Getting 12- and 8-Fiber MPO Polarity Right

Base-8 vs. Base-12 MPO Connectivity

MPO connectors come in multimode and single-mode versions with various fiber counts and either male (with pins) or female (without pins). The pins ensure precise alignment when connectors are mated. Active equipment typically uses male MPO interfaces (with pins), so any MPO cable plugged directly into the equipment must be female (without pins) to prevent damage to the transceiver. MPO connectors also feature a key on the top side of the connector and a white dot on the side to indicate Position 1.

The 12-fiber MPO connector has historically been a popular choice for consolidating duplex backbone cabling in the data center. As parallel-optics applications emerged, which transmit and receive over multiple fibers, 12-fiber MPOs gained popularity for end-to-end links. However, these applications did not require all 12 fibers. For example, early 40GBASE-SR4 and 100GBASE-SR4 applications introduced in 2010 and 2015, respectively, used only 8 fibers: 4 transmitting and 4 receiving at 10 Gb/s or 25 Gb/s.

Initial deployments of 8-fiber applications often used 12-fiber MPO connectivity, leaving the four center fibers inactive, essentially wasting them. This inefficiency prompted manufacturers to develop 8-fiber MPO connectivity. To differentiate, the industry designated the two solutions as Base-8 and Base-12. The Base-8 MPO connector uses the same form factor as the Base-12 MPO, but with the four center fibers removed.

Data centers with existing Base-12 MPO infrastructure can upgrade to 8-fiber applications without wasting fibers by using MPO conversion patch cords or cassettes. For example, a conversion cord or cassette could break out two Base-12 MPOs (24 fibers total) to three Base-8 connectors (also 24 fibers total).

While Base-12 MPO connectivity remains common for consolidating backbone cabling in duplex applications, it is now rarely deployed for parallel optics. This is because most current parallel optics applications use 8 fibers:

• • 100 Gig: 4 fibers transmitting and 4 receiving at 25 Gb/s
• • 200 Gig: 4 fibers transmitting and 4 receiving at 50 Gb/s
• • 400 Gig: 4 fibers transmitting and 4 receiving at 100 Gb/s

The upcoming IEEE 802.3dj standard will also support 800 Gig over 8 single-mode fibers, with 4 fibers transmitting and 4 receiving at 200 Gb/s.

MPO Polarity Basics

Fiber optic polarity is critical for successful high-speed data transmission in MPO-based parallel optics links. When polarity isn’t maintained, the transmit (Tx) signal at one end won’t match the corresponding receiver (Rx) at the other end, resulting in a non-functional channel.

There are three primary methods for polarity: Method A, Method B, and Method C. These polarity methods remain the same whether the parallel optics link uses Base-12 or Base-8 connectivity.

Method A Polarity

Method A uses Type A straight-through MPO trunk cables with a key-up connector on one end and a key-down connector on the other. This configuration ensures that Position 1 on one end of the link arrives in Position 1 on the other end.

Type A adapters, which mate two connectors key-up to key-down, are used with a Type A MPO patch cord on one end and a Type B MPO patch cord on the other end to ensure that the Tx corresponds with the correct Rx. The Type B cord is key-up to key-up, which results in the fiber in Position 1 arriving at Position 12. Per TIA standards, only one Type B patch cord is allowed in a link.

Method B Polarity

Method B uses Type B reversed MPO trunk cables, which feature key-up connectors on both ends. This configuration causes the fiber at Position 1 to arrive at Position 12 at the other end.

Method B simplifies 12-fiber MPO polarity by using Type B trunk cables with Type B adapters, which mate two connectors key-up to key-up. Type B patch cords at both ends of the link, resulting in a key-up-to-key-up arrangement throughout the channel. Since the same type of patch cords is used uniformly throughout the channel, any uncertainty about cord selection is eliminated.

Method C Polarity

Method C uses Type C cross-over MPO trunk cables that flip the pairs: the fiber in Position 1 arrives at Position 2 at the opposite end, and the fiber in Position 2 arrives at Position 1, and so on.

While Type C MPO trunk cables work well for duplex applications, Method C is not recommended for parallel optics. Supporting parallel optics with Method C requires Type A adapters (key-up to key-down) with a complex Type C MPO cross-over patch cord at one end to reverse the pair flip and a Type B patch cord at the other end. Furthermore, Type C trunks and patch cables are not readily available.

The Angled End Face Conundrum

All single-mode MPO connectors use angled physical contact (APC) end faces, which feature an 8-degree angle to improve reflectance. It’s now common to find APC multimode MPO connectors in the data center for high-speed 400 Gig and 800 Gig applications that are more sensitive to back reflections.

While Method B polarity is recommended because it uses the same patch cord types on both ends, mating ACP MPO connectors requires a key-up to key-down configuration for proper end face alignment. That is why Type A adapters are almost always used in Method B polarity — often referred to as a “Modified Method B.” With Type A adapters, the orientation of the Type B cable changes, ensuring a key-up to key-down configuration to accommodate angled end faces.

How to Ensure Proper Polarity

Polarity method should be determined during the design phase to ensure the correct components are purchased. Unfortunately, confusion surrounding polarity often leads to the discovery of incorrect cables or patch cords after installation. Since pre-terminated MPO assemblies and jumpers are typically made to order (and often nonreturnable), ordering the wrong polarity can lead to unexpected project delays and costs.

The Fluke Networks MultiFiber™ Pro Tester allows you to test individual patch cords, permanent links, and channels for correct polarity (not to mention being able to automatically scan and test all 8 or 12 fibers for insertion loss results in just 6 seconds). So, if you mistakenly order and install Type B patch cords on both ends when your trunk cables are Type A, the MultiFiber Pro will alert you to the problem.

Instead of having to reorder patch cords with the correct polarity (and wait for them) or maintain a large inventory of different polarity types, some UPC multimode MPO connectors allow you to change polarity in the field. This capability can save money and prevent delays by correcting mistakes on-site.

• • US Conec’s MTP Pro: The key position on this connector can be changed between key up and key down using a special tool. The connector is inserted into the tool’s polarity-change port until it clicks, then removed.
• • Panduit’s PanMPO: Polarity is changed using an interchangeable housing that is easily removed, rotated 180 degrees, and replaced to change the key position between key up and key down. 

The PanMPO and MTP PRO also allow you to change gender on both UPC and APC single-mode and multimode connectors by removing or adding pins, so you can always ensure female-to-male mating at active equipment and adapters.  Gender can also be changed on Senko’s MPO connectors, as well as connectors offered by several connectivity manufacturers that license the US Conec and Senko connectors. Just remember that polarity cannot be changed on APC connectors due to the angled end face.

继续学习

• • 获取我们的最新电子书：当今数据中心的光纤测试最佳实践
• • 多芯推拉式 (MPO) 连接器
• • Fiber Polarity for Duplex Applications
• • MPOs in the Data Center and How to Test Them
• • 查看我们的光纤测试仪、工具和故障排查指南