straight (for MM) or Angled (for SMF). The connectors also have a Keys (or grooves) at the top or bottom which keeps them in place after being connected. Lastly there is a White mark on the side of the connectors which indicates where the Fiber position 1 is. All of these connector features help users maintain the proper Polarity required across different applications, ensuring the transmit and receiving ends are interconnected properly. Because there are so many combinations of these features for one MPO cable it is important to understand when and how they are used.

Anatomy of an MTP/MPO Connector

The Three 12Fiber Polarity/Pinout Methods
There are three polarity standards defined by TIA 568 (Telecommunications Industry Association), these are Method A, Method B, and Method C. These methods are defined in more detail below.

The Three 12Fiber Polarity Methods

Method A MTP/MPO
Type A aka Method A is known as a straight through cable, it has a key up on one connector end and a key down on the other connector end of the cable. This design allows the fibers to have the same position at each end of the cable, for example Fiber1 from one end will connect to Fiber1 on the other end.

Method B MTP/MPO
Type B aka Method B is known as a reverse cable, it has key up orientation on both connector ends. This design allows each fiber to mate with the opposite end, for example Fiber1 from one end will connect to Fiber12 on the other end. See Type B 12Fiber diagram below for more details.

Method C MTP/MPO
Type C aka Method C is known as a pairs flipped cable, it has a key up on one end and a key down on the other end of the cable. This design allows each adjacent fiber on one end to be flipped on the other end, for example Fiber1 from one end will connect to Fiber2 on the other end. See Type C 12Fiber diagram below for more details.

Polarity Connectivity Examples
The TIA standard also explains that all MTP/MPO polarity methods should use a Duplex patch cable to complete the fiber circuit. There are two types of Duplex fiber patch cable options A to A connection and A to B connection.

Fiber Jumper Polarity A to A and A to B

Since the goal is for the transmit (Tx) signal to connect to the receiving (Rx) equipment, the tables below shows the most common component combinations to achieve this.

Applications With Cassettes and MTP-MPO
10Gig Duplex and 40/100 Gig Parallel Applications

Commonly Used 12 strand Applications
Method A cables are used in Duplex applications along with a standard A-B LC/SC patch cord on one end and A-A LC/SC patch cord on the other end to set the proper Tx/Rx positions required.
Type A cables are also used on one end of 40/100 Gig applications to connect the patch panel ports to their corresponding transceiver ports. This setup will need a Type B MPO patch cord on the other end of the channel.

Method B cables are used on both ends of 40/100 Gig applications to connect patch panel ports to their corresponding ports. This would mean the same type of patch cords would be needed for both Duplex and Parallel applications, this is why Method B is most often recommended.

Method C is used when you need to break the MTP/MPO cable out to Duplex LC/SC connectors and need to connect those up to mate to Tx or Rx you will have to use a Pairwise reversal. The alternate way to accomplish this is by handling the reversal on the MTP/MPO breakout modules on either end, this method is preferred since you will be able to convert the same cable for parallel optics if needed.

Conclusion
Due to the increased migration to 40G/100G/400G in Data Centers, MTP/MPO cables are the ideal high density cabling solution, but pose complex polarity questions. These polarity issues can be managed by selecting the correct type of combination of MTP/MPO cable, Connectors, Cassettes, and Fiber Patch cables. As shown in the tables above, regardless of the method selected, there will need to be a pair-wise flip somewhere in the link.
When dealing with MTP/MPO cables it is important to keep the following top of mind: Know how many products you will need to connect and how much loss your network can tolerate, and design with as few components as possible.

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