FOA Guide 

Topic: Prefabricated Fiber Optic Cable Systems

Table of Contents: The FOA Reference Guide To Fiber Optics

Alternatives To Fiber Optic Termination In The Field:
Prefabricated Cabling Systems

    Is it necessary to field terminate fiber optic cabling systems? Not necessarily. If one has a good layout design and can accurately determine cable lengths between termination points, it's possible to have a complete cabling system factory made and delivered to the worksite ready to install and plug together.

    Prefabricated cable assemblies have been around for decades. In the early days of fiber optics for premises links, making a good field termination was more difficult than today. Many connectors used metal ferrules to hold the fiber that were hard to glue to the glass fiber. These metal connectors were difficult to properly polish because the metal contaminated the polishing films, sometimes requiring wet polishing. And the tolerances on the machined ferrules make low loss connectors a hit-or-miss proposition.

    Many cable and patchcord manufacturers offer a cable termination service. You specified what kind and number of fibers you needed, the cable and connector type and the length and the manufacturer would supply a completed assembly. And they guaranteed the connector and cable losses. There was no field termination needed.

    However, it was very important to accurately specify the cable length. That could be difficult in a premises environment, where cables go up and down walls, over ceilings and around obstacles. The usual method was to have plenty of extra length and store the excess in service loops in out of the way places like above the ceiling.
Nexans, prefab 1
Nexans, prefab    Another problem was how to protect the connectors becasue of the handling required during the installation process. Several manufacturers designed custom boots that fit over the connectors for protection and included a pulling loop attached to the strength members of the cable. (Photos shown courtesy of Nexans)

    This worked fairly well, especially for small fiber count cables of the breakout type where the jacketed simplex fibers provided protection to each connector. Distribution cables required more care because the connectors directly attached to the tight-buffered fibers had less protection. But under any circumstances, the total fiber count was limited because of the size of the fiber optic connectors common to the era made bunches of them inside a pulling boot bulky and hard to pull.
prefab modules
    The next development was creating a modular system that used multifiber cables with mass termination connectors (generally MTPs) and plug-in modules at the patch panel.  Ribbon cable has become a standard item, widely used in many large fiber count cables. Ribbon connectors, especially the MTP, have improved in performance enough to allow them to be used in mass terminations of pre-terminated cables. With the MTP, 12 fibers can be terminated in a connector hardly more bulky than a typical SC, which is easily pulled and installed. Using 12 fiber ribbon cables and MTP connectors, 12 connector modules could be easily prefabricated and installed. Complete modular systems included a module at each end and prefabricated ribbon cables for installation between termination points. (Photos courtesy Corning Cable Systems)

    Verizon’s desire to reduce installation time and cost in their fiber to the home programs (FiOS) spurred the development of outside plant prefab fiber optic cabling systems. The Verizon need was a result of volume. When you need to install connections to several million homes per year, small savings in time and labor cost become very important. In addition, finding enough experienced installation techs or training them is not easy.

MTP rugged    Manufacturers like Corning Cable Systems actually came up with a simple, elegant solution. Create a rugged, weather-sealed plastic shell around the MTP connector that provides protection before, during and after installation. Multifiber cables are installed by regular fiber techs from the central offices to near the homes, where mating drop boxes with 4, 6, 8 or 12 terminations are available. The FTTH tech plugs a cable into the drop box on one end an a network interface box on the home and the fiber installation is completed.

    Another technical development has made installing prefab cable assemblies indoors, bend-insensitive fiber. This allows for smaller cables that are more flexible and fit in smaller pathways. In fact bend-insensitive fiber is used in some outdoor cables for the same reason.

    But is a cabling system based on a concept developed for applications numbering in the millions cost effective for a single custom system? The only generalization that can be made is a factory-made product will probably be higher in component cost and lower in installation labor cost. For any given installation, it’s really necessary to price it both ways (being realistic on installation costs) for a cost comparison. Then the customer should consider the intangibles.

   Prefabricated systems are faster to install so if the cabling is going into a building currently occupied, say an airport, it may cause less work disruption. If the location is in a big city where labor costs are high, the factory-made system may be lower cost. If good design drawings are available, like in new construction, the engineering time to develop the cable plant design may be much lower.

Handling And Cleaning, Testing
    Prefab cables are not immune to problems. They must be handled carefully to avoid damage during unpacking and installation. Connectors must be carefully cleaned before connecting to other cables, modules or equipment. Testing often requires special reference cables and procedures for use with the multipin MTP/MPO connectors. Manufacturers generally supply information on handling these cables and their directions should be followed.

Here are other FOA Guide pages that might be helpful:

Table of Contents: The FOA Reference Guide To Fiber Optics


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