Most contractors assume it is necessary to field-terminate fiber optic cabling systems as part of every installation, but they are all looking for alternatives. The first alternative most people consider is to use prepolished/splice connectors, which use a mechanical splice to terminate the fiber. Another option, considered the best solution for single-mode fiber, is to fusion splice a factory-made pigtail onto the end of each fiber. Some manufacturers now offer something in between these two methods: prepolished/splice connectors that attach by fusion splicing, which provides significantly better performance without the need for the splice closures required for pigtails.
Another alternative, if you have a good cable system design so you can accurately determine cable placement between termination points, is to have a complete cabling system factory-made and delivered to the work site 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 difficult. Some cable and patchcord manufacturers offered 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 complete assembly.
The cables were shipped with the terminated ends protected by boots, which had pulling loops attached to protect the connectors during the installation process. This worked fairly well, especially for small fiber count cables. But the total fiber count was limited because the fiber optic connector size—such as the ST and SC common to the era—made bundles of them inside a pulling boot bulky and hard to pull. The advent of the LC connector reduced connector bulk considerably, so cables with larger fiber counts could be accommodated.
The next development in prefab cabling systems came with the introduction of the multifiber MTP connector. This single connector—not much bigger than a SC connector—terminated 12 fibers in a row using one connector, greatly reducing the bulk at the end of the cable. The MTP made pulling the cable from location to location much easier, even through conduit. The MTP connected on either end to breakout cables (MTP to 12 single fiber connectors, such as a LC, SC or ST) or to rack-mounted modules at a patch panel that has 12 LC, SC or ST connections.
Another development, ribbon cable, has become a standard item in fiber optics, widely used in many large fiber count cables. When combined with ribbon connectors, this product creates a compact cable assembly, which is easily pulled and installed in any premises cabling application.
What has influenced these developments? Verizon’s desire to reduce installation time and cost in the company’s fiber-to-the-home programs (FiOS) has spurred the development of outside plant prefab fiber optic cabling systems. Verizon’s need was a result of volume of usage. 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 experienced installation techs and training them is not easy.
Manufacturers, such as Corning Cable Systems, came up with a simple, elegant solution: Create a rugged, weather-sealed plastic shell around the small MTP connector that provides protection before, during and after installation.
Regular cables can be installed by fiber installation techs from the central offices to near the homes, connecting to mating drop boxes with 4, 6, 8 or 12 terminations. The fiber-to-the-home (FTTH) tech plugs a cable into the drop box on one end and a network interface box on the home, completing the fiber installation.
Designing a prefab cabling system
Obviously, installing a prefab cabling system is a different process than not using prefab, but how about designing the system? A regular system requires only figuring out how many meters or feet of cable, how many connectors should be needed, and what quantities of related supplies and hardware are necessary. You order enough cable length to ensure you have more than what you need and extra connectors to cover yield in termination. Often, cables even are ordered with extra fibers to have spares.
But a prefab system is more complex. You still need to know how many fibers you need for every cable link, but you will be rounding off the numbers, since you must order cables in multiples of six or 12 fibers. You need good drawings of the routings for the cables and locations of the modules at each end. Obstacles that require moving the cable route will require additional length, as it may need to go up and down to ceilings and under floors. Obviously, ordering a cable too short is a big problem, but ordering too long means you have to find places for storage loops. Therefore, it’s very important to have good drawings and make several site visits to know what the cable route really looks like. You also need to specify the types of termination modules that will be used at various locations, as they will be ordered with the cable as an integral part of the system.
Fortunately, manufacturers that offer prefab systems, such as Corning, have experienced design engineers that work with you to specify your system. Their experience will be important in ensuring you don’t make expensive mistakes. If you work with the manufacturer, by the time you order the system, it will be properly specified.
Installation and test
Here you will find a big difference in a prefab cabling system. You will have to carefully despool and pull or place cables. The ends of the cables will be bulkier than unterminated cables and several people may be needed to place the cable. Pulling in conduit requires connecting to the pulling loops provided on the assembly and exercising care to keep pulling tension within limits. If pulling lubricant is needed, it should be a type approved by the cable manufacturer and properly cleaned up. Connectors should always be kept covered with dust caps and appropriate cleaners should be used before making final connections.
Testing can be simpler, since the system has been tested at the factory. After the cables have been connected to the end modules, they can be tested end-to end using standard insertion loss tests per OFSTP-14. Just remember when testing that the link contains two MTP connections in addition to the fiber and connectors on each end. If your test equipment is in good calibration, your results should be close to the manufacturer’s test results.
What about the economics?
Can a prefab cabling system be profitable for a contractor as well as cost effective for the customer? A factory-made cabling system will probably be higher in component cost and lower in installation labor cost. The advantage to the contractor is the cable can be installed by less experienced personnel, even those not trained in fiber optic termination. And the contractor will probably spend more time carefully going over the cable plant with the customer and the manufacturer, which is billable time.
Prefabricated systems are faster to install, so if the cabling is going into a building currently occupied, say an airport or large office building, it may cause less work disruption or take less overtime for after-hours installation.
More fiber users are considering preterminated fiber optic cabling systems, so it is to your advantage to learn more about them. Companies such as Corning Cable Systems have information on their Web sites that can provide detailed information on their prefabricated cable system products and even offer planning guides to designers.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.JimHayes.com.