Fiber Systems Best Practices for Ensuring Polarity of Array-Based Fiber Optic Channels 1
Fiber Systems: Polarity Best Practices Introduction Data centers are the central location for data interchange and are found in enterprises, government offices, schools, universities, hospitals, and other networked server farms. The ease of turning nearly any location into an information interchange hub has been enabled by the development of array-based fiber optic cabling systems. Ribbon fiber cables, array-based fiber connectors, and packaged breakout assemblies (i.e.
Fiber Systems: Polarity Best Practices Responding to the growing adoption of fiber cabling based on array connectivity, TIA/EIA developed and published an addendum to the standards governing cabling systems. TIA/EIA-568-B.1 Addendum 7, Guidelines for Maintaining Polarity Using Array Connectors, outlines three recommended methods for assuring correct transmit-to-receive polarity over serial duplex fiber circuits using ribbon cables and array connectors.
Fiber Systems: Polarity Best Practices Male Array Connector Pins Array Adapter Key Up No Pins Female Array Connector White Dot Fiber Position White Dot 1 12 Key Up Key Down Pin Location Figure 3. Array Connector Schematic (left) and End View (right) Cassettes. A cassette is a packaged, pre-terminated cabling assembly that transitions from small form factor ribbon-style fiber cables and array connectors to traditional single-fiber connectors (see Figure 4).
Fiber Systems: Polarity Best Practices Understanding Array-Based Polarity The basic strategy for designing polarity into an array-based serial duplex fiber optic system is to think of each ribbon cable as a series of two-fiber optical circuits that require polarity management. At some point along each circuit the fiber in odd-numbered array positions (i.e., the “transmit” position) must cross its duplex counterpart and end in an even-numbered position (i.e.
Fiber Systems: Polarity Best Practices Method A Method A employs Key Up to Key Down Adapters to link straight-through Key Up to Key Down ribbon cables to fiber cassettes (see Figure 7). This method maintains registration of Fiber 1 throughout the permanent link: Fiber 1 in the near-end cassette mates to Fiber 1 in the ribbon cable assembly, which mates to Fiber 1 in the remote cassette.
Fiber Systems: Polarity Best Practices Method B Method B employs Key Up to Key Up Adapters to link straight-through Key Up to Key Up ribbon cables to fiber cassettes (see Figure 8). Method B cassettes are identical to Method A cassettes except that they use Key Up to Key Up adapters. The fiber channel is completed by flipping the second cassette (i.e., rotating it 180 degrees) and re-numbering it, and then utilizing straight “A-to-B” patch cords at the beginning and end of the link.
Fiber Systems: Polarity Best Practices Method C Method C employs Key Up to Key Down Adapters to link a special Key Up to Key Down ribbon assembly to Method A cassettes (see Figure 9). In this method, polarity is achieved by using a ribbon cable that has the pair-wise flip built in to individual fiber pairs, and the fiber channel is completed by utilizing straight “A-to-B” patch cords at the beginning and end of the link.
Fiber Systems: Polarity Best Practices Which Method? In complex, high-density fiber installations, the advantages of array connectors can be lost if large-scale confusion and loss of local control result. Also, intermixing the components of these methods may result in incorrect polarity.
Fiber Systems: Polarity Best Practices Conclusion Ensuring fiber polarity over an array-based system requires a significant number of decisions early in the design process. The challenge for the system designer is ensuring that polarity of connections is observed from end to end, particularly in systems that contain many interconnect points. Constant attention is required to ensure that polarity decisions are adhered to by installers and other network stakeholders. TIA/EIA-568-B.
