System polarity is -- and has always been -- a critical part of a structured wiring design.
May 1, 2001
Optical fiber communication systems are based on fiber pairs. Optical transceivers have two ports: one for “transmit” and one for “receive”. In order for a system to properly function, the “transmit” port from one end of a link must be connected to the “receive” port of the other.
Optical fiber connection technology used to be based primarily on single-conductor, or simplex, connector designs. Although channels were routed in pairs, connections were made one fiber at a time. Polarity mismatches could easily be corrected at the patch panel simply by exchanging the positions of the “transmit” and “receive” connectors.
The fiber distribution data interface (FDDI) duplex connector was the first duplex optical connector to be endorsed by a standard. The bulky design of this connector accommodated a set of exchangeable keys that could be field-installed to obtain the proper polarity.
The more compact SC duplex connector style was later adopted by the ANSI/EIA/TIA-568-A, Commercial Building Telecommunications Cabling Standard, but this is most often employed as two simplex connectors clipped together. Field implementation or correction of polarity mismatch is quite easily accomplished with the SC duplex by exchanging the two connectors within the clip.
UPDATING THE STANDARD
ANSI/EIA/TIA-568-A will be updated in the near future and will be published as three separate documents: 568-B.1, Commercial Building Telecommunications Standard; 568-B.2, Commercial Building Telecommunications Cabling Standard Part 2: Balanced Twisted-Pair Cabling; and ANSI/TIA-568-B.3, Optical Fiber Cabling Components Standard. The TIA-568-B.3 document has been published. The other two documents have completed TIA ballot and will be published in the near future.
When ANSI/EIA/TIA-568-B.1 is published, polarity will be addressed for SC duplex connector systems with Consecutive Fiber Numbering (CFN). CFN is depicted in the standard in Figure 1.
CONSECUTIVE FIBER NUMBERING
CFN maintains polarity by rotating the adapters 180 degrees on one end of each link, and maintaining fiber colouring with consecutive numbering (i.e., the blue fiber in position one, and the orange fiber in position two on both ends of the link). This means the adapter will be oriented with the key up on one end of the link and the key down on the other end. The polarity is achieved on the front of the patch panel by the orientation of the patch cords. (Please see Figure 2 above. Note the rotation of the adapters and the continuity of the orientation of the fibers on the back, or cable, side of the panels).
For other duplex connectors that meet the requirements of ANSI/EIA/TIA-568-B.3 (such as the MT-RJ), CFN or Reverse-Pair Positioning (RPP) may be implemented. However, there is no diagram in the standard for RPP. The TIA-TR 42.8 Telecommunications Optical Fiber Cable System subcommittee has approved the publication of a Telecommunications Systems Bulletin (TSB) on RPP to clarify its implementation. The TSB may have a diagram similar to Figure 3 below:
With RPP, fiber polarity is achieved by reversing the position in which the coloured fibers are plugged into the back side of the panel, on one end of the link. On one end of the link, “blue” will be in position 1 and “orange” in position 2; on the other end, “blue” will be in position 2 and “orange” in position 1.
One of the above methodologies should be used for all links within the system (i.e., MC-IC, IC-HC, HC-WA). If every link is implemented in the same manner, polarity will be correct regardless of the number of cross connects, interconnects or the manner in which the end user connects the patchcords. Figure 4 depicts RPP with an interconnect.
THE MT-RJ CONNECTOR
The MT-RJ connector incorporates multiple fibers in a single body or ferrule, and offers easy and quick installation, reduced size and increased packing density. Polarity can be easily reversed once the connector is installed on a cable. The MT-RJ connector is designed so that the transmit fiber is located on the left side of the key, looking at the tip of the connector with the key facing up. For a system to operate properly, the fiber that is in the “transmit” position on one end of the system must be in the “receive” position on the other end. Standard, factory-assembled MT-RJ jumpers are manufactured so that the fiber in the “transmit” position on one end is in the “receive” position on the other.
A typical communications link incorporates two jumpers — one on each end, with each running from a patch panel to a transceiver. Using two jumpers results in either two changes of fiber position or no net change at all. It is therefore necessary to incorporate a change of position in the structured wiring between the two patch panels to achieve a net end-to-end fiber position reversal.
To facilitate either the CFN or RPP method, MT-RJ dual-keyed adapters are available for use in patch panels. These adapters have two key positions on the back side of the connector panel only. The special adapters allow the installer to implement CFN on the backside of the patch panel, while the front side is single-keyed so the user can only connect to the structured wiring in one way. Even if RFP is used (and no position reversal is necessary in the adapter), the double-slotted adapters are useful for correcting connectorization polarity errors if they occur.
System polarity has always been a critical part of a structured wiring design. Duplex SFF connectors (such as the MT-RJ) will require that a polarity scheme be planned and selected in advance of an installation. The double-slot feature on the back side of MT-RJ patch panels allows polarity to be adjusted at that point, if necessary, and still remain fixed and foolproof in the completed installation.CS
Doug Coleman is Manager, Technology and Standards – Private Networks at Corning Cable Systems, Corning, NY. He is active in the development of optical specifications at the Insulated Cable Engineering Association (ICEA), the Society of Cable Telecommunications Engineers (SCTE) and TIA/EIA, IEC, IEEE and Fiber Channel standards groups.
Angela May, RCDD, is OEM Marketing Specialist at Corning Cable Systems. Since joining Corning in 1993, Ms. May has served as production team leader for the supervision of cable assemblies at the company’s Specialty Cable Plant, and as field engineer in the Engineering Services department.