May 2, 2024

    Cable Assembly Types and Polarity

    SEL_05.2024_Blog Cable Assemblies Banners_v1_Homepage Feature

    An Integral Component of any Network

    With ever-expanding LAN and Data Center bandwidth requirements, network data transmission speeds, latency, link loss, application optimization, and ease-of-install/maintenance/MACs have become paramount. Fiber optic cables and cable assemblies are the backbone of these high-speed and often ultra-dense networks. 

    Most network installers and operators use cable assemblies daily, but the selection and polarity of cable assemblies are still often a mystery. In this blog, we will look at two of the most fundamental and commonly misunderstood aspects of cable assemblies:

    • The differences between Simplex and Duplex cable assemblies.
    • Assembly polarity and its role in effective network communication.


    Simplex vs. Duplex

    Assembly Structural Differences

    Although often associated only with fiber count within the assembly itself, the difference between a Simplex cable assembly and a Duplex cable assembly, both of which are tight-buffered and have the same jacketing, strength members and are available in single-mode and multimode, is that Simplex cables (see example A) have only one fiber with only one end being the transmitter (Tx) and the other end the only receiver (Rx). In contrast, Duplex cables most commonly have two individual fiber cables, joined in a zip-cord fashion, allowing one fiber to transmit from point A to B and the other from point B to A, making both ends of the assembly both a transmitter (Tx) and a receiver (Rx). This type of Duplex cable is called “full-duplex” (see example B). 

    Note: While not the norm due to increased overall system cost, single-fiber/single-strand cables are another form of duplex cables, which is “half-duplex” (see example C). These cables can transmit and receive by transferring data over two wavelengths in opposite directions but not simultaneously, causing a small lag over their full-duplex counterparts. 


    SEL_05_2024_Blog Cable Assemblies_Images_a-simplexSEL_05_2024_Blog Cable Assemblies_Images_b-duplexSEL_05_2024_Blog Cable Assemblies_Images_c-halfduplex


    Which Assembly to Use for the Application

    Choosing between simplex and duplex cables will depend on the specific needs of your network and the application in question.

    Simplex Cables (Single-mode and multimode) are designed for applications requiring only one-way data transmission, often to an information output device. They are simple to install and maintain and highly resistant to electromagnetic interference (EMI) and radio frequency interference (RFI). Simplex cables have the added benefit of functioning as a “pass-through,” allowing any devices in the sequence to communicate with all other devices in that network.

    Duplex Cables (Single-mode and multimode) are generally used as interconnectors or cross-connects between fiber optic transceivers or for standard optical devices requiring a simultaneous bidirectional data transfer. Thanks to increased bandwidth capacity, duplex cables are often used in telecom and workstation environments. However, they are more complex to install and maintain, more susceptible to interference, and allow only the two connected devices to communicate at once.




    In its simplest form, fiber polarity is the direction data/a light pulse takes from traveling through a cable, point A to point B. For polarity to be maintained and, thereby the connection between the devices achieved, a fiber optic link’s transmit signal (Tx) at the end of the cable must match the corresponding receiver (Rx) at the other end.


    It is important to note that the TIA-568-D standard outlines two types of fiber links: serial duplex signal connections and parallel signal (MPO/MPT) connections. 



    Full-Duplex Assembly Polarity Maintenance

    Two types of duplex fiber patch cords are defined in the TIA standards: A-to-A type (cross-over) shown in Example D and A-to-B (straight-through) shown in Example D


    SEL_05_2024_Blog Cable Assemblies_Images_v2_AAFiberCatchCable

    Example D

    Note: A-to-A patch cords are not commonly deployed and should be used only, when necessary, as part of a polarity method


    To help network operators and installers maintain proper channel-wide polarity, TIA standards recommend the A-B polarity scenario for duplex patch cords. This straight-through connection maintains the A-B polarity in a duplex channel (Example E).


    SEL_05_2024_Blog Cable Assemblies_Images_v2_ABFiberPatchCable

    Example E

    Note: Every fiber connector is keyed to prevent fiber rotation during connector-mating guaranteeing the correct Tx and Rx connection.


    MPO/MPT Assembly Polarity Maintenance

    The TIA has defined three different polarity methods to maintain fiber polarity when using multi-fiber MPO/MTP array patch cords. Each method uses different types of MPO cables: Type A, B, and C are used for the three different connectivity Methods, A, B, and C, respectively. With three different polarity methods and different patch cords for each, mistakes are common.


    Method A

    Method A uses Type A (straight-through) MPO cables with a key-up connector on one end and a key-down connector on the other end so that the fiber located in Position 1 (Tx) arrives at Position 1 (Tx) at the other end.


    SEL_05_2024_Blog Cable Assemblies_Images_v2_typeA-straightthrough


    Method B

    Method B uses key-up connectors on both ends to achieve the transceiver-receiver flip so that the fiber located in Position 1 (Tx) arrives at Position 12 (Rx) at the opposite end, the fiber located in Position 2 (Rx) arrives at Position 11 (Tx) at the opposite end, and so on.


    SEL_05_2024_Blog Cable Assemblies_Images_v2_typeB-cross


    Method C

    Like Method A, Method C uses a key-up connector on one end and a key-down connector on the other end. However, the flip happens within the cable itself, where each pair of fibers is flipped so that the fiber in Position 1 (Tx) arrives at Position 2 (Rx) at the opposite end, and the fiber in Position 2 (Rx) arrives at Position 1 (Tx).


    SEL_05_2024_Blog Cable Assemblies_Images_v2_typeC-crosspair



    SEL Cable Assemblies

    Sumitomo Electric Lightwave (SEL), an industry leader in fiber, cabling techniques, and connectorization, has one of the widest selections of cable assemblies: patch-cords, jumpers, rip-cords, trunks, and interconnects of any industry-leading fiber optic solutions provider to include the often-illusive exact length cable assemblies.

    Learn more about our selection of assemblies:

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