Published In November 2000
Yes, optical fiber has its own version of tip and ring. With the acceptance of small-form factor (SFF) duplex connectors in the published TIA/EIA-568-B.3 Standard (Optical Fiber Cabling Component Standard) and the pending approval of ANSI/TIA/EIA-568-B.1 Standard (Commercial Building Telecommunications Cabling Standard), the optical fiber version of tip and ring has been redefined. It’s a simple matter of managing and ensuring that you have transmit-to-receive connectivity (polarity). Unlike copper, where the proper transmit-to-receive orientation is normally managed by the local area network (LAN) electronics, fiber systems require the cabling infrastructure installer to manage this connectivity at the time of installation. However, once the fiber cabling is installed correctly, the need to manage polarity is taken care of—the same patch/equipment cords can be used anywhere in the network and all the LAN electronic interfaces have the same polarity orientation. The fiber polarity on a small form factor (SFF) connector system is just like that of the 568SC (Duplex SC) originally established in TIA/EIA-568-A in 1995. However, many of the SFF connectors use a jack-plug versus plug-adapter-plug connection. This required a variation to the original guidelines. Both methods achieve the same result. Because the standard for “reverse-adapter positioning” is explained in the standard in detail, the second method “reverse-fiber pair positioning” is explained below. Both the SFF jack (used on installed cabling) and the SFF plug (used on patch cords) have two fiber positions (Position A and Position B),which are used to designate each fiber in the connector. Figure 1 shows Positions A and B, which are labeled on the jack in accordance with TIA. The SFF jacks and plugs have been designed so that the connection of a jack and plug causes a crossover within the connection, i.e., Position A of the plug connects to Position B of the jack. “Crossover” does not mean there is a physical crossing of the fibers; it simply means that odd-numbered fibers are Position A at one end and Position B at the other end (thus even numbered fibers are Position B at one end and Position A at the other end). Fibers operate in pairs just like copper wire; however, they are not color-coded with a white tracer. Because the jack-to-plug connection itself actually performs a crossover and the patch cords provide a crossover, the network will have the correct polarity as long as the installed cabling provides a crossover. In other words, it is impossible to have an even number of crossovers if the installed cabling is crossed-over. The TIA provides recommendations for the A-B and B-A orientation at various portions of the network, i.e., outlet or cross-connect (patch panel). These orientations within the TIA document are stated as if viewed from the front (user-side) of the jack. Since this article is written from an installer’s viewpoint, the orientation is presented from the back (installer-side) of the jack because this is where the rubber meets the road. However, these exact guidelines do not have to be followed to ensure polarity. A crossover (A to B and B to A) must be provided on each particular cabling segment. Horizontal (FttD) installations A horizontal cross-connect (patch panel) to telecommunications outlet jack installation follows these recommendations. If installing a four-fiber or higher fiber count cable to the outlet, then the odd-numbered fibers would be done the same as the blue fiber and the even-numbered fibers like the orange fiber. Backbone installations Backbone installations, whether intrabuilding (inside) or interbuilding (campus), also require a cabling crossover for the SFF cabling system. The backbone typically involves a patch panel on each end with a high-fiber-count cable between them. The same methodology applies if the patch panel is oriented vertically. The fibers in telecommunications room No. 1 are in A-B orientation and in telecommunications room No. 2 are in B-A orientation. Note that the fibers are organized from right to left (when viewed from the installer side). Also this figure is based on the SFF jack being in the same orientation (latch up) on both ends. The bottom line is: while tip goes to tip and ring to ring with copper, A goes to B and B goes to A with optical fiber. Products that clearly identify positions A and B and those that allow switching the fibers during installation without cutting off the connector offer a significant benefit to the installer in managing polarity. BEAM is director of systems marketing at AMP NETCONNECT Systems. He can be reached at (336) 727-5784 or firstname.lastname@example.org.